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8-channel, 16-bit, simultaneous sampling adc with power scaling, 110.8 khz bw data sheet ad7761 rev. 0 document feedbac k information furnished by analog devices is believed to be accurate and reliable. however, no responsibility is assumed by analog devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. specifications subject to change without notice. no license is granted by implication or otherwise under any patent or patent rights of analog devices. trademarks and registered trademarks are the property of their respective owners. one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781.329.4700 ?2016 analog devices, inc. all rights reserved. technical suppor t www.analog.com features precision ac and dc performance 8-channel simultaneous sampling 256 ksps adc output data rate per channel 97.7 db dynamic range 110.8 khz input bandwidth (?3 db bandwidth (bw)) C120 db thd, typical 1 lsb inl, 1 lsb offset error, 5 lsb gain error optimized power dissipation vs. noise vs. input bandwidth selectable power, speed, and input bandwidth fast: highest speed; 110.8 khz bw, 51.5 mw per channel median: half speed, 55.4 khz bw, 27.5 mw per channel focus: lowest power, 13.8 khz bw, 9.375 mw per channel input bw range: dc to 110.8 khz programmable input bandwidth/sampling rates cyclic redundancy check (crc) error checking on data interface daisy-chaining linear phase digital filter low latency sinc5 filter wideband brick wall filter: 0.005 db ripple to 102.4 khz analog input precharge buffers power supply avdd1 = 5 v, avdd2 = 2.25 v to 5.0 v iovdd = 2.5 v to 3.3 v or iovdd = 1.8 v 64-lead lqfp package, no exposed pad temperature range: ?40c to +105c applications data acquisition systems: usb/pxi/ethernet instrumentation and industrial control loops audio test and measurement vibration and asset condition monitoring 3-phase power quality analysis sonar high precision medical electroencephalogram (eeg)/ electromyography (emg)/electrocardiogram (ecg) functional block diagram adc output data serial interface digital filter engine wideband low ripple filter sinc5 low latency filter spi control interface 1.8v ldo sync_in 1.8v ldo buffered vcm vcm ain1+ ch 1 ain1? ain2+ ch 2 ain2? ain3+ ch 3 ain3? ain4+ ch 4 ain4? ain5+ ch 5 ain5? ain6+ ch 6 ain6? ain7+ ch 7 ain7? ain0+ ch 0 ain0? vcm 8 precharge reference buffers sync_out start reset format1 format0 drdy dclk st0/cs pin/spi st1/sclk dec0/sdo dec1/sdi avss xtal2/mclk xtal1 mode3/gpio3 to mode0/gpio0 filter/gpio4 - ? adc - ? adc - ? adc - ? adc - ? adc - ? adc - ? adc - ? adc dout7 avdd1a, avdd1b dgnd ad7761 iovdd dregcap regcapa, regcapb a vdd2a, avdd2b refx+ refx? offset, gain phase correction offset, gain phase correction offset, gain phase correction offset, gain phase correction offset, gain phase correction offset, gain phase correction offset, gain phase correction offset, gain phase correction dout1 14285-001 dout0 dout2 dout3 dout4 dout5 dout6 p p p p p p p p p p p p p p p p 16 analog input precharge buffers (p) figure 1.
ad7761 data sheet rev. 0 | page 2 of 69 table of contents features .............................................................................................. 1 ? applications ....................................................................................... 1 ? functional block diagram .......................................................... 1 ? revision history ............................................................................... 2 ? general description ......................................................................... 3 ? specifications ..................................................................................... 4 ? timing specifications .................................................................. 9 ? 1.8 v iovdd timing specifications ....................................... 10 ? absolute maximum ratings .......................................................... 14 ? thermal resistance .................................................................... 14 ? esd caution ................................................................................ 14 ? pin configuration and function descriptions ........................... 15 ? typical performance characteristics ........................................... 19 ? terminology .................................................................................... 25 ? theory of operation ...................................................................... 26 ? clocking, sampling tree, and power scaling ............................. 26 ? noise performance and resolution .......................................... 27 ? applications information .............................................................. 29 ? power supplies ............................................................................ 30 ? device configuration ................................................................ 31 ? pin control mode ....................................................................... 31 ? spi control .................................................................................. 34 ? spi control functionality ......................................................... 35 ? spi control mode extra diagnostic features ........................ 37 ? circuit information ........................................................................ 38 ? core signal chain ....................................................................... 38 ? analog inputs .............................................................................. 39 ? vcm ............................................................................................. 40 ? reference input ........................................................................... 40 ? clock selection ........................................................................... 40 ? digital filtering ........................................................................... 40 ? decimation rate control .......................................................... 42 ? antialiasing ................................................................................. 42 ? calibration ................................................................................... 43 ? data interface .................................................................................. 45 ? setting the format of data output .......................................... 45 ? adc conversion output: header and data .......................... 46 ? functionality ................................................................................... 54 ? gpio functionality .................................................................... 54 ? register map details (spi control) ............................................. 55 ? register map ............................................................................... 55 ? channel standby register ......................................................... 57 ? channel mode a register ......................................................... 57 ? channel mode b register ......................................................... 58 ? channel mode select register .................................................. 58 ? power mode select register ...................................................... 59 ? general device configuration register .................................. 59 ? data control: soft reset, sync, and single-shot control register ........................................................................................ 60 ? interface configuration register .............................................. 61 ? digital filter ram built in self test (bist) register ............ 61 ? status register ............................................................................. 62 ? revision identification register ............................................... 62 ? gpio control register .............................................................. 62 ? gpio write data register ......................................................... 63 ? gpio read data register .......................................................... 63 ? analog input precharge buffer enable register channel 0 to channel 3 .................................................................................... 64 ? analog input precharge buffer enable register channel 4 to channel 7 .................................................................................... 64 ? positive reference precharge buffer enable register ............ 65 ? negative reference precharge buffer enable register .......... 65 ? offset registers ........................................................................... 65 ? gain registers ............................................................................. 66 ? sync phase offset registers ...................................................... 66 ? adc diagnostic receive select register ................................ 66 ? adc diagnostic control register ........................................... 67 ? modulator delay control register ........................................... 68 ? chopping control register ....................................................... 68 ? outline dimensions ....................................................................... 69 ? ordering guide .......................................................................... 69 ? revision history 4/16revision 0: initial version
data sheet ad7761 rev. 0 | page 3 of 69 general description the ad7761 is an 8-channel, simultaneous sampling - analog- to-digital converter (adc) with a - modulator and digital filter per channel, enabling synchronized sampling of ac and dc signals. the ad7761 achieves 97.7 db dynamic range at a maximum input bandwidth of 110.8 khz, co mbined with typical performance of 1 lsb integral nonlinearity (inl), 1 lsb offset error, and 5 lsb gain error. the ad7761 user can trade off input bandwidth, output data rate, and power dissipation. select one of three power modes to optimize for noise targets and power consumption. the flexibility of the ad7761 allows it to become a reusable platform for low power dc and high performance ac measurement modules. the ad7761 has three modes: fast mode (256 ksps maximum, 110.8 khz input bandwidth, 51.5 mw per channel), median mode (128 ksps maximum, 55.4 khz input bandwidth, 27.5 mw per channel) and focus mode (32 ksps maximum, 13.8 khz input bandwidth, 9.375 mw per channel). the ad7761 offers extensive digital filtering capabilities, such as a wideband, low 0.005 db pass-band ripple, antialiasing low- pass filter with sharp roll-off, and 105 db attenuation at the nyquist frequency. frequency domain measurements can use the wideband linear phase filter. this filter has a flat pass band (0.005 db ripple) from dc to 102.4 khz at 256 ksps, from dc to 51.2 khz at 128 ksps, or from dc to 12.8 khz at 32 ksps. the ad7761 also offers sinc response via a sinc5 filter, a low latency path for low bandwidth, and low noise measurements. the wideband and sinc5 filters can be selected and run on a per channel basis. within these filter options, the user can improve the dynamic range by selecting from decimation rates of 32, 64, 128, 256, 512, and 1024. the ability to vary the decimation filtering optimizes noise performance to the required input bandwidth. embedded analog functionality on each adc channel makes design easier, such as a precharge buffer on each analog input that reduces analog input current and a precharge reference buffer per channel reduces input current and glitches on the reference input terminals. the device operates with a 5 v avdd1a and avdd1b supply, a 2.25 v to 5.0 v avdd2a and avdd2b supply, and a 2.5 v to 3.3 v or 1.8 v iovdd supply (see the 1.8 v iovdd operation section for specific requirements for operating at 1.8 v iovdd). the device requires an external reference; the absolute input reference voltage range is 1 v to avdd1 ? avss. for the purposes of clarity within this data sheet, the avdd1a and avdd1b supplies are referred to as avdd1 and the avdd2a and avdd2b supplies are referred to as avdd2. for the negative supplies, avss refers to the avss1a, avss1b, avss2a, avss2b, and avss pins. the specified operating temperature range is ?40c to +105c. the device is housed in a 10 mm 10 mm 64-lead lqfp package with a 12 mm 12 mm printed circuit board (pcb) footprint. throughout this data sheet, multifunction pins, such as xtal2/mclk, are referred to either by the entire pin name or by a single function of the pin, for example mclk, when only that function is relevant.
ad7761 data sheet rev. 0 | page 4 of 69 specifications avdd1a = avdd1b = 4.5 v to 5.5 v, avdd2a = avdd2b = 2.0 v to 5.5 v, iovdd = 1.72 v to 1.88 v and 2.25 v to 3.6 v, avss = dgnd = 0 v, refx+ = 4.096 v and refx? = 0 v, mclk = 32.768 mhz, analog input precharge buffers on, reference precharge buffers off, wideband filter, f chop = f mod /32, t a = ?40c to +105c, unless otherwise noted. table 1. parameter test conditions/comments min typ max unit adc speed and performance output data rate (odr), per channel 1 fast 8 256 ksps median 4 128 ksps focus 1 32 ksps ?3 db bandwidth fast, wideband filter 110.8 khz median, wideband filter 55.4 khz focus, wideband filter 13.8 khz data output coding twos complement, msb first no missing codes 2 16 bits dynamic performance decimation by 32, 256 ksps odr dynamic range shorted input, wideband filter 97.3 97.7 db signal-to-noise ratio (snr) 1 khz, ?0.5 dbfs, sine wave input sinc5 filter 97.3 97.9 db wideband filter 97.3 97.7 db signal-to-noise-and-distortion ratio (sinad) 1 khz, ?0.5 dbfs, sine wave input 97.3 97.7 db total harmonic distortion (thd) 1 khz, ?0.5 dbfs, sine wave input ?120 ?107 db spurious-free dynamic range (sfdr) 126 dbc intermodulaton distortion (imd) f ina = 9.7 khz, f inb = 10.3 khz second order ?125 db third order ?124 db accuracy inl 3 endpoint method 1 1.5 lsb offset error 4 1 2 lsb gain error 4 t a = 25c 5 40 lsb gain drift vs. temperature 2 0.01 0.02 lsb/c vcm pin output with respect to avss (avdd1 ? avss)/2 v load regulation ?v out /?i l 400 v/ma voltage regulation applies to the following vcm output options only: v cm = ?v out /?(avdd1 ? avss)/2, v cm = 1.65 v, and v cm = 2.5 v 5 v/v short-circuit current 30 ma analog inputs see the analog input section differential input voltage range v ref = (refx+) ? (refx?) ?v ref +v ref v input common-mode range 2 avss avdd1 v absolute analog input voltage limits 2 avss avdd1 v analog input current unbuffered differential component 48 a/v common-mode component 17 a/v precharge buffer on 5 ?20 a input current drift unbuffered 5 na/v/c precharge buffer on 31 na/c
data sheet ad7761 rev. 0 | page 5 of 69 parameter test conditions/comments min typ max unit external reference reference voltage v ref = (refx+) ? (refx?) 1 avdd1 ? avss v absolute reference voltage limits 2 precharge reference buffers off avss ? 0.05 avdd1 + 0.05 v precharge reference buffers on avss avdd1 v average reference current fast mode precharge reference buffers off 72 a/v/channel precharge reference buffers on 16 a/v/channel average reference current drift fast mode precharge reference buffers off 1.7 na/v/c precharge reference buffers on 49 na/v/c common-mode rejection 95 db digital filter response low ripple wideband filter filter = 0 decimation rate up to six selectable decimation rates 32 1024 group delay latency 34/odr sec settling time complete settling 68/odr sec pass-band ripple 2 0.005 db pass band 0.005 db bandwidth 0.4 odr hz ?0.1 db bandwidth 0.409 odr hz ?3 db bandwidth 0.433 odr hz stop band frequency attenuation > 105 db 0.499 odr hz stop band attenuation 105 db sinc5 filter filter = 1 decimation rate up to six selectable decimation rates 32 1024 group delay latency 3/odr sec settling time complete settling 7/odr sec pass band ?3 db bandwidth 0.204 odr hz rejection ac power supply rejection ratio (psrr) v in = 0.1 v, avdd1 = 5 v, avdd2 = 5 v, iovdd = 2.5 v avdd1 90 db avdd2 100 db iovdd 75 db dc psrr v in = 1 v avdd1 100 db avdd2 118 db iovdd 90 db analog input common-mode rejection ratio (cmrr) dc v in = 0.1 v 95 db ac up to 10 khz 95 db crosstalk ?0.5 dbfs input on adjacent channels ?120 db clock crystal frequency 8 32.768 34 mhz external clock (mclk) see the clock selection section 32.768 mhz duty cycle for data sheet performance 50:50 % mclk pulse width 2 functionality logic low 12.2 ns logic high 12.2 ns cmos clock input voltage see the logic inputs parameter high, v inh low, v inl
ad7761 data sheet rev. 0 | page 6 of 69 parameter test conditions/comments min typ max unit lvds clock 2 r l = 100 differential input voltage 100 650 mv common-mode input voltage 800 1575 mv absolute input voltage 1.88 v adc reset 2 adc start-up time after reset 6 time to first drdy , fast mode, decimation by 32 1.58 1.66 ms minimum reset low pulse width t mclk = 1/mclk 2 t mclk logic inputs input voltage 2 high, v inh 0.65 iovdd v low, v inl 2.25 v < iovdd < 3.6 v 0.7 v 1.72 v < iovdd < 1.88 v 0.4 v hysteresis 2 2.25 v < iovdd < 3.6 v 0.04 0.09 v 1.72 v < iovdd < 1.88 v 0.04 0.2 v leakage current ?10 +0.03 +10 a reset pin 7 ?10 +10 a logic outputs output voltage 2 high, v oh i source = 200 a 0.8 iovdd v low, v ol i sink = 400 a 0.4 v leakage current floating state ?10 +10 a output capacitance floating state 10 pf system calibration 2 full-scale calibration limit 1.05 v ref v zero-scale calibration limit ?1.05 v ref v input span 0.4 v ref 2.1 v ref v power requirements power supply voltage avdd1 ? avss 4.5 5.0 5.5 v avdd2 ? avss 2.0 2.25 to 5.0 5.5 v avss ? dgnd ?2.75 0 v iovdd ? dgnd 1.72 1.8 or 2.5 to 3.3 3.6 v power supply currents maximum output data rate, cmos mclk, eight doutx signals, all supplies at maximum voltages, all channels in channel mode a eight channels active fast mode avdd1 current precharge reference buffers off/on 36/57.5 40/64 ma avdd2 current 37.5 40 ma iovdd current wideband filter 63 69 ma sinc5 filter 2 27 29 ma median mode avdd1 current precharge reference buffers off/on 18.5/29 ma avdd2 current 21.3 ma iovdd current wideband filter 34 ma sinc5 filter 16 ma
data sheet ad7761 rev. 0 | page 7 of 69 parameter test conditions/comments min typ max unit focus mode avdd1 current precharge reference buffers off/on 5.1/8 ma avdd2 current 9.3 ma iovdd current wideband filter 12.5 ma sinc5 filter 8 ma four channels active 2 fast mode avdd1 current precharge reference buffers off/on 18.2/28.8 20.3/32.5 ma avdd2 current 18.8 20.3 ma iovdd current wideband filter 43.5 47.7 ma sinc5 filter 17 18.6 ma median mode avdd1 current precharge reference buffers off/on 9.3/14.7 ma avdd2 current 10.7 ma iovdd current wideband filter 24.5 ma sinc5 filter 11 ma focus mode avdd1 current precharge reference buffers off/on 2.7/4.1 ma avdd2 current 4.7 ma iovdd current wideband filter 10 ma sinc5 filter 6.5 ma two channels active 2 fast mode avdd1 current precharge reference buffers off/on 9.3/14.7 10.5/16.6 ma avdd2 current 9.5 10.5 ma iovdd current wideband filter 34 36.7 ma sinc5 filter 12 13.5 ma median mode avdd1 current precharge reference buffers off/on 4.8/7.5 ma avdd2 current 5.5 ma iovdd current wideband filter 19.5 ma sinc5 filter 8.5 ma focus mode avdd1 current precharge reference buffers off/on 1.52/2.2 ma avdd2 current 2.4 ma iovdd current wideband filter 8.6 ma sinc5 filter 5.8 ma standby mode all channels disabled (sinc5 filter enabled) 6.5 8 ma sleep mode full power-down (serial peripheral interface (spi) mode only) 0.73 1.2 ma crystal excitation current extra current in iovdd when using an external crystal compared to using the cmos mclk 540 a
ad7761 data sheet rev. 0 | page 8 of 69 parameter test conditions/comments min typ max unit power dissipation external cmos mclk, all channels active, mclk = 32.768 mhz, all channels in channel mode a full operating mode analog precharge buffers on wideband filter fast avdd1 = 5 v, avdd2 = iovdd = 2.5 v, precharge reference buffers off 2 412 446 mw avdd1 = 5 v, avdd2 = iovdd = 3.3 v, precharge reference buffers on 2 600 645 mw avdd1 = 5.5 v, avdd2 = 5.5 v, iovdd = 3.6 v, precharge reference buffers off 631 681 mw avdd1 = 5.5 v, avdd2 = 5.5 v, iovdd = 1.88 v, precharge reference buffers off 2 524 571 mw median avdd1 = 5 v, avdd2 = iovdd = 2.5 v, precharge reference buffers off 220 mw avdd1 = 5 v, avdd2 = iovdd = 3.3 v, precharge reference buffers on 320 mw avdd1 = 5.5 v, avdd2 = 5.5 v, iovdd = 3.6 v, precharge reference buffers off 341 mw avdd1 = 5.5 v, avdd2 = 5.5 v, iovdd = 1.88 v, precharge reference buffers off 284 mw focus avdd1 = 5 v, avdd2 = iovdd = 2.5 v, precharge reference buffers off 75 mw avdd1 = 5 v, avdd2 = iovdd = 3.3 v, precharge reference buffers on 107 mw avdd1 = 5.5 v, avdd2 = 5.5 v, iovdd = 3.6 v, precharge reference buffers off 124 mw avdd1 = 5.5 v, avdd2 = 5.5 v, iovdd = 1.88 v, precharge reference buffers off 99 mw sinc5 filter 2 fast avdd1 = 5 v, avdd2 = iovdd = 2.5 v, precharge reference buffers off 325 355 mw avdd1 = 5 v, avdd2 = iovdd = 3.3 v, precharge reference buffers on 475 525 mw avdd1 = 5.5 v, avdd2 = 5.5 v, iovdd = 3.6 v, precharge reference buffers off 501 545 mw avdd1 = 5.5 v, avdd2 = 5.5 v, iovdd = 1.88 v, precharge reference buffers off 455 495 mw median avdd1 = 5 v, avdd2 = iovdd = 2.5 v, precharge reference buffers off 175 mw avdd1 = 5 v, avdd2 = iovdd = 3.3 v, precharge reference buffers on 260 mw avdd1 = 5.5 v, avdd2 = 5.5 v, iovdd = 3.6 v, precharge reference buffers off 277 mw avdd1 = 5.5 v, avdd2 = 5.5 v, iovdd = 1.88 v, precharge reference buffers off 248 mw
data sheet ad7761 rev. 0 | page 9 of 69 parameter test conditions/comments min typ max unit focus avdd1 = 5 v, avdd2 = iovdd = 2.5 v, precharge reference buffers off 65 mw avdd1 = 5 v, avdd2 = iovdd = 3.3 v, precharge reference buffers on 95 mw avdd1 = 5.5 v, avdd2 = 5.5 v, iovdd = 3.6 v, precharge reference buffers off 108 mw avdd1 = 5.5 v, avdd2 = 5.5 v, iovdd = 1.88 v, precharge reference buffers off 94 mw standby mode all channels disabled; avdd1 = 5 v, avdd2 = iovdd = 2.5 v 14.5 mw avdd1 = 5 v, avdd2 = iovdd = 3.3 v 21 mw avdd1 = avdd2 = 5.5 v, iovdd = 3.6 v 23.5 29 mw avdd1 = 5.5 v, avdd2 = 5.5 v, iovdd = 1.88 v 12.5 mw sleep mode full power-down (spi mode only); avdd1 = 5 v, avdd2 = iovdd = 2.5 v 1.8 mw avdd1 = 5 v, avdd2 = iovdd = 3.3 v 2.5 mw avdd1 = avdd2 = 5.5 v, iovdd = 3.6 v 2.7 6.5 mw avdd1 = 5.5 v, avdd2 = 5.5 v, iovdd = 1.88 v 1.5 mw 1 the output data rate ranges refer to the programmable decimation ra tes available on the ad7761 for a fixed mclk rate of 32.768 m hz. varying mclk rates allow users a wider variation of odr. 2 these specifications are not production tested but are suppo rted by characterization data at initial product release. 3 the maximum inl specification is guaranteed by design and characte rization testing prior to releas e. this specification is not production tested. 4 following a system zero-scale calibration, the offset error is in the order of the noise for the pr ogrammed output data rate s elected. a system full-scale calibration reduces the gain error to the order of the noise for the programmed output data rate. 5 ?25 a is measured when the analog inpu t is close to either the avdd1 or avss r ail. the input current reduces as the common-mo de voltage approaches (avdd1 ? avss)/2. the analog input current scales wi th the mclk frequency and device power mode. 6 for lower mclk rates or higher decimation rates, use table 31 and table 32 to calculate any additional delay before the first drdy e pulse. 7 the reset e pin has an internal pull-up device to iovdd. timing specifications avdd1a = avdd1b = 5 v, avdd2a = avdd2b = 5 v, iovdd = 2.25 v to 3.6 v, input logic 0 = dgnd, input logic 1 = iovdd; c load = 10 pf on the dclk pin, c load = 20 pf on the other digital outputs; refx+ = 4.096 v, t a = ?40c to +105c. see table 4 and table 5 for timing specifications at 1.8 v iovdd. table 2. data interface timing 1 parameter description test conditions/comments min typ max unit mclk master clock 1.15 34 mhz f mod modulator frequency fast mode mclk/4 hz median mode mclk/8 hz focus mode mclk/32 hz t 1 drdy high time t dclk = t 8 + t 9 t dclk ? 10% 28 ns t 2 dclk rising edge to drdy rising edge 2 ns t 3 dclk rising to drdy falling ?3.5 0 ns t 4 dclk rise to doutx valid 1.5 ns t 5 dclk rise to doutx invalid ?3 ns t 6 doutx valid to dclk falling 9.5 t dclk /2 ns t 7 dclk falling edge to doutx invalid 9.5 t dclk /2 ns t 8 dclk high time, dclk = mclk/1 50:50 cmos clock t dclk /2 t dclk /2 (t dclk /2) + 5 ns t 8a = dclk = mclk/2 t mclk = 1/mclk t mclk ns t 8b = dclk = mclk/4 2 t mclk ns t 8c = dclk = mclk/8 4 t mclk ns
ad7761 data sheet rev. 0 | page 10 of 69 parameter description test conditions/comments min typ max unit t 9 dclk low time dclk = mclk/1 50:50 cmos clock (t dclk /2) ? 5 t mclk /2 t dclk /2 ns t 9a = dclk = mclk/2 t mclk ns t 9b = dclk = mclk/4 2 t mclk ns t 9c = dclk = mclk/8 4 t mclk ns t 10 mclk rising to dclk rising cmos clock 30 ns t 11 setup time of dout6 and dout7 14 ns t 12 hold time of dout6 and dout7 0 ns t 13 start low time 1 t mclk ns t 14 mclk to sync_out valid cmos clock sync_out retime_en bit disabled; measured from falling edge of mclk 4.5 22 ns sync_out retime_en bit enabled; measured from rising edge of mclk 9.5 27.5 ns t 15 sync_in setup time cmos clock 0 ns t 16 sync_in hold time cmos clock 10 ns 1 these specifications are not production tested but are suppo rted by characterization data at initial product release. table 3. spi control interface timing 1 parameter description test conditions/comments min typ max unit t 17 sclk period 100 ns t 18 cs falling edge to sclk rising edge 26.5 ns t 19 sclk falling edge to cs rising edge 27 ns t 20 cs falling edge to data output enable 22.5 40.5 ns t 21 sclk high time 20 50 ns t 22 sclk low time 20 50 ns t 23 sclk falling edge to sdo valid 15 ns t 24 sdo hold time after sclk falling 7 ns t 25 sdi setup time 0 ns t 26 sdi hold time 6 ns t 27 sclk enable time 0 ns t 28 sclk disable time 0 ns t 29 cs high time 10 ns t 30 cs low time f mod = mclk/4 1.1 t mclk ns f mod = mclk/8 2.2 t mclk ns f mod = mclk/32 8.8 t mclk ns 1 these specifications are not production tested but are suppo rted by characterization data at initial product release. 1.8 v iovdd timing specifications avdd1a = avdd1b = 5 v, avdd2a = avdd2b = 5 v, iovdd = 1.72 v to 1.88 v (dregcap tied to iovdd), input logic 0 = dgnd, input logic 1 = iovdd, c load = 10 pf on dclk pin, c load = 20 pf on other digital outputs, t a = ?40c to +105c. table 4. data interface timing 1 parameter description test conditions/comments min typ max unit mclk master clock 1.15 34 mhz f mod modulator frequency fast mode mclk/4 hz median mode mclk/8 hz focus mode mclk/32 hz t 1 drdy high time t dclk ? 10% 28 ns t 2 dclk rising edge to drdy rising edge 2 ns
data sheet ad7761 rev. 0 | page 11 of 69 parameter description test conditions/comments min typ max unit t 3 dclk rising to drdy falling ?4.5 0 ns t 4 dclk rise to doutx valid 2.0 ns t 5 dclk rise to doutx invalid ?4 ns t 6 doutx valid to dclk falling 8.5 t dclk /2 ns t 7 dclk falling edge to doutx invalid 8.5 t dclk /2 ns t 8 dclk high time, dclk = mclk/1 50:50 cmos clock t dclk /2 t dclk /2 (t dclk /2) + 5 ns t 8a = dclk = mclk/2 t mclk ns t 8b = dclk = mclk/4 2 t mclk ns t 8c = dclk = mclk/8 4 t mclk ns t 9 dclk low time dclk = mclk/1 50:50 cmos clock (t dclk /2) ? 5 t mclk /2 t dclk /2 ns t 9a = dclk = mclk/2 t mclk ns t 9b = dclk = mclk/4 2 t mclk ns t 9c = dclk = mclk/8 4 t mclk ns t 10 mclk rising to dclk rising cmos clock 37 ns t 11 setup time dout6 and dout7 14 ns t 12 hold time dout6 and dout7 0 ns t 13 start low time 1 t mclk ns t 14 mclk to sync_out valid cmos clock sync_out retime_en bit disabled; measured from falling edge of mclk 10 31 ns sync_out retime_en bit enabled; measured from rising edge of mclk 15 37 ns t 15 sync_in setup time cmos clock 0 ns t 16 sync_in hold time cmos clock 11 ns 1 these specifications are not production tested but are suppo rted by characterization data at initial product release. table 5. spi control interface timing 1 parameter description test conditions/comments min typ max unit t 17 sclk period 100 ns t 18 cs falling edge to sclk rising edge 31.5 ns t 19 sclk falling edge to cs rising edge 30 ns t 20 cs falling edge to data output enable 29 54 ns t 21 sclk high time 20 50 ns t 22 sclk low time 20 50 ns t 23 sclk falling edge to sdo valid 16 ns t 24 sdo hold time after sclk falling 7 ns t 25 sdi setup time 0 ns t 26 sdi hold time 10 ns t 27 sclk enable time 0 ns t 28 sclk disable time 0 ns t 29 cs high time 10 ns t 30 cs low time f mod = mclk/4 1.1 t mclk ns f mod = mclk/8 2.2 t mclk ns f mod = mclk/32 8.8 t mclk ns 1 these specifications are not production tested but are suppo rted by characterization data at initial product release.
ad7761 data sheet rev. 0 | page 12 of 69 timing diagrams drdy dclk d23 doutx t 1 t odr = 1/odr t 7 t 6 t 5 t 4 t 3 t 2 t 8 t 9 14285-002 figure 2. data interface timing diagram t 8a t 8b t 9a mclk dclk = mclk/2 dclk = mclk/4 dclk = mclk/8 t 10 t 9b t 8c t 9c 14285-003 figure 3. mclk to dclk divider timing diagram t 11 t odr t 12 drdy dclk dout6 and dout7 14285-004 figure 4. daisy-chain setup and hold timing diagram t 13 t 14 mclk start s ync_out 14285-005 figure 5. asynchronous start and sync_out timing diagram
data sheet ad7761 rev. 0 | page 13 of 69 t 16 t 15 mclk s ync_in t 15 14285-006 figure 6. synchronous sync_in e pulse timing diagram cs sclk sdo msb t 18 t 17 t 21 t 22 t 23 t 24 t 20 t 19 14285-007 t 30 figure 7. spi serial read timing diagram cs sclk sdi msb lsb t 18 t 25 t 26 14285-008 t 30 figure 8. spi serial write timing diagram cs sclk t 28 t 29 t 27 14285-009 figure 9. sclk enable and disable timing diagram
ad7761 data sheet rev. 0 | page 14 of 69 absolute maximum ratings table 6. parameter rating avdd1, avdd2 to avss 1 ?0.3 v to +6.5 v avdd1 to dgnd ?0.3 v to +6.5 v iovdd to dgnd ?0.3 v to +6.5 v iovdd, dregcap to dgnd (iovdd tied to dregcap for 1.8 v operation) ?0.3 v to +2.25 v iovdd to avss ?0.3 v to +7.5 v avss to dgnd ?3.25 v to +0.3 v analog input voltage to avss ?0.3 v to avdd1 + 0.3 v reference input voltage to avss ?0.3 v to avdd1 + 0.3 v digital input voltage to dgnd ?0.3 v to iovdd + 0.3 v digital output voltage to dgnd ?0.3 v to iovdd + 0.3 v operating temperature range ?40c to +105c storage temperature range ?65c to +150c pb-free temperature, soldering reflow (10 sec to 30 sec) 260c maximum junction temperature 150c maximum package classification temperature 260c 1 transient currents of up to 100 ma do not cause scr latch-up. stresses at or above those listed under absolute maximum ratings may cause permanent damage to the product. this is a stress rating only; functional operation of the product at these or any other conditions above those indicated in the operational section of this specification is not implied. operation beyond the maximum operating conditions for extended periods may affect product reliability. thermal resistance ja is specified for the worst case conditions, that is, a device soldered in a circuit board for surface-mount packages. table 7. thermal resistance package type ja jc unit jedec board layers 64-lead lqfp 38 9.2 c/w 2p2s 1 2p2s is a jedec standard pcb confi guration per jedec standard jesd51-7. esd caution
data sheet ad7761 rev. 0 | page 15 of 69 pin configuration and fu nction descriptions 64 ain0+ 63 ain0? 62 avss2a 61 regcapa 60 avdd2a 59 vcm 58 clk_sel 57 pin/spi 56 format0 55 format1 54 avss 53 avdd2b 52 regcapb 51 avss2b 50 ain4? 49 ain4+ 47 ain5+ 46 avss1b 45 avdd1b 42 ain6? 43 ref2+ 44 ref2? 48 ain5? 41 ain6+ 40 ain7? 39 ain7+ 37 start 36 sync_in 35 iovdd 34 dregcap 33 dgnd 38 sync_out 2 ain1+ 3 avss1a 4 avdd1a 7 ain2? 6 ref1+ 5 ref1? 1 ain1? 8 ain2+ 9 ain3? 10 ain3+ 12 mode0/gpio0 13 mode1/gpio1 14 mode2/gpio2 15 mode3/gpio3 16 st0/cs 11 filter/gpio4 17 st1/sclk 18 dec1/sdi 19 dec0/sdo 20 dout7 21 dout6 22 dout5 23 dout4 24 dout3 25 dout2 26 dout1 27 dout0 28 dclk 29 drdy 30 reset 31 xtal1 32 xtal2/mclk ad7761 top view (not to scale) 14285-010 figure 10. pin configuration table 8. pin function descriptions pin no. mnemonic type 1 description 1 ain1? ai negative analog input to adc channel 1. 2 ain1+ ai positive analog input to adc channel 1. 3 avss1a p negative analog supply. this pin is nominally 0 v. 4 avdd1a p analog supply voltage, 5 v 10% with respect to avss. 5 ref1? ai reference input negative. ref1? is the negative reference terminal for channel 0 to channel 3. the ref1? voltage range is from avss to (avdd1 ? 1 v). decouple this pin to avss with a high quality capacitor, and maintain a low impedance between this capacitor and pin 3. 6 ref1+ ai reference input positive. ref1+ is the positive reference terminal for channel 0 to channel 3. the ref1+ voltage range is from (avss + 1 v) to avdd1. apply an external reference differential between ref1+ and ref1? in the range from 1 v to |avdd1 ? avss|. decouple this pin to avss with a high quality capacitor, and maintain a low impedance between this capacitor and pin 3. 7 ain2? ai negative analog input to adc channel 2. 8 ain2+ ai positive analog input to adc channel 2. 9 ain3? ai negative analog input to adc channel 3. 10 ain3+ ai positive analog input to adc channel 3. 11 filter/gpio4 di/o filter select/general-purpose input/output 4. in pin control mode, this pin selects the filter type. set this pin to logic 1 for the sinc5 filter. this sinc 5 filter is a low latency filter, and is best for dc applications or where a user has specialized postfiltering implemented off chip. set this pin to logic 0 for the wideband low ripple filter response. this filter has a steep transition band and 105 db stop band attenuation. full attenuation at nyquist (odr/2) means that no aliasing occurs at odr/ 2 out to the first chopping zone. when in spi control mode, this pin can be used as a general-purpos e input/output (gpio4).
ad7761 data sheet rev. 0 | page 16 of 69 pin no. mnemonic type 1 description 12, 13, 14, 15 mode0/gpio0, mode1/gpio1, mode2/gpio2, mode3/gpio3 di/di/o mode selection/general-purpose input/output pin 0 to pin 3. in pin control mode, the modex pins set the mode of operation for all adc channels, controlling power consumption, dclk frequency, and th e adc conversion type, allowing one-shot conversion operation. in spi control mode, the gpiox pins, in addition to the filter/gpio4 pin, form five general- purpose input/output pins (gpio4 to gpio0). 16 st0/cs di standby 0/chip select input. in pin control mode, a logic 1 places channel 0 to channel 3 into standby mode. in spi control mode, this pin is the active low chip select input to the spi control interface. 17 st1/sclk di standby 1/serial clock input. in pin control mode, a logic 1 places channel 4 to channel 7 into standby mode. in spi control mode, this pin is the serial clock input pin for the spi control interface. 18 dec1/sdi di decimation rate cont rol input 1/serial data input. in pin control mode, the dec0 and dec1 pins configure the decimation rate for all adc channels. in spi control mode, this pin is the serial data input pin used to write data to the ad7761 register bank. 19 dec0/sdo di/o decimation rate co ntrol input 0/serial data output. in pin control mode, the dec0 and dec1 pins configure the decimation rate for all adc channels. in spi control mode, this pin is the serial data output pin, allowing readback from the ad7761 registers. 20 dout7 di/o conversion data output 7. this pin is synchronous to dclk and framed by drdy e . this pin acts as a digital input from a separate ad7761 device if configured in a synchronized multidevice daisy chain when the formatx pins are configured as 01. to use the ad7761 in a daisy chain, hardwire the formatx pins as either 01, 10, or 11, depending on the best interfacing format for the application. when formatx is set to 10 or 11, connect this pin to ground through a pull-down resistor. 21 dout6 di/o conversion data output 6. this pin is synchronous to dclk and framed by drdy e . this pin acts as a digital input from a separate ad7761 device if configured in a synchronized multidevice daisy chain. to use this pin in a daisy chain, ha rdwire the formatx pins as either 01, 10, or 11, depending on the best interfacing format for the application. 22 dout5 do conversion data output 5. this pin is synchronous to dclk and framed by drdy . 23 dout4 do conversion data output 4. this pin is synchronous to dclk and framed by drdy . 24 dout3 do conversion data output 3. this pin is synchronous to dclk and framed by drdy . 25 dout2 do conversion data output 2. this pin is synchronous to dclk and framed by drdy . 26 dout1 do conversion data output 1. this pin is synchronous to dclk and framed by drdy . 27 dout0 do conversion data output 0. this pin is synchronous to dclk and framed by drdy 28 dclk do adc conversion data clock. this pin clocks conv ersion data out to the digital host (dsp/fpga). this pin is synchronous with drdy e and any conversion data outp ut on dout0 to dout7 and is derived from the mclk signal. this pin is unrelated to the control spi interface. 29 drdy do data ready. periodic signal o utput framing the conversion results from the eight adcs. this pin is synchronous to dclk and dout0 to dout7. 30 reset e di hardware asynchronous reset input. after the devi ce is fully powered up, it is recommended to perform a hard or soft reset. 31 xtal1 di input 1 for crystal or connection to an lvds clock. when clk_sel is 0, connect xtal1 to dgnd. 32 xtal2/mclk di input 2 for cmos or crystal/lvds sampling cloc k. see the clk_sel pin for the details of this configuration. external crystal: xtal2 is connected to the external crystal. lvds: a second lvds input is connected to this pin. cmos clock: this pin operates as an mclk input. cmos input with logic level of iovdd/dgnd. 33 dgnd p digital ground. this pin is nominally 0 v. 34 dregcap ao digital low dropout (ldo) regulator output. deco uple this pin to dgnd with a high quality, low esr, 10 f capacitor. for optimum performa nce, use a decoupling capacitor with an esr specification between 50 m and 400 m. this pin is not for use in circuits external to the ad7761 . for 1.8 v iovdd operation, connect this pin to iovdd via an external trace to provide power to the digital processing core.
data sheet ad7761 rev. 0 | page 17 of 69 pin no. mnemonic type 1 description 35 iovdd p digital supply. this pin sets the logic levels for all interface pins. iovdd also powers the digital processing core via the digital ldo when iovdd is at least 2.25 v. for 1.8 v iovdd operation, connect this pin to dregcap via an external trace to provide power to the digital processing core. 36 sync_in di synchronization input. sync_in receives the synchronous signal from sync_out . it is used in the synchronization of any ad7761 that requires simultaneous sampling or is in a daisy chain. ignore the start and sync_out functions if the sync_in pin is connected to the system synchronization pulse. this signal pulse must be synchronous to the mclk clock domain. 37 start di start signal. the start pulse synchronizes the ad7761 to other devices. the signal can be asynchronous. the ad7761 samples the input and then outputs a sync_out pulse. this sync_out pulse must be routed to the sync_in pin of this device, and any other ad7761 devices that must be synchronized together. this means that the user does not need to run the adcs and their digital host from the same cloc k domain, which is useful when there are long traces or back planes between the adc and the contro ller. if this pin is not used, it must be tied to a logic 1 through a pull-up resistor. 38 sync_out do synchronization output. this pin operates only when the start input is used. when using the start input feature, the sync_out pin must be connected to sync_in via an external trace. sync_out is a digital output that is synchronous to the mclk signal; the synchronization signal driven in on start is internally synchronized to the mclk signal and is driven out on sync_out . sync_out can also be routed to other ad7761 devices requiring simultaneous sampling and/or daisy-chaining, ensuring synchronization of devices related to the mclk clock domain. it must then be wired to drive the sync_in pin on the same ad7761 and on the other ad7761 devices. 39 ain7+ ai positive analog input to adc channel 7. 40 ain7? ai negative analog input to adc channel 7. 41 ain6+ ai positive analog input to adc channel 6. 42 ain6? ai negative analog input to adc channel 6. 43 ref2+ ai reference input positive. ref2+ is the positive reference terminal for channel 4 to channel 7. the ref2+ voltage range is from (avss + 1 v) to avdd1. apply an external reference differential between ref2+ and ref2? in the range from 1 v to |avdd1 ? avss|. decouple this pin to avss with a high quality capacitor, and maintain a low impedance between this capacitor and pin 46. 44 ref2? ai reference input negative. ref2? is the negative reference terminal for channel 4 to channel 7. the ref2? voltage range is from avss to (avdd1 ? 1 v). decouple this pin to avss with a high quality capacitor, and maintain a low impedance between this capacitor and pin 46. 45 avdd1b p analog supply voltage. this pin is 5 v 10% with respect to avss. 46 avss1b p negative analog supply. this pin is nominally 0 v. 47 ain5+ ai positive analog input to adc channel 5. 48 ain5? ai negative analog input to adc channel 5. 49 ain4+ ai positive analog input to adc channel 4. 50 ain4? ai negative analog input to adc channel 4. 51 avss2b p negative analog supply. this pin is nominally 0 v. 52 regcapb ao analog ldo regulator output. decoup le this pin to avss with a 1 f capacitor. 53 avdd2b p analog supply voltage. this pin is 2 v to 5.5 v with respect to avss. 54 avss p negative analog supply. this pin is nominally 0 v. 55, 56 format1, format0 di format selection pins. hardwire the formatx pins to the required values in pin control and spi control mode. these pins set the number of dout x pins used to output adc conversion data. the formatx pins are checked by the ad7761 on power-up; the ad7761 then remains in this data output configuration. 57 pin /spi di pin control/spi control. this pin sets the control method. logic 0 = pin control mode for the ad7761 . pin control mode allows pin strapped configuration of the ad7761 by tying logic input pins to the required logic levels. tie the logic pins (mode0 to mode4, dec0 and dec1, and filter) as required for the configuration. logic 1 = spi control mode for the ad7761 . use the spi control interface signals (cs e , sclk, sdi, and sdo) for reading and writing to the ad7761 memory map.
ad7761 data sheet rev. 0 | page 18 of 69 pin no. mnemonic type 1 description 58 clk_sel di clock select. 0 = pull this pin low for the cmos clock option. the clock is applied to pin 32 (connect pin 31 to dgnd). 1 = pull this pin high for the crystal or lvds clock option. the crystal or lvds clock is applied to pin 31 and pin 32. the lvds option is available only in spi control mode. a write is required to enable the lvds clock option. 59 vcm ao common-mode voltage output. this pin outputs (av dd1 ? avss)/2, which is 2.5 v by default in pin control mode. configure this pin to (avdd1 ? avss)/2, 2.5 v, 2.14 v, or 1.65 v in spi control mode. when driving capacitive loads larger than 0.1 f, it is recommended to place a 50 series resistor between this pin and the capacitive load for stability. 60 avdd2a p analog supply voltage. this pin is 2 v to 5.5 v with respect to avss. 61 regcapa ao analog ldo regulator output. decoup le this pin to avss with a 1 f capacitor. 62 avss2a p negative analog supply. this pin is nominally 0 v. 63 ain0? ai negative analog input to adc channel 0. 64 ain0+ ai positive analog input to adc channel 0. 1 ai is analog input, p is power, di/o is digital input/output, di is digital input, do is digital output, and ao is analog outp ut.
data sheet ad7761 rev. 0 | page 19 of 69 typical performance characteristics avdd1 = 5 v, avdd2 = 2.5 v, avss = 0 v, iovdd = 2.5 v, v ref = 4.096 v, t a = 25c, wideband filter, decimation = 32, mclk = 32.768 mhz, analog input precharge buffers on, precharge reference buffers off, unless otherwise noted. ?200 ?180 ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 10 100k 10k 1k 100 amplitude (db) frequency (hz) 14285-011 snr = 97.7db thd = ?123.8db figure 11. fft, fast mode, wi deband filter, ?0.5 dbfs ?200 ?180 ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 10 100k 10k 1k 100 amplitude (db) frequency (hz) 14285-012 snr = 97.7db thd = ?125.1db figure 12. fft, median mode, wideband filter, ?0.5 dbfs ?200 ?180 ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 10 110k 1k 100 amplitude (db) frequency (hz) 14285-013 snr = 97.7db thd = ?128.3db figure 13. fft, focus mode, wi deband filter, ?0.5 dbfs ?200 ?180 ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 10 100k 10k 1k 100 amplitude (db) frequency (hz) 14285-017 snr = 97.8db thd = ?124.2db figure 14. fft, fast mode, sinc5 filter, ?0.5 dbfs ?200 ?180 ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 10 100k 10k 1k 100 amplitude (db) frequency (hz) 14285-018 snr = 97.9db thd = ?125.1db figure 15. fft, median mode, sinc5 filter, ?0.5 dbfs ?200 ?180 ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 10 110k 1k 100 amplitude (db) frequency (hz) 14285-019 snr = 97.9db thd = ?128.3db figure 16. fft, focus mode, sinc5 filter, ?0.5 dbfs
ad7761 data sheet rev. 0 | page 20 of 69 ?200 ?180 ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 55000 500 50 amplitude (db) frequency (hz) 14285-023 snr = 97.9db thd = ?119.9db figure 17. fft one-shot mode, sinc5 filter, median mode, decimation = 64, ?0.5 dbfs, sync_in frequency = mclk/4000 ?200 ?180 ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 100 100k 10k 1k amplitude (db) frequency (hz) 14285-024 second-order imd = ?125.2db third-order imd = ?120.8 db figure 18. imd with input signals at 9.7 khz and 10.3 khz ?180 ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 01234567 amplitude (db) channel 14285-016 f in = 3.15khz interferer (1khz) on all other channels figure 19. crosstalk ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 0 5 10 15 20 25 30 35 40 thd and thd + n (db) snr (db) mclk frequency (mhz) 14285-027 90 91 92 93 94 95 96 97 98 99 f in = 1khz snr, fast thd, fast thd + n, fast figure 20. snr, thd, thd + n vs. mclk frequency ?140 ?120 ?100 thd (db) ?80 ?60 ?40 ?20 0 10 100 1k input frequency (hz) 10k 100k fast median focus 14285-028 figure 21. thd vs. input frequency, three power modes, wideband filter ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 ?40 ?30 ?20 ?10 0 thd and thd + n (db) input amplitude (dbfs) 14285-030 thd + n thd figure 22. thd and thd + n vs. input am plitude, wideband filter, fast mode
data sheet ad7761 rev. 0 | page 21 of 69 inl error (lsb) input voltage (v) ?v ref 0v +v ref 14285-032 ?1.0 ?0.8 ?0.6 ?0.4 ?0.2 0 0.2 0.4 0.6 0.8 1.0 fast median focus figure 23. inl error vs. input voltage inl error (lsb) input voltage (v) 14285-033 ?v ref 0v +v ref ?1.0 ?0.8 ?0.6 ?0.4 ?0.2 0 0.2 0.4 0.6 0.8 1.0 +25c 0c +85c +105c ?40c figure 24. inl error vs. input voltage for various temperatures, fast mode 14285-034 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 ?2.0 ?1.8 ?1.6 ?1.4 ?1.2 ?1.0 ?0.8 ?0.6 ?0.4 ?0.2 0 0.2 0.4 0.6 0.8 1.0 number of occurrences offset error (lsb) figure 25. offset error distribution (approximately 2500 devices sampled) 14285-036 0 5 10 15 20 25 30 35 ?16 ?14 ?12 ?10 ?8 ?6 ?4 ?2 0 2 4 6 8 10 12 14 16 number of occurrences gain error ( lsb) +105c +25c ?40c figure 26. gain error distribution (100 devices sampled) 14285-037 0 5 10 15 20 25 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 number of occurrences gain error (lsb) figure 27. channel to channel gain er ror matching (100 devices sampled) ?180 ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 10 100 1k 10k 100k 1m 10m ac cmrr (db) input frequency (hz) 14285-038 figure 28. ac cmrr vs. input frequency
ad7761 data sheet rev. 0 | page 22 of 69 0 ?100 ?90 ?80 ?70 ?60 ?50 ?40 ?30 ?20 ?10 100 1k 10k 100k 1m 10m ac psrr (db) frequency (hz) 14285-039 ch0 ch1 ch2 ch3 ch4 ch5 ch6 ch7 dclk = 32.768mhz avdd1 = 5v + 100mv p-p figure 29. ac psrr vs. frequency, avdd1 0 ?100 ?90 ?80 ?70 ?60 ?50 ?40 ?30 ?20 ?10 100 1k 10k 100k 1m 10m ac psrr (db) frequency (hz) 14285-040 ch0 ch1 ch2 ch3 ch4 ch5 ch6 ch7 dclk = 32.768mhz avdd2 = 5v + 100mv p-p figure 30. ac psrr vs. frequency, avdd2 ?110 ?100 ?90 ?80 ?70 ?60 ?50 ?40 ?30 ?20 ?10 0 100 1k 10k 100k 1m 10m ac psrr (db) frequency (hz) iovdd = 1.8v, dclk = 32.768mhz, ch7 iovdd = 2.5v, dclk = 32.768mhz, ch7 iovdd = 1.8v, dclk = 8.192mhz, ch7 iovdd = 2.5v, dclk = 8.192mhz, ch7 14285-041 figure 31. ac psrr vs. frequency, iovdd ?180 ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 20 0.1 ?0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 normalized input frequency ( f in / f odr ) amplitude (db) focus median fast 14285-042 figure 32. wideband filter profile, amplitude vs. f in /f odr doutx (code) ainx (v) samples 14285-043 ?4 ?3 ?2 ?1 0 1 2 3 4 5 0 1020304050607080 ainx doutx 0 10000 20000 30000 40000 50000 60000 70000 figure 33. step response, wideband filter ?0.005 ?0.004 ?0.003 ?0.002 ?0.001 0 0.001 0.002 0.003 0.004 0.005 0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 amplitude (db) normalized input frequency ( f in / f odr ) focus median fast 14285-044 figure 34. wideband filter ripple
data sheet ad7761 rev. 0 | page 23 of 69 ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 0123456 amplitude (db) normalized input frequency ( f in / f odr ) resolution limit = ?110db 14285-045 figure 35. sinc5 filter profile, amplitude vs. f in /f odr 0 5 10 15 20 25 30 doutx (code) ainx (v) samples 14285-046 ?4 ?3 ?2 ?1 0 1 2 3 4 5 ainx doutx 10000 0 20000 30000 40000 50000 60000 70000 figure 36. step response, sinc5 filter ?40 ?30 ?20 ?10 0 10 20 30 40 50 60 ? 40 25 105 temperature (c) analog input current (a) common-mode component, no precharge (a/v) differential component, no precharge (a/v) total current, precharge on (a) 14285-047 figure 37. analog input current vs. temperature, analog input precharge buffers on/off ?80 ?70 ?60 ?50 ?40 ?30 ?20 ?10 0 ?40 25 105 reference input current (a/v/channel) temperature (c) fast, no precharge median, no precharge focus, no precharge fast with precharge median with precharge focus with precharge 14285-048 figure 38. reference input current vs. temperature, reference precharge buffers on/off 120 100 80 60 40 20 0 2.47 2.46 2.45 2.44 2.43 2.42 number of occurrences v cm (v) 14285-049 avdd1 = 5v, avss = 0v vcm_vsel = 10 part to part distribution figure 39. vcm output voltage distribution 0 5 10 15 20 25 30 35 40 ?40 ?25 ?10 5 20 35 50 65 80 95 110 supply current (ma) temperature (c) focus fast median 14285-050 figure 40. supply current vs. temperature, avdd1
ad7761 data sheet rev. 0 | page 24 of 69 0 5 10 15 20 25 30 35 40 ?40 ?25 ?10 5 20 35 50 65 80 95 110 supply current (ma) temperature (c) focus fast median 14285-051 figure 41. supply current vs. temperature, avdd2 ?40 ?25 ?10 5 20 35 50 65 80 95 110 supply current (ma) temperature (c) 0 10 20 30 40 50 60 70 focus, sinc5 focus, wideband fast, sinc5 fast, wideband median, sinc5 median, wideband 14285-052 figure 42. supply current vs. temperature, iovdd 0 50 100 150 200 250 300 350 400 450 500 ?40 25 105 total power (mw) fast, sinc5 filter median, sinc5 filter focus, sinc5 filter fast, wideband filter median, wideband filter focus, wideband filter temperature (c) 14285-053 figure 43. total power vs. temperature
data sheet ad7761 rev. 0 | page 25 of 69 terminology ac common-mode rejection ratio (ac cmrr) ac cmrr is defined as the ratio of the power in the adc output at frequency, f, to the power of a sine wave applied to the common- mode voltage of ainx+ and ainx? at frequency, f s . ac cmrr (db) = 10log( pf / pf s ) where: pf is the power at frequency, f , in the adc output. pf s is the power at frequency, f s , in the adc output. gain error the first transition (from 100 000 to 100 001) occurs at a level ? lsb above nominal negative full scale (?4.0959375 v for the 4.096 v range). the last transition (from 011 110 to 011 111) occurs for an analog voltage 1? lsb below the nominal full scale (+4.0959375 v for the 4.096 v range). the gain error is the deviation of the difference between the actual level of the last transition and the actual level of the first transition from the difference between the ideal levels. gain error drift gain error drift is defined as the gain error change due to a temperature change of 1c. it is expressed in parts per million per degree celsius. integral nonlinearity (inl) error inl error refers to the deviation of each individual code from a line drawn from negative full scale through positive full scale. the point used as negative full scale occurs ? lsb before the first code transition. positive full scale is defined as a level 1? lsb beyond the last code transition. the deviation is measured from the middle of each code to the true straight line. intermodulation distortion (imd) with inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities creates distortion products at the sum and difference frequencies of mfa and nfb, where m, n = 0, 1, 2, 3, and so on. intermodulation distortion terms are those for which neither m or n are equal to 0. for example, the second-order terms include (fa + fb) and (fa ? fb), and the third-order terms include (2fa + fb), (2fa ? fb), (fa + 2fb), and (fa ? 2fb). the ad7761 is tested using the ccif standard, where two input frequencies near to each other are used. in this case, the second- order terms are usually distanced in frequency from the original sine waves, and the third-order terms are usually at a frequency close to the input frequencies. as a result, the second-order and third-order terms are specified separately. the calculation of the intermodulation distortion is as per the thd specification, where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the sum of the fundamentals, expressed in decibels. least significant bit (lsb) the least significant bit, or lsb, is the smallest increment that can be represented by a converter. for a fully differential input adc with n bits of resolution, the lsb expressed in volts is as follows: lsb (v) = (2 v ref )/2 n where: v ref is the difference voltage between the refx+ and refx? pins. n = 16. offset error offset error is the difference between the ideal midscale input voltage (0 v) and the actual voltage producing the midscale output code. power supply rejection ratio (psrr) variations in power supply affect the full-scale transition but not the linearity of the converter. psrr is the maximum change in the full-scale transition point due to a change in the power supply voltage from the nominal value. signal-to-noise ratio (snr) snr is the ratio of the rms value of the actual input signal to the rms sum of all other spectral components below the nyquist frequency, excluding harmonics and dc. the value for snr is expressed in decibels. signal-to-noise-and-distortion ratio (sinad) sinad is the ratio of the rms value of the actual input signal to the rms sum of all other spectral components below the nyquist frequency, including harmonics but excluding dc. the value for sinad is expressed in decibels. spurious-free dynamic range (sfdr) sfdr is the difference, in decibels, between the rms amplitude of the input signal and the peak spurious signal (excluding the first five harmonics). total harmonic distortion (thd) thd is the ratio of the rms sum of the first five harmonic components to the rms value of a full-scale input signal and is expressed in decibels.
ad7761 data sheet rev. 0 | page 26 of 69 theory of operation the ad7761 is an 8-channel, simultaneously sampled, low noise, - adc. each adc within the ad7761 employs a - modulator whose clock runs at a frequency of f mod . the modulator samples the inputs at a rate of 2 f mod , to convert the analog input into an equivalent 16-bit digital representation, and these samples therefore represent a quantized version of the analog input signal. the - conversion technique is an oversampled architecture. this oversampled approach spreads the quantization noise over a wide frequency band (see figure 44). to reduce the quantization noise in the signal band, the high order modulator shapes the noise spectrum so that most of the noise energy is shifted out of the band of interest (see figure 45). the digital filter that follows the modulator removes the large out of band quantization noise (see figure 46). for further information on the basics as well as more advanced concepts of - adcs, see the mt-022 tutorial and the mt-023 tutorial . digital filtering has certain advantages over analog filtering. first, it is insensitive to component tolerances and the variation of component parameters over time and temperature. and because digital filtering on the ad7761 occurs after the analog-to-digital conversion, it can remove some of the noise injected during the conversion process; analog filtering cannot remove noise injected during conversion. second, the digital filter combines low pass-band ripple with a steep roll-off and high stop-band attenuation, while also maintaining a linear phase response, which is difficult to achieve in an analog filter implementation. quantization noise f mod /2 band of interest 14285-054 figure 44. - adc quantization noise (linear scale x-axis) noise shaping band of interest 14285-055 f mod /2 figure 45. - adc noise shaping (linear scale x-axis) digital filter cutoff frequency band of interest 14285-056 f mod /2 figure 46. - adc digital filter cutoff frequency (linear scale x-axis) clocking, sampling tr ee, and power scaling the ad7761 includes multiple adc cores. each of these adcs receives the same master clock signal, mclk. the mclk signal can be sourced from one of three options: a cmos clock, a crystal connected between the xtal1 and xtal2 pins, or in the form of an lvds signal. the mclk signal received by the ad7761 is used to define the modulator clock rate, f mod , and in turn the sampling frequency of the modulator of 2 f mod . the same mclk signal is also used to define the digital output clock, dclk. the f mod and dclk internal signals are synchronous with mclk. figure 47 shows the clock tree from the mclk input to the modulator, the digital filter, and the dclk output. there are divider settings for mclk and dclk. these dividers, in conjunction with the power mode and digital filter decimation settings, are key to the operation of the ad7761. the ad7761 has the ability to scale power consumption vs. the input bandwidth or desired noise. the user controls two para- meters to achieve this: mclk division and power mode. when combined, these two settings determine the clock frequency of the modulator (f mod ) and the bias current supplied to each modulator. the power mode (fast, median, or focus) sets the noise, speed capability, and current consumption of the modulator. the power mode is the dominant control for scaling the power consumption of the adc. all settings of mclk division and power mode apply to all adc channels. dclk_div 00: dclk = mclk/8 01: dclk = mclk/4 10: dclk = mclk/2 11: dclk = mclk/1 mclk_div mclk/4 mclk/8 mclk/32 power modes: fast median focus decimation rates = 32, 64, 128, 256, 512, 1024 digital filter dclk drdy doutx data interface control adc modulator 14285-057 figure 47. sampling structure, defined by the mclk, dclk_div, and mclk_div settings the modulator clock frequency (f mod ) is determined by selecting one of three clock divider settings: mclk/4, mclk/8, or mclk/32. although the mclk division and power modes are independent settings, there are restrictions that must be adhered to. a valid range of modulator frequencies exists for each power mode. table 9 describes this recommended range, which allows the device to achieve the best performance while minimizing power consumption. the ad7761 specifications do not cover the performance and function beyond the maximum f mod for a given power mode.
data sheet ad7761 rev. 0 | page 27 of 69 for example, to maximize the speed of conversion or input bandwidth in fast mode, an mclk of 32.768 mhz is required and mclk_div = 4 must be selected for a modulator frequency of 8.192 mhz. table 9. recommended f mod range for each power mode power mode recommended f mod (mhz), mclk = 32.768 mhz focus 0.036 to 1.024 median 1.024 to 4.096 fast 4.096 to 8.192 control of the settings for the power mode, the modulator frequency, and the data clock frequency differs in pin control mode vs. spi control mode. in spi control mode, the user can program the power mode, the mclk divider (mclk_div), and the dclk frequency using register 0x04 and register 0x07 (see table 38 and table 41 for register information). independent selection of the power mode and mclk_div allows full freedom in the mclk speed selection to achieve a target modulator frequency. in pin control mode, the modex pins determine the power mode, the modulator frequency, and the dclk frequency. the modulator frequency tracks the power mode. this means that f mod is fixed at mclk/32 for focus mode, mclk/8 for median mode, and mclk/4 for fast mode (see table 18). example of power vs. noise performance optimization depending on the bandwidth of interest for the measurement, the user can choose a strategy of either lowest current consumption or highest resolution. this choice is due to an overlap in the coverage of each power mode. the ad7761 offers the ability to balance the mclk division ratio with the rate of decimation (averaging) set in the digital filter. lower power can be achieved by using lower modulator clock frequencies. conversely, the highest resolution can be achieved by using higher modulator clock frequencies and maximizing the amount of oversampling. as an example, consider a system constraint with a maximum available mclk of 16 mhz. the system is targeting a measure- ment bandwidth of approximately 25 khz with the wideband filter, setting the output data rate of the ad7761 to 62.5 khz. because of the low mclk frequency available and the system power budget, median power mode is used. in median power mode, this 25 khz input bandwidth can be achieved by setting the mclk division and decimation ratio to balance, using two configurations (configuration a and configuration b). this flexibility is possible in spi control mode only. configuration a to maximize the dynamic range, use the following settings: ? mclk = 16 mhz ? median power ? f mod = mclk/4 ? decimation = 64 (digital filter setting) ? odr = 62.5 khz this configuration maximizes the available decimation rate (or oversampling ratio) for the bandwidth required and mclk rate available. the decimation averages the noise from the modulator, maximizing the dynamic range. configuration b to minimize power, use the following settings: ? mclk = 16 mhz ? median power ? f mod = mclk/8 ? decimation = 32 (digital filter setting) ? odr = 62.5 khz this configuration reduces the clocking speed of the modulator and the digital filter. when compared to configuration a, configuration b saves 48 mw of power. the trade-off in the case of configuration b is that the digital filter must run at a 2 lower decimation rate. this 2 reduction in decimation rate (or oversampling ratio) results in a 3 db reduction in the dynamic range vs. config- uration a. clocking out the adc conversion results (dclk) the ad7761 dclk is a divided version of the master clock input. as shown in figure 47, the dclk_div setting determines the speed of the dclk. dclk is a continuous clock. the user can set the dclk frequency rate to one of four divisions of mclk: mclk/1, mclk/2, mclk/4, and mclk/8. because there are eight channels and 24 bits of data per conversion, the conversion time and the setting of dclk directly determine the number of data output lines that are required via the formatx pin settings. thus, the intended minimum decimation and desired dclk_div setting must be understood prior to choosing the setting of the formatx pins. noise performance and resolution table 10 and table 11 show the noise performance for the wideband and sinc5 digital filters of the ad7761 for various output data rates and power modes. the noise values specified are typical for the bipolar input range with an external 4.096 v reference (v ref ). the lsb size with 4.096 v reference is 125 v, and is calculated as follows: lsb (v) = (2 v ref )/2 16
ad7761 data sheet rev. 0 | page 28 of 69 table 10. wideband filter noise: performance vs. output data rate output data rate (ksps) ?3 db bandwidth (khz) rms noise (v) fast mode 256 110.8 11.58 128 55.4 7.77 64 27.7 5.42 32 13.9 3.82 16 6.9 2.72 8 3.5 1.94 median mode 128 55.4 11.36 64 27.7 7.6 32 13.9 5.3 16 6.9 3.74 8 3.5 2.64 4 1.7 1.87 focus mode 32 13.9 11.28 16 6.9 7.54 8 3.5 5.25 4 1.7 3.71 2 0.87 2.62 1 0.43 1.85 table 11. sinc5 filter noise: perf ormance vs. output data rate output data rate (ksps) ?3 db bandwidth (khz) rms noise (v) fast mode 256 52.224 7.83 128 26.112 5.43 64 13.056 3.82 32 6.528 2.71 16 3.264 1.93 8 1.632 1.39 median mode 128 26.112 7.68 64 13.056 5.3 32 6.528 3.72 16 3.264 2.64 8 1.632 1.87 4 0.816 1.33 focus mode 32 6.528 7.65 16 3.264 5.26 8 1.632 3.7 4 0.816 2.61 2 0.408 1.85 1 0.204 1.31
data sheet ad7761 rev. 0 | page 29 of 69 applications information the ad7761 offers users a multichannel platform measurement solution for ac and dc signal processing. flexible filtering allows the ad7761 to be configured to simultaneously sample ac and dc signals on a per channel basis. power scaling allows users to trade off the input bandwidth of the measurement vs. the current consumption. this ability, coupled with the flexibility of the digital filtering, allows the user to optimize the energy efficiency of the measurement, while still meeting power, bandwidth, and performance targets. key capabilities that allow users to choose the ad7761 as their platform high resolution adc are highlighted as follows: ? eight fully differential or pseudo differential analog inputs. ? fast throughput simultaneous sampling adcs catering for input signals up to 110.8 khz. ? three selectable power modes (fast, median, and focus) for scaling the current consumption and input bandwidth of the adc for optimal measurement efficiency. ? analog input precharge and reference precharge buffers reduce the drive requirements of external amplifiers. ? control of reference and analog input precharge buffers on a per channel basis. ? wideband, low ripple, digital filter for ac measurement. ? fast sinc5 filter for precision low frequency measurement. ? two channel modes, defined by the user selected filter choice, and decimation ratios, can be defined for use on different adc channels. this enables optimization of the input bandwidth versus the signal of interest. ? option of spi or pin strapped control and configuration. ? offset, gain, and phase calibration registers per channel. ? common-mode voltage output buffer for use by a driver amplifier. ? on-board avdd2 and iovdd ldos for the low power, 1.8 v, internal circuitry. refer to figure 48 and table 12 for the typical connections and minimum requirements to get started using the ad7761. table 13 shows the typical power and performance of the ad7761 for the available power modes, for each filter type. 14285-058 adc data serial interface spi control interface v out v in v in adr4540 avdd1a, avdd1b avss a vdd2a, avdd2b regcapa, regcapb precharge buffers ain7+ ain7? ain0+ ain0? vcm ad7761 dregcap iovdd refx? refx+ 5v ada4940-1/ ada4940-2 - ? adc sinc5 low latency filter wideband low ripple filter sync_in drdy sync_out start reset formatx dclk dout6 dout7 st0/cs st1/sclk dec0/sdo dec1/sdi filter/gpio4 mode3/gpio3 to mode0/gpio0 xtal2/mclk xtal1 pin/spi dout0 dout1 dout2 dout3 dout4 dout5 ada4841-1 ? + suggested op amps: fast mode: ada4896-2 or ada4807-2 median mode: ada4940-2 or ada4807-2 focus mode: ada4805-2 figure 48. typical connection diagram
ad7761 data sheet rev. 0 | page 30 of 69 table 12. requirements to operate the ad7761 requirement description power supplies 5 v avdd1 supply, 2.25 v to 5 v avdd2 supply, 1.8 v or 2.5 v to 3.3 v iovdd supply ( adp7104/ adp7118 ) external reference 2.5 v, 4.096 v, or 5 v ( adr4525 , adr4540 , or adr4550) external driver amplifiers the ada4896-2 , the ada4940-1 / ada4940-2 , the ada4805-2 , and the ada4807-2 external clock crystal or a cmos/lvds clock for the adc modulator sampling fpga or dsp input/output voltage of 2.5 v to 3.6 v, or 1.8 v (see the 1.8 v iovdd operation section) table 13. speed, dynamic range, thd, and power overview; eight channels active, decimate by 32 1 power mode output data rate (ksps) thd (db) sinc5 filter wideband filter dynamic range (db) bandwidth (khz) power dissipation (mw per channel) dynamic range (db) bandwidth (khz) power dissipation (mw per channel) fast 256 ?115 97.7 52.224 41 97.7 110.8 52 median 128 ?120 97.7 26.112 22 97.7 55.4 28 focus 32 ?120 97.7 6.528 8.5 97.7 13.9 9.5 1 analog precharge buffers on, precharge reference buffers and vcm disabled, typical values, avdd1 = 5 v, avdd2 = iovdd = 2.5 v, v ref = 4.096 v, mclk = 32.768 mhz, dclk = mclk/4, t a = 25c. power supplies the ad7761 has three independent power supplies: avdd1 (given by the avdd1a pin and the avdd2a pin), avdd2 (given by the avdd2a pin and the avdd2b pin), and iovdd. the reference potentials for these supplies are avss and dgnd. tie all the avss supply pins (avss1a, avss1b, avss2a, avss2b, and avss) to the same potential with respect to dgnd. avdd1a, avdd1b, avdd2a, and avdd2b are referenced to this avss rail. iovdd is referenced to dgnd. the supplies can be powered within the following ranges: ? avdd1 = 5 v 10%, relative to avss ? avdd2 = 2 v to 5.5 v, relative to avss ? iovdd (with internal regulator) = 2.25 v to 3.6 v, relative to dgnd ? iovdd (bypassing regulator) = 1.72 v to 1.88 v, relative to dgnd ? avss = ?2.75 v to 0 v, relative to dgnd the avdd1a and avdd1b (avdd1) supplies power the analog front end, reference input, and common-mode output circuitry. avdd1 is referenced to avss, and all avdd1 supplies must be tied to the same potential with respect to avss. if avdd1 supplies are used in a 2.5 v split supply configuration, the adc inputs are truly bipolar. when using split supplies, reference the absolute maximum ratings, which apply to the voltage allowed between the avss and iovdd supplies. the avdd2a and avdd2b (avdd2) supplies connect to internal 1.8 v analog ldo regulators. the regulators power the adc core. avdd2 is referenced to avss, and all avdd2 supplies must be tied to the same potential with respect to avss. the voltage on avdd2 can range from 2 v (minimum) to 5.5 v (maximum), with respect to avss. iovdd powers the internal 1.8 v digital ldo regulator. this regulator powers the digital logic of the adc. iovdd also sets the voltage levels for the spi interface of the adc. iovdd is referenced to dgnd, and the voltage on iovdd can vary from 2.25 v (minimum) to 3.6 v (maximum), with respect to dgnd. iovdd can also be configured to run at 1.8 v. in this case, iovdd and dregcap must be tied together and must be within the range of 1.72 v (minimum) to 1.88 v (maximum), with respect to dgnd. see the 1.8 v iovdd operation section for more information on operating the ad7761 at 1.8 v iovdd. recommended power supply configuration analog devices, inc., has a wide range of power management products to meet the requirements of most high performance signal chains. an example of a power solution that uses the adp7118 is shown in figure 49. the adp7118 provides positive supply rails for optimal converter performance, creating either a single 5 v, 3.3 v, or dual avdd1 and avdd2/iovdd, depending on the required supply configuration. the adp7118 can operate from input voltages of up to 20 v. adp7118 ldo 12v input 5v: avdd1x 3.3v: avdd2x/iovdd adp7118 ldo 14285-059 figure 49. power supply configuration alternatively, the adp7112 or adp7104 can be selected for powering the ad7761 . refer to the an-1120 application note for more information regarding low noise ldo performance and power supply filtering.
data sheet ad7761 rev. 0 | page 31 of 69 1.8 v iovdd operation the ad7761 contains an internal 1.8 v ldo on the iovdd supply to regulate the iovdd down to the operating voltage of the digital core. this internal ldo allows the internal logic to operate efficiently at 1.8 v and the input/output logic to operate at the level set by iovdd. the iovdd supply is rated from 2.25 v to 3.6 v for normal operation, and 1.8 v for ldo bypass setup. 14285-060 37 start 36 sync_in 35 iovdd 34 dregcap 33 dgnd 38 sync_out 28 dclk 29 drdy 30 reset 31 xtal1 32 xtal2/mclk 1.8v iovdd supply figure 50. dregcap and iovdd connection diagram for 1.8 v iovdd operation users can bypass the ldo by shorting the dregcap pin to iovdd (see figure 50), which pulls the internal ldo out of regulation and sets the internal core voltage and input/output logic levels to the iovdd level. when bypassing the internal ldo, the maximum operating voltage of the iovdd supply is equal to the maximum operating voltage of the internal digital core, which is 1.72 v to 1.88 v. analog supply internal connectivity the ad7761 has two analog supply rails, avdd1 and avdd2, which are both referred to avss. these supplies are completely separate from the digital pins, iovdd, dregcap, and dgnd. to achieve optimal performance and isolation of the adcs, more than one device pin supplies these analog rails to the internal adcs. ? avss1a (pin 3) and avss2a (pin 62) are internally connected. ? avss (pin 54) is connected to the substrate, and is connected internally to avss1b (pin 46) and avss2b (pin 51). ? the following supply and reference input pins are separate on chip: avdd1a, avdd1b, avdd2a, avdd2b, ref1+, ref1?, ref2+, and ref2?. the details of which individual supplies are shorted internally are given in this section for informational purposes. in general, connect the supplies as described in the power supplies section. device configuration the ad7761 has independent paths for reading data from the adc conversions and for controlling the device functionality. for control, the device can be configured in either of two modes. the two modes of configuration are as follows: ? pin control mode: pin strapped digital logic inputs (which allows a subset of the configurability options) ? spi control mode: over a 3-wire or 4-wire spi interface (complete configurability) on power-up, the state of the pin /spi pin determines the mode used. immediately after power-up, the user must apply a soft or hard reset to the device when using either control mode. interface data format when operating the device, the data format of the serial inter- face is determined by the formatx pins. table 28 shows that each adc can be assigned a doutx pin, or, alternatively, the data can be arranged to share the doutx pins in a time division multiplexed manner. for more details, see the data interface section. pin control mode pin control mode eliminates the need for an spi communication interface. when a single known configuration is required by the user, or when only limited reconfiguration is required, the number of signals that require routing to the digital host can be reduced using this mode. pin control mode is useful in digitally isolated applications where minimal adjustment of the configuration is needed. pin control offers a subset of the core functionality and ensures a known state of operation after power-up, reset, or a fault condition on the power supply. in pin control mode, the analog input precharge buffers are enabled by default for best performance. the reference input precharge buffers are disabled in pin control mode. after any change to the configuration in pin control mode, the user must provide a sync signal to the ad7761 by applying the appropriate pulse to the start pin or the sync_in pin to ensure that the configuration changes are applied correctly to the adc and the digital filters. setting the filter the filter function chooses between the two filter settings. in pin control mode, all adc channels use the same filter type, which is selected by the filter pin, as shown in table 14. table 14. filter control pin logic level function 1 sinc5 filter selected 0 wideband filter selected
ad7761 data sheet rev. 0 | page 32 of 69 setting the decimation rate pin control mode allows selection from four possible decimation rates. the decimation rate is selected via the dec1 and dec0 pins. the chosen decimation rate is used on all adc channels. table 15 shows the truth table for the decx pins. table 15. decimation rate control pins truth table dec1 dec0 decimation rate 0 0 32 0 1 64 1 0 128 1 1 1024 operating mode the mode3 to mode0 pins determine the configuration of all channels when using pin control mode. the variables controlled by the modex pins are shown in table 16. the user selects how much current the device consumes, the sampling speed of the adc (power mode), how fast the adc result is received by the digital host (dclk_div), and how the adc conversion is initiated (conversion operation). figure 51 illustrates the inputs used to configure the device in pin control mode. table 16. modex pins: variables for control control variable possible settings sampling speed/power consumption power mode fast median focus data clock output frequency (dclk_div) dclk = mclk/1 dclk = mclk/2 dclk = mclk/4 dclk = mclk/8 conversion operation standard conversion one-shot conversion the modex pins map to 16 distinct settings. the settings are selected to optimize the use cases of the ad7761 , allowing the user to reduce the dclk frequency for lower, less demanding power modes and selecting either the one-shot or standard conversion modes. see table 18 for the complete selection of operating modes that are available via the modex pins in pin control mode. the power mode setting automatically scales the bias currents of the adc and divides the applied mclk signal to the correct setting for that mode. note that this is not the same as using spi control mode, where separate bit fields exist to control the bias currents of the adc and mclk division. in pin control mode, the modulator rate is fixed for each power mode to achieve the best performance. table 17 shows the modulator division for each power mode. table 17. modulator rate, pin control mode power mode modulator rate, f mod fast mclk/4 median mclk/8 focus mclk/32 diagnostics pin control mode offers a subset of diagnostics features. internal errors are reported in the status header output with the data conversion results for each channel. internal crc errors, memory map flipped bits, and external clocks not detected are reported by bit 7 of the status header and indicate that a reset is required. the status header also reports filter not settled, filter type, and filter saturated signals. users can determine when to ignore data by monitoring these error flags. for more information on the status header, see the adc conversion output: header and data section. pin control mode pin/spi = low option to select between filters to dsp/ fpga channel standby ch 0 to ch 3 standby ch 4 to ch 7 standby output data format 1 channel per pin 4 channels per pin 8 channels per pin decimation rates /32 /64 /128 /1024 mode configuration mode 0x0 to mode 0xf set up via 4 pins dec0/ dec1 filter dout7 dout1 dout0 format1 format0 st1 ad7761 st0 pin/spi mode0 mode1 mode2 mode3 14285-061 figure 51. pin configurable functions
data sheet ad7761 rev. 0 | page 33 of 69 table 18. modex selection details: pin control mode mode hex. mode3 mode2 mode1 mode0 power mode dclk frequency data conversion 0x0 0 0 0 0 focus mclk/1 standard 0x1 0 0 0 1 focus mclk/2 standard 0x2 0 0 1 0 focus mclk/4 standard 0x3 0 0 1 1 focus mclk/8 standard 0x4 0 1 0 0 median mclk/1 standard 0x5 0 1 0 1 median mclk/2 standard 0x6 0 1 1 0 median mclk/4 standard 0x7 0 1 1 1 median mclk/8 standard 0x8 1 0 0 0 fast mclk/1 standard 0x9 1 0 0 1 fast mclk/2 standard 0xa 1 0 1 0 fast mclk/4 standard 0xb 1 0 1 1 fast mclk/8 standard 0xc 1 1 0 0 focus mclk/1 one-shot 0xd 1 1 0 1 median mclk/1 one-shot 0xe 1 1 1 0 fast mclk/2 one-shot 0xf 1 1 1 1 fast mclk/1 one-shot configuration example in the example shown in table 20, the lowest current consumption is used, and the ad7761 is connected to an fpga. the formatx pins are set such that all eight data outputs, dout0 to dout7, connect to the fpga. for the lowest power, the lowest dclk frequency is used. the input bandwidth is set through the combination of selecting decimation by 64 and selecting the wideband filter. odr = f mod decimation ratio where: f mod is mclk/32 for focus mode (see table 17). decimation ratio = 64. thus, for this example, where mclk = 32.768 mhz, odr = (32.768 mhz/32) 64 = 16 khz minimizing the dclk frequency means selecting dclk = mclk/8, which results in a 4 mhz dclk signal. the period of dclk in this case is 1/4 mhz = 250 ns. the data conversion on each doutx pin is 24 bits long. the conversion data takes 24 250 ns = 6 s to be output. all 24 bits must be output within the odr period of 1/16 khz, which is approximately 64 s. in this case, the 6 s required to read out the conversion data is well within the 64 s between conversion outputs. therefore, this combination, which is summarized in table 20, is viable for use. channel standby table 19 shows how the user can put channels into standby mode. set either st0 or st1 to logic 1 to place banks of four channels into standby mode. when in standby mode, the disabled channels hold their position in the output data stream. the 8-bit header and 16-bit conversion result are set to all zeros when the adc channels are set to standby. the vcm voltage output is associated with the channel 0 circuitry. if channel 0 is put into standby mode, the vcm voltage output is also disabled for maximum power savings. channel 0 must be enabled while vcm is being used externally to the ad7761. the crystal excitation circuitry is associated with the channel 4 circuitry. if channel 4 is put into standby mode, the crystal circuitry is also disabled for maximum power savings. channel 4 must be enabled while the external crystal is used on the ad7761 . table 19. truth table for the ad7761 st0 and st1 pins st1 st0 function 0 0 all channels operational. 0 1 channel 0 to channel 3 in standby. channel 4 to channel 7 operational. 1 0 channel 4 to channel 7 in standby. channel 0 to channel 3 operational. 1 1 all channels in standby.
ad7761 data sheet rev. 0 | page 34 of 69 spi control the ad7761 has a 4-wire spi interface that is compatible with qspi?, microwire?, and dsps. the interface operates in spi mode 0. in spi mode 0, sclk idles low, the falling edge of cs e clocks out the msb, the falling edge of sclk is the drive edge, and the rising edge of sclk is the sample edge. this means that data is clocked out on the falling/drive edge and data is clocked in on the rising/sample edge. drive edge sample edge 14285-062 figure 52. spi mode 0 sclk edges accessing the adc register map to use spi control mode, set the pin /spi pin to logic high. the spi control operates as a 16-bit, 4-wire interface, allowing read and write access. figure 53 shows the interface format between the ad7761 and the digital host. the spi serial control interface of the ad7761 is an independent path for controlling and monitoring the device. there is no direct link to the data interface. the timing of mclk and dclk is not directly related to the timing of the spi control interface. however, the user must ensure that the spi reads and writes satisfy the minimum t 30 specification (see table 3 and table 5) so that the ad7761 can detect changes to the register map. spi access is ignored during the period immediately after a reset. allow the full adc start-up time after reset (see table 1) to elapse before accessing the ad7761 over the spi interface. table 20. modex example selection mode hex mode3 mode2 mode1 mode0 power mode dclk frequency data conversion 0x3 0 0 1 1 focus mclk/8 standard scl k sdi d0 d1 d2 d3d4d5 d6 d7a0a1a2 a3 a4a5a6 r/w sd o d0 d1d2d3d4d5d6d7 00000000 cs 14285-063 figure 53. write/read command
data sheet ad7761 rev. 0 | page 35 of 69 spi interface details each spi access frame is 16 bits long. the msb (bit 15) of the sdi command is the r/ w bit; 1 = read and 0 = write. bits[14:8] of the sdi command are the address bits. the spi control interface uses an off frame protocol. this means that the master (fpga/dsp) communicates with the ad7761 in two frames. the first frame sends a 16-bit instruction (r/ w , address, and data) and the second frame is the response where the ad7761 sends 16 bits back to the master. during the master write command, the sdo output contains eight leading zeros, followed by eight bits of data, as shown in figure 53. figure 54 shows the off frame protocol. register access responses are always offset by one cs frame. in figure 54, the response (read resp 1) to the first command (cmd 1) is output by the ad7761 during the following cs frame at the same time as the second command (cmd 2) is being sent. sclk sdi sdo cs cmd 1 cmd 2 read resp 1 14285-064 figure 54. off frame protocol spi control interface error handling the ad7761 spi control interface detects whether it has received an illegal command. an illegal command is a write to a read only register, a write to a register address that does not exist, or a read from a register address that does not exist. if any of these illegal commands are received by the ad7761 , the device responds with an error output of 0x0e00. spi reset configuration after a power-on or reset, the ad7761 default configuration is set to the following low current consumption settings: ? focus power mode with mclk/32. ? interface configuration of dclk = mclk/8, header output enabled, and crc disabled. ? filter configuration of channel mode a and channel mode b is set to sinc5 and decimation = 1024. channel mode select is set to 0x00, and all channels are assigned to channel mode a. ? channel configuration of channel 0 to channel 7 is enabled, with the analog input buffers enabled. the reference precharge buffers are disabled. the offset, gain, and phase calibration are set to the zero position. ? continuous conversion mode is enabled. spi control functionality spi control offers the superset of flexibility and diagnostics to the user. the following sections highlight the functionality and diagnostics offered when spi control is used. after any change to these configuration register settings, the user must provide a sync signal to the ad7761 through either the spi_sync command, or by applying the appropriate pulse to the start pin or sync_in pin to ensure that the configuration changes are applied correctly to the adc and digital filters. channel configuration the ad7761 has eight fully differential analog input channels. the channel configuration registers allow the channels to be individually configured to adapt to the measurement required on that channel. channels can be enabled or disabled using the channel standby register, register 0x00. analog input and reference precharge buffers can be assigned per input terminal. gain, offset, and phase calibration can be controlled on a per channel basis using the calibration registers. see the per channel calibration gain, offset, and sync phase section for more information. channel modes in spi control mode, the user can set up two channel modes, channel mode a (register 0x01), and channel mode b (register 0x02). each channel mode register can have a specific filter type and decimation ratio. using the channel mode select register (register 0x03), the user can assign each channel to either channel mode a or channel mode b, which maps that mode to the required adc channels. these modes allow different filter types and decimation rates to be selected and mapped to any of the adc channels. when different decimation rates are selected on different channels, the ad7761 outputs a data ready signal at the fastest selected decimation rate. any channel that runs at a lower output data rate is updated only at that slower rate. in between valid result data, the data for that channel is set to zero and the repeated data bit is set in the header status bits to distinguish it from a real conversion result (see the adc conversion output: header and data section for more information). on the ad7761 , consider channel mode a as the primary group. in this respect, it is recommended that there always be at least one channel assigned to channel mode a. if all eight channels of the ad7761 are assigned to channel mode b, conversion data is not output on the data interface for any of the channels. table 21. channel mode a/channel mode b, register 0x01 and register 0x02 bits bit name setting description reset access 3 filter_type_x filter output 0x1 rw 0 wideband filter 1 sinc5 filter [2:0] dec_rate_x decimation rate 0x5 rw 000 to 101 32 to 1024
ad7761 data sheet rev. 0 | page 36 of 69 table 22. channel mode selection, register 0x03 bits bit name setting description reset access [7:0] ch_x_mode channel x 0x0 rw 0 mode a 1 mode b reset over spi control interface two successive commands must be written to the ad1 data control register to initiate a full reset of the device over the spi interface. this action fully resets all registers to the default conditions. details of the commands and their sequence are shown in table . after a reset over the spi control interface, the ad1 responds to the first command sent to the device with xe. this response, in addition to the fact that all registers have assumed their default values, indicates that the software reset succeeded. sleep mode sleep mode puts the ad1 into its lowest power mode. in sleep mode, all adcs are disabled and a large portion of the digital core is inactive. the ad1 spi remains active and is available to the user when in sleep mode. write to register x, bit to exit sleep mode. for the lowest power consumption, select the sinc5 filter before entering sleep mode. channel standby mode for efficient power usage, place selected channels into standby mode when not in use. setting the bits in register x disables the corresponding channel (see table ). for maximum power savings, switch disabled channels to the sinc5 filter using the channel mode configurations, which disables some clocks associated with the wideband filters of those channels. the vcm voltage output is associated with the channel circuitry. if channel is put into standby mode, the vcm voltage output is also disabled for maximum power savings. channel must be enabled while vcm is being used externally to the ad1 . the crystal excitation circuitry is associated with the channel circuitry. if channel is put into standby mode, the crystal circuitry is also disabled for maximum power savings. channel must be enabled while the external crystal is used on the ad1 . clocking selections the internal modulator frequency (f mod ) used by each of the adcs in the ad1 is derived from the externally applied mclk signal. the mclk division bits allow the user to control the ratio between the mclk frequency and the internal modulator clock frequency. this control allows the user to select the division ratio that is best for their configuration. the appropriate clock configuration depends on the power mode, the decimation rate, and the base mclk frequency available in the system. see the clocking, sampling tree section for further information on setting mclkdiv correctly. mclk source selection the following clocking options are available as the mclk input source in spi control mode: ? lvds ? external crystal ? cmos input mclk setting clk_sel to logic low configures the ad7761 for correct operation using a cmos clock. setting clk_sel to logic high enables the use of an external crystal. if clk_sel is set to logic high and bit 3 of register 0x04 is also set, the application of an lvds clock signal to the mclk pin is enabled. lvds clocking is exclusive to spi control mode and requires the register selection for operation (see table 38). the dclk rate is derived from mclk. dclk division (the ratio between mclk and dclk) is controlled in the interface configuration selection register, register 0x07 (see table 41). interface configuration the data interface is a master output interface, where adc conversion results are output by the ad1 at a rate based on the mode selected. the interface consists of a data clock (dclk), the data ready ( drd e ) framing output, and the data output pins (dout0 to dout7). the interface can be configured to output conversion data on one, two, or eight of the doutx pins. the doutx configuration for the ad7761 is selected using the formatx pins (see table 28). the dclk rate is a direct division of the mclk input and can be controlled using bits[1:0] of register 0x07. the minimum dclk rate can be calculated as dclk (minimum) = output data rate channels per doutx 24 bits where mclk dclk . with eight adcs enabled, an mclk rate of 32.768 mhz, an odr of 256 ksps, and two doutx channels, dclk (minimum) is 256 ksps 4 channels per doutx 24 bits = 24.576 mhz where dclk = mclk /1. for more information on the status header, crc, and interface configuration, see the data interface section. crc protection the ad1 can be configured to output a crc message per channel every or 1 samples. this function is available only with spi control. crc is enabled in the interface control register, register x (see the crc check on data interface section). adc synchronization over spi the adc synchronization over spi allows the user to request a synchronization pulse to the adcs over the spi interface. to initiate the synchronization in this manner, write to bit in register x twice.
data sheet ad7761 rev. 0 | page 37 of 69 first, the user must write a 0, which sets sync_out low, and then write a 1 to set the sync_out logic high again. the spi_sync command is recognized after the last rising edge of sclk in the spi instruction, where the spi_sync bit is changed from low to high. the spi_sync command is then output synchronously to the ad7761 mclk signal on the sync_out pin. the user must connect the sync_out signal to the sync_in pin on the pcb. 14285-065 start sync_in dout0 dout1 drdy mclk spi interface sync_out dsp/ fpga ad7761 synchronization logic digital filter master clock iovdd figure 55. connection diagram for synchronization using spi_sync the sync_out pin can also be routed to the sync_in pins of other ad7761 devices, allowing simultaneous sampling to occur across larger channel count systems. any daisy-chained system of ad7761 devices requires that all adcs be synchronized. in a daisy-chained system of ad7761 devices, two successive synchronization pulses must be applied to guarantee that all adcs are synchronized. two synchronization pulses are also required in a system of more than one ad7761 device sharing a single mclk signal, where the drdy pin of only one device is used to detect new data. as per any synchronization pulse present on the sync_in pin, the digital filters of the ad7761 are reset by the spi_sync command. the full settling time of the filters must then elapse before valid data is output on the data interface. analog input precharge buffers the ad7761 contains precharge buffers on each analog input to ease the drive requirements on the external amplifier. each analog input precharge buffer can be enabled or disabled using the analog input precharge buffer registers (see table 48 and table 49). reference precharge buffers the ad7761 contains reference precharge buffers on each reference input to ease the drive requirements on the external reference and help to settle any nonlinearity on the reference inputs. each reference precharge buffer can be enabled or disabled using the reference precharge buffer registers (see table 50 and table 51). per channel calibration gain, offset, and sync phase the user can adjust the gain, offset, and sync phase of the ad7761 . these options are available only in spi control mode. further register information and calibration instructions are available in the offset registers section, the gain registers section, and the sync phase offset registers section. see the calibration section for information on calibration equations. gpios the ad7761 has five general-purpose input/output (gpio) pins available when operating in spi control mode. for further information on gpio configuration, see the gpio functionality section. spi control mode extra diagnostic features ram built in self test the ram built in self test (bist) is a coefficient check for the digital filters. the ad7761 dsp path uses some internal memories for storing data associated with filtering and calibration. a user may, if desired, initiate a built in self test (bist) of these memories. normal conversions are not possible while bist is running. the test is started by writing to the bist control register, register 0x08. the results and status of the test are available in the status register, register 0x09 (see table 43). normal adc conversion is disrupted when this test is run. a synchronization pulse is required after this test is complete to resume normal adc operation. revision identification number the ad7761 contains an identification register that is accessible in spi control mode, the revision identification register. this register is an excellent way to verify the correct operation of the serial control interface. register information is available in the revision identification register section. diagnostic meter mode the diagnostic metering mode can be used to verify the functionality of each adc by internally passing a positive full- scale, midscale, or negative full-scale voltage to the adc. the user can then read the resulting adc conversion result to determine that the adc is operating correctly. to configure adc conversion diagnostics, see the adc diagnostic receive select register section and the adc diagnostic control register section.
ad7761 data sheet rev. 0 | page 38 of 69 circuit information core signal chain each adc channel on the ad7761 has an identical signal path from the analog input pins to the data interface. figure 57 shows a top level implementation of the core signal chain. each adc channel has its own - modulator that oversamples the analog input and passes the digital representation to the digital filter block. the modulator sampling frequency (f mod ) ranges are explained in the clocking, sampling tree, and power scaling section. the data is filtered, scaled for gain and offset (depending on user settings), and then output on the data interface. control of the flexible settings for the signal chain is provided by either using the pin control or the spi control mode, set at power-up by the state of the pin /spi input pin. the ad7761 can use up to a 5 v reference and converts the differential voltage between the analog inputs (ainx+ and ainx?) into a digital output. the analog inputs can be configured as either differential or pseudo differential inputs. as a pseudo differential input, either ainx+ or ainx? can be connected to a constant input voltage (such as 0 v, avss, or some other reference voltage). the adc converts the voltage difference between the analog input pins into a digital code on the output. using a common- mode voltage of avdd1/2 for the analog inputs, ainx+ and ainx?, maximizes the adc input range. the 16-bit conversion result is in twos complement, msb first, format. figure 56 shows the ideal transfer functions for the ad7761. adc power modes the ad7761 has three selectable power modes. in pin control mode, the modulator rate and power mode are tied together for best performance. in spi control mode, the user can select the power mode and modulator mclk divider settings. the choice of power modes gives more flexibility to control the bandwidth and power dissipation for the ad7761 . table 9 shows the recommended f mod frequencies for each power mode, and table 38 shows the register information for the ad7761. 100 ... 000 100 ... 001 100 ... 010 011 ... 101 011 ... 110 011 ... 111 adc code (twos complement) analog input +fs ? 1.5lsb +fs ? 1lsb ?fs + 1lsb ?fs ?fs + 0.5lsb 14285-066 figure 56. adc ideal transfer functions (fs is full scale) table 23. output codes and ideal input voltages description analog input (ainx+ ? (ainx?)) v ref = 4.096 v digital output code, twos complement (hex.) fs ? 1 lsb +4.095875 v 0x7fff midscale + 1 lsb +125 v 0x0001 midscale 0 v 0x0000 midscale ? 1 lsb ?125 v 0xffff ?fs + 1 lsb ?4.095875 v 0x8001 ?fs ?4.096 v 0x8000 digital filter dclk pin/spi drdy doutx a inx+ a inx? filter/gpio4 cs sclk sdo sdi mode3/gpio3 to mode0/gpio0 control block pin control spi control data interface control - ? modulator precharge buffer esd protection control option pin or spi signal chain for single channel mclk start sync_out sync_in reset 14285-067 figure 57. top level core signal chain and control
data sheet ad7761 rev. 0 | page 39 of 69 analog inputs figure 58 shows the ad7761 analog front end. the esd protection diodes that are designed to protect the adc from some short duration overvoltage and esd events are shown on the signal path. the analog input is sampled at twice the modulator sampling frequency, f mod , which is derived from mclk. by default, the adc internal sampling capacitors, cs1 and cs2, are driven by a per channel analog input precharge buffer to ease the driving requirement of the external network. bps 0+ avdd1 ain0+ cs2 phi 0 phi 1 phi 1 phi 0 cs1 ain0? avss avss avdd1 bps 0? 14285-068 figure 58. analog front end the analog input precharge buffers provide the initial approximate charging of the switched capacitor network for 25% of the sampling phase. during this first phase, the bypass switches, bps 0+ and bps 0?, remain open. for the remaining 75% of the sampling phase, the bypass switches are closed, and the fine accuracy settling charge is provided by the external source. phi 0 and phi 1 represent the modulator clock sampling phases that switch the input signals onto the sampling capacitors, cs1 and cs2. the analog input precharge buffers reduce the switching kickback from the sampling stage to the external circuitry. the precharge buffer reduces the average input current by a factor of eight, and makes the input current more signal independent, to reduce the effects of sampling distortion. this reduction in drive requirements allows pairing of the ad7761 with lower power, lower bandwidth front-end driver amplifiers such as the ada4940-1 / ada4940-2 . ?400 ?300 ?200 ?100 0 100 200 300 400 0123456 a in (a) input voltage (v diff ) 14285-069 unbuffered ainx+ unbuffered ainx? figure 59. analog input current (a in ) vs. input voltage, analog input precharge buffer off, v cm = 2.5 v, f mod = 8.192 mhz ?30 ?25 ?20 ?15 ?10 ?5 0 01234 a in (a) input voltage (v diff ) 14285-070 precharge buffered ainx+ precharge buffered ainx? figure 60. analog input current (a in ) vs. input voltage, analog input precharge buffer on, v cm = 2.5 v, f mod = 8.192 mhz the analog input precharge buffers can be turned on/off by means of a register write to register 0x11 and register 0x12 (precharge buffer register 1 and precharge buffer register 2, respectively). each analog input precharge buffer is selectable per channel. in pin control mode, the analog input precharge buffers are always enabled for optimum performance. when the analog input precharge buffers are disabled, the analog input current is sourced completely from the analog input source. the unbuffered analog input current is calculated from two components: the differential input voltage on the analog input pair, and the analog input voltage with respect to avss. with the precharge buffers disabled, for 32.768 mhz mclk in fast mode with f mod = mclk/4, the differential input current is approximately 48 a/v and the current with respect to ground is approximately 17 a/v. for example, if the precharge buffers are off, with ain1+ = 5 v, and ain1? = 0 v, estimate the current in each input pin as follows: ain1+ = 5 v 48 a/v + 5 v 17 a/v = 325 a ain1 ? = ?5 v 48 a/v + 0 v 17 a/v = ?240 a when the precharge buffers are enabled, the absolute voltage with respect to avss determines the majority of the current. the maximum input current of approximately ?25 a is measured when the analog input is close to either the avdd1 or avss rails. with either precharge buffers enabled or disabled, the analog input current scales linearly with the modulator clock rate. the analog input current vs. input voltage is shown in figure 59. full settling of the analog inputs to the adc requires the use of an external amplifier. pair amplifiers such as the ada4805-2 for focus mode, the ada4805-2 or ada4940-1 / ada4940-2 for median mode, and the ada4807-2 or ada4896-2 for fast mode with the ad7761 (see table 24 for details on some of these pairings.). running the ad7761 in median and focus modes or reducing the mclk rate reduces the load and speed requirements of the amplifier; therefore, lower power amplifiers can be paired with the analog inputs to achieve the optimum signal chain efficiency.
ad7761 data sheet rev. 0 | page 40 of 69 table 24. amplifier pairing options power mode amplifier amplifier power (mw/channel) 1 analog input precharge buffer total power (amplifier + ad7761) (mw/channel) 1 fast ada4896-2 40.6 on 92.1 fast ada4807-2 13.6 on 65.1 median ada4805-2 7.5 on 35.0 focus ada4805-2 7.525 on 16.9 1 typical power at 25c. vcm the ad7761 provides a buffered common-mode voltage output on pin 59. this output can bias up analog input signals. by incorporating the vcm buffer into the adc, the ad7761 reduces component count and board space. in pin control mode, the vcm potential is fixed to (avdd1 ? avss)/2, and is enabled by default. in spi control mode, configure the vcm potential using the general configuration register (register 0x05). the output can be enabled or disabled, and set to (avdd1 ? avss)/2, 1.65 v, 2.14 v, or 2.5 v, with respect to avss. the vcm voltage output is associated with the channel 0 circuitry. if channel 0 is put into standby mode, the vcm voltage output is also disabled for maximum power savings. channel 0 must be enabled while vcm is being used externally to the ad7761. reference input the ad7761 has two differential reference input pairs. ref1+ and ref1? are the reference inputs for channel 0 to channel 3, and ref2+ and ref2? are for channel 4 to channel 7. the absolute input reference voltage range is 1 v to avdd1 ? avss. like the analog inputs, the reference inputs have a precharge buffer option. each adc has an individual buffer for each refx+ and refx?. the precharge buffers help reduce the burden on the external reference circuitry. in pin control mode, the reference precharge buffers are off by default. in spi control mode, the user can enable or disable the reference precharge buffers. in the case of unipolar analog supplies, in spi control mode, the user can achieve the best performance and power efficiency by enabling only the refx+ buffers. the reference input current scales linearly with the modulator clock rate. for 32 mhz mclk and mclk/4 fast mode, the differential input current is ~72 a/v per channel, unbuffered, and ~16 a/v per channel with the precharge buffers enabled. with the precharge buffers off, refx+ = 5 v, and refx? = 0 v, refx = 5 v 72 a/v = 360 a with the precharge buffers on, refx+ = 5 v, and refx? = 0 v, refx = 5 v 16 a/v = 80 a for the best performance and headroom, it is recommended to use a 4.096 v reference such as the adr444 or the adr4540 . for the best performance at high sampling rates, it is recommended to use an external reference drive amplifier such as the ada4841-1 or the ad8031. clock selection the ad7761 has an internal oscillator used for initial power-up of the device. after the ad7761 completes the start-up routine, the device normally transfer control of the internal clocking to the externally applied mclk. the ad7761 counts the falling edges of the external mclk over a given number of internal clock cycles to determine if the clock is valid and at least a frequency of 1.15 mhz. if there is a fault with the external mclk, the transfer of control does not occur, the ad7761 outputs an error in the status header, and the clock error bit is set in the device status register. no conversion data is output and a reset is required to exit this error state. three clock source input options are available to the ad7761: external cmos, crystal oscillator, or lvds. the clock is selected on power-up and is determined by the state of the clk_sel pin. if clk_sel = 0, the cmos clock option is selected and the clock is applied to pin 32 (pin 31 is tied to dgnd). if clk_sel = 1, the crystal or lvds option is selected and the crystal or lvds is applied to pin 31 and pin 32. the lvds option is available only in spi control mode. an spi write to bit 3 of register 0x04 enables the lvds clock option. digital filtering the ad7761 offers two types of digital filters. in spi control mode, these filters can be chosen on a per channel basis. in pin control mode, only one filter can be selected for all channels. the digital filters available on the ad7761 are ? sinc5 low latency filter, ?3 db at 0.204 odr ? wideband low ripple filter, ?3 db at 0.433 odr both filters can be operated in one of six different decimation rates, allowing the user to choose the optimal input bandwidth and speed of the conversion vs. the desired power mode or resolution.
data sheet ad7761 rev. 0 | page 41 of 69 sinc5 filter most precision - adcs use a sinc filter. the sinc5 filter offered in the ad7761 enables a low latency signal path useful for dc inputs, for control loops, or where other specific postprocessing is required. the sinc5 filter path offers the lowest noise and power consumption. the sinc5 filter has a ?3 db bw of 0.204 odr. table 11 contains the noise performance for the sinc5 filter across power modes and decimation ratios. 0 ?20 ?40 ?60 ?80 ?100 ?120 ?140 ?160 ?180 ?200 0 2 4 6 8 10 12 14 16 amplitude (db) normalized input frequency ( f in / f odr ) 14285-071 figure 61. sinc5 filter frequency response (decimation = 32) the settling times for the ad7761 when using the sinc5 filter are shown in table 32. wideband low ripple filter the wideband filter has a low ripple pass band, within 0.005 db of ripple, of 0.4 odr. the wideband filter has full attenuation at 0.499 odr (nyquist), maximizing antialias protection. the wideband filter has a pass-band ripple of 0.005 db and a stop band attenuation of 105 db from nyquist out to f chop . for more information on antialiasing and f chop aliasing, see the antialiasing section. the wideband filter is a very high order digital filter with a group delay of approximately 34/odr. after a synchronization pulse, there is an additional delay from the sync_in rising edge to fully settled data. the settling times for the ad7761 when using the wideband filter are shown in table 31. see table 10 for the noise performance of the wideband filter across power modes and decimation rates. 0 ?10 ?30 ?50 ?70 ?90 ?100 ?110 ?120 ?130 ?140 00 . 2 0.1 0.3 0.4 0.5 0.6 0.7 0.90.8 1.0 amplitude (db) normalized input frequency ( f in / f odr ) ?20 ?40 ?60 ?80 14285-072 figure 62. wideband filter frequency response amplitude (db) ?0.010 0.010 0.008 ?0.008 0.006 ?0.006 0.004 ?0.004 0.002 ?0.002 0 0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 normalized input frequency ( f in / f odr ) 14285-073 figure 63. wideband fi lter pass-band ripple 1.2 1.0 0.8 0.6 0.4 0.2 0 ?0.2 0 1020304050607080 amplitude (db) output data rate samples 14285-074 figure 64. wideband filter step response
ad7761 data sheet rev. 0 | page 42 of 69 decimation rate control the ad7761 has programmable decimation rates for the digital filters. the decimation rates allow the user to reduce the measure- ment bandwidth, reducing the speed but increasing the resolution. when using the spi control, control the decimation rate on the ad7761 through the channel mode registers. these registers set two separate channel modes with a given decimation rate and filter type. each adc is mapped to one of these modes via the channel mode select register. table 25 details both the decimation rates available, and the filter types for selection, within channel mode a and channel mode b. in pin control mode, the decimation ratio is controlled by the dec0 and dec1 pins; see table 15 for the decimation configuration in pin control mode. table 25. channel mode x registers, register 0x01 and register 0x02 bits name logic value decimation rate 3 filter_type_x 0 wideband filter 1 sinc5 filter [2:0] dec_rate_x 000 32 001 64 010 128 011 256 100 512 101 1024 110 1024 111 1024 antialiasing because the ad7761 is a switched capacitor, discrete time adc, the user may wish to employ external analog antialiasing filters to protect against foldback of out of band tones. within this section, an out of band tone refers to an input fre- quency greater than the pass band frequency specification of the digital filter that is applied at the analog input. when designing an antialiasing filter for the ad7761 , three main aliasing regions must be taken into account. after the alias requirements of each zone are understood, the user can design an antialiasing filter to meet the needs of the specific application. the three zones for consideration are related to the modulator sampling frequency, the modulator chopping frequency, and the modulator saturation point. modulator sampling frequency the ad7761 modulator signal transfer function includes a notch, at odd multiples of f mod , to reject tones or harmonics related to the modulator clock. the modulator itself attenuate signals at frequencies of f mod , 3 f mod , 5 f mod , and so on. for an mclk frequency of 32.768 mhz, the attenuation is approx- imately 35 db in fast mode, 41 db in median mode, and 53 db in focus mode. attenuation is increased by 6 db across each power mode, with every halving of the mclk frequency, for example, when reducing the clock from 32.768 mhz to 16.384 mhz. the modulator has no rejection to signals that are at frequencies in zones around 2 f mod and all even multiples of f mod . signals at these frequencies are aliased by the ad7761 . for the ad7761, the first of these zones that requires protection is at 2 f mod . because typical switch capacitor, discrete time - modulators provide no protection to aliasing at the frequency, f mod , the ad7761 provides a distinct advantage in this regard. figure 65 shows the frequency response of the modulator and wideband digital filter to out of band tones at the analog input. figure 65 shows the magnitude of an alias that is seen in band vs. the frequency of the signal sampled at the analog input. the relationship between the input signal and the modulator frequency is expressed in a normalized manner as a ratio of the input signal (f in ) to the modulator frequency (f mod ). this data demonstrates the adc frequency response relative to out of band tones when using the wideband filter. the input frequency (f in ) is swept from dc to 20 mhz. in fast mode, using an 8.192 mhz f mod frequency, the x-axis spans ratios of f in /f mod from 0 to 2.44 (equivalent to f in of 0 hz to 20 mhz). a similar characteristic occurs in median and focus modes. the notch appears in figure 65 with the input frequency (f in ) at f mod (designated at f in /f mod = 1.00 on the x-axis). an input at this frequency is attenuated by 35 db, which adds to the attenuation of any external antialiasing filter, thus reducing the frequency roll- off requirement of the external filter. if the plot is swept further in frequency, the notch is seen to recur at f in /f mod = 3.00. the point where f in = 2 f mod (designated on the x-axis at 2.00) offers 0 db attenuation, indicating that all signals falling at this frequency alias directly back into the adc conversion results, in accordance with the sampling theory. the ad7761 wideband digital filter also offers an added protection against aliasing. because the wideband filter has full attenuation at the nyquist frequency (f odr /2, where f odr = f mod / decimation rate), input frequencies, and in particular harmonics of input frequencies, that may fall close to f odr /2, do not fold back into the passband of the ad7761 .
data sheet ad7761 rev. 0 | page 43 of 69 ?140 ?130 ?120 ?110 ?100 ?90 ?80 ?70 ?60 ?50 ?40 ?30 ?20 ?10 0 0 0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000 1.125 1.250 1.375 1.500 1.625 1.750 1.875 2.000 2.125 2.250 2.375 2.500 alias magnitude (db) with respect to in- band magnitude f chop = f mod /32 f chop = f mod /8 f in / f mod 14285-075 figure 65. rejection of out of band input tones, wideband filter, decimation = 32, f mod = 8.192 mhz, analog input sweep from dc to 20 mhz modulator chopping frequency figure 65 plots two scenarios that relate to the chopping frequency of the ad7761 modulator. the ad7761 uses a chopping technique in the modulator similar to that of a chopped amplifier to remove offset, offset drift, and 1/f noise. the ad7761 default chopping rate is f mod /32. in pin control mode, the chop frequency is hardwired to f mod /32. in spi control mode, the user can select the chop frequency to be either f mod /32 or f mod /8. as shown in figure 65, the stop band rejection of the digital filter is reduced at frequencies that relate to even multiples of the chopping frequency (f chop ). all other out of band frequencies (excluding those already discussed relating to the modulator clock frequency, f mod ) are rejected by the stop band attenuation of the digital filter. an out of band tone with a frequency in the range of (2 f chop ) f 3db , where f 3db is the filter bandwidth employed, is attenuated to the envelope determined by the chop frequency setting (see figure 65), and aliased into the pass band. out of band tones near additional even multiples of f chop (that is, n f chop , where n is an even integer), are attenuated and aliased in the same way. chopping at f mod /32 offers the best performance for noise, offset, and offset drift for the ad7761. for ac performance it may be useful to select chopping at f mod /8 as this moves the first chopping tone to a higher frequency. however, chopping at f mod /8 may lead to slightly degraded noise (approximately 1 db loss in dynamic range) and offset performance compared to the default chop rate of f mod /32. table 26 shows the aliasing achieved by different order antialiasing filter options at the critical frequencies of f mod /32 and f mod /8 for chop aliasing, f mod /16 for modulator saturation, and 2 f mod for the first zone with 0 db attenuation. it assumes the corner frequency of the antialiasing filter is at f mod /64, which is just above the maximum input bandwidth that the ad7761 digital filter can pass when using a decimate by 32 filter setting. table 26. external antialiasing filter attenuation rc filter f mod /32 (db) f mod /16 (db) f mod /8 (db) 2 f mod (db) first order ?6 ?12 ?18 ?42 second order ?12 ?24 ?36 ?84 third order ?18 ?36 ?54 ?126 modulator saturation point a - modulator can be considered a standard control loop, employing negative feedback. the control loop works to ensure that the average processed error signal is very small over time. it uses an integrator to remember preceding errors and force the mean error to be zero. as the input signal rate of change increases with respect to the modulator clock, f mod , a larger voltage feedback error is processed. above a certain frequency, the error begins to saturate the modulator. for the ad7761 , the modulator may saturate for full-scale input frequencies greater than f mod /16, depending on the rate of change of input signal, input signal amplitude, and reference input level. a half power input tone at f mod /8 also causes the modulator to saturate. in applications where there may be high amplitude and frequency out of band tones, a first-order antialiasing filter is required with a ?3 db corner frequency set at f mod /16 to protect against modulator saturation. for example, if operating the ad7761 at full speed and using a decimation rate of 32 to achieve an output data rate of 256 ksps, the modulator rate is equal to 8.192 mhz. in this instance, to protect against saturation, set the antialiasing filter ?3 db corner frequency to 512 khz. calibration in spi control mode, the ad7761 offers users the ability to adjust offset, gain, and phase delay on a per channel basis. offset adjustment the chx_offset_msb, chx_offset_mid, and chx_ offset_lsb registers are 24 bit, signed twos complement registers for channel offset adjustment. if the channel gain setting is at its ideal nominal value of 0x5 55555, an lsb of offset register adjustment changes the digital output by ?1/192 lsbs. for example, changing the offset register from 0 to 4800 changes the digital output by ?25 lsbs. because offset calibration occurs before gain calibration, the ratio of 1/192 changes linearly with gain adjustment via the channel x gain registers (see table 53). after a reset or power cycle, the offset register values revert to the default factory setting.
ad7761 data sheet rev. 0 | page 44 of 69 gain adjustment each adc channel has an associated gain coefficient. the coefficient is stored in three single-byte registers split up as msb, mid, and lsb. each of the gain registers are factory programmed. nominally, this gain is around the value 0x555555 (for an adc channel). the user can overwrite the gain register setting. however, after a reset or power cycle, the gain register values revert to the hard coded programmed factory setting. calculate the approximate result that is output using the following formula: 42 21 2 300,194,4 1024 (2 3 ?? ? ? ? ? ? ? ? ? ?? ? ? gain offset) v v data ref in where: offset is the offset register setting. gain is the gain register setting. sync phase offset adjustment the ad7761 has one synchronization signal for all channels. the sync phase offset register allows the user to vary the phase delay on each channel relative to the synchronization edge received on the sync_in pin. by default, all adc channels react simultaneously to the sync_in pulse. the sync phase registers can be programmed to equalize known external phase differences on the adc input channels, relative to one another. the range of phase compensation is limited to a maximum of one conversion cycle, and the resolution of the correction depends on the decimation rate in use. table 27 displays the resolution and register bits used for phase offset for each decimation ratio. table 27. phase delay resolution decimation ratio resolution steps sync phase offset register bits 32 1/f mod 32 [7:3] 64 1/f mod 64 [7:2] 128 1/f mod 128 [7:1] 256 1/f mod 256 [7:0] 512 2/f mod 256 [7:0] 1024 4/f mod 256 [7:0] adjusting the sync phase of channels can affect the time to the first drdy pulse after the sync pulse, as well as the time to bit 6 of the header status (filter not settled data bit) being cleared, that is, the time to settled data. if all channels are using the sinc5 filter, the time to the first drdy pulse is not affected by the adjustment of the sync phase offset, assuming that at least one channel has zero sync phase offset adjustment. if all channels have a nonzero sync phase offset setting, the time to the first drdy pulse is delayed according to the channel that has the least offset applied. channels with a sync offset adjustment setting that delays the internal sync signal, relative to other channels, may not output settled data until after the next drdy pulse. in other words, there may be a delay of one odr period between the settled data being output by the ad7761 for the channels with added phase delay. if all channels are using the wideband filter, the time to the first drdy pulse and the time to settled data is delayed according to the channel with the maximum phase delay setting. in this case, the interface waits for the latest channel and outputs data for all channels when that channel is ready.
data sheet ad7761 rev. 0 | page 45 of 69 data interface setting the format of data output the data interface format is determined by setting the formatx pins. the logic state of the formatx pins is read on power-up and determines how many data lines (doutx) the adc conversions are output on. because the formatx pins are read on power-up of the ad7761 and the device remains in this output configuration, this function must always be hardwired and cannot be altered dynamically. table 28, figure 66, figure 67, and figure 68 show the formatting configuration for the digital output pins on the ad7761. calculate the minimum required dclk rate for a given data interface configuration as follows: dclk (minimum) = output data rate channels per doutx 24 where mclk dclk . for example, if mclk = 32.768 mhz, with two doutx lines, dclk (minimum) = 256 ksps 4 channels per doutx 24 = 24.576 mhz therefore, dclk = mclk/1 is required. alternatively, if mclk = 32.768 mhz, with eight doutx lines, dclk (minimum) = 256 ksps 1 channel per doutx 24 = 6.144 mhz therefore, dclk = mclk/4 (dclk = 8.192 mhz) is sufficient. higher dclk rates make it easier to receive the conversion data from the ad7761 with a lower number of doutx lines; however, there is a trade-off against adc offset performance with higher dclk frequencies. for the best offset and offset drift performance, use the lowest dclk frequency possible. the user can choose to reduce the dclk frequency by an appropriate selection of mclk frequency, dclk divider, and/or the number of doutx lines used. table 28. formatx truth table format1 format0 description 0 0 each adc channel outputs on its own dedicated pin. dout0 to dout7 are in use. 0 1 the adcs share the dout0 and dout1 pins: channel 0 to channel 3 output on dout0. channel 4 to channel 7 output on dout1. the adc channels share data pins in time division multiplexed (tdm) output. dout0 and dout1 are in use. 1 x all channels output on the dout0 pin, in tdm output. only dout0 is in use. dout0 dclk drdy daisy-chaining is not possible in this format dgnd format0 each adc has a dedicated doutx pin 0 0 ad7761 format1 ch 0 dout1 ch 1 dout7 ch 7 14285-076 figure 66. formatx = 00, eight data output pins dout1 dout0 dclk drdy format1 daisy-chaining is possible in this format dgnd format0 iovdd channel 0 to channel 3 output on dout0 channel 4 to channel 7 output on dout1 1 0 ad7761 14285-077 figure 67. formatx = 01, two data output pins
ad7761 data sheet rev. 0 | page 46 of 69 dout0 dclk drdy format1 daisy-chaining is possible in this format format0 iovdd channel0 to channel7 output on dout0 1 1 ad7761 14285-078 figure 68. formatx = 10 or 11, one data output pin adc conversion output: header and data the ad7761 data is output on the dout0 to dout7 pins, depending on the formatx pins. the actual structure of the data output for each adc result is shown in figure 69. each adc result comprises 24 bits. the first eight bits are the header status bits, which contain status information and the channel number. the names of each of the header status bits are shown in table 29, and their functions are explained in the subsequent sections. this header is followed by a 16-bit adc output in twos complement coding, msb first. transitions on the drdy and the doutx pins are aligned with the rising edge of dclk. see figure 2 and table 2 for details. adc data n n ? 1 16 bits 8 bits doutx drdy header n 14285-079 figure 69. adc output: 8-bit header, 16-bit adc conversion data table 29. header status bits bit bit name 7 chip_error 6 filter not settled 5 repeated data 4 filter type 3 filter saturated [2:0] channel id[2:0] chip error the chip error bit indicates that a serious error has occurred. if this bit is set, a reset is required to clear this bit. this bit indicates that the external clock is not detected, a memory map bit has unexpectedly changed state, or an internal crc error has been detected. in the case where an external clock is not detected, the conversion results are output as all zeros regardless of the analog input voltages applied to the adc channels. filter not settled after power-up, reset, or synchronization, the ad7761 clears the digital filters and begins conversion. due to the weighting of the digital filters, there is a delay from the first conversion to fully settled data. the settling times for the ad7761 when using the wideband and sinc5 filters are shown in table 31 and table 32, respectively. this bit is set if this settling delay has not yet elapsed. repeated data if different channels use different decimation rates, data outputs are repeated for the slower speed channels. in these cases, the header is output as normal with the repeated data bit set to 1, and the following repeated adc result is output as all zeros. this bit indicates that the conversion result of all zeros is not real; it indicates that there is a repeated data condition because two different decimation rates are selected. this condition can only occur during spi control of the ad7761. filter type in pin control mode, all channels operate using one filter selection. the filter selected in pin control mode is determined by the logic level of the filter pin. in spi control mode, the digital filters can be selected on a per channel basis, using the mode registers. this header bit is 0 for channels using the wideband filter, and 1 for channels using the sinc5 filter. filter saturated the filter saturated bit indicates that the filter output is clipping at either positive or negative full scale. the digital filter clips if the signal goes beyond the specification of the filter; it does not wrap. the clipping may be caused by the analog input exceeding the analog input range, or by a step change in the input, which may cause overshoot in the digital filter. clipping may also occur when the combination of the analog input signal and the channel gain register setting causes the signal seen by the filter to be higher than the analog input range. channel id the channel id bits indicate the adc channel from which the succeeding conversion data originates (see table 30). table 30. channel id vs. channel number channel channel id 2 cha nnel id 1 channel id 0 channel 0 0 0 0 channel 1 0 0 1 channel 2 0 1 0 channel 3 0 1 1 channel 4 1 0 0 channel 5 1 0 1 channel 6 1 1 0 channel 7 1 1 1
data sheet ad7761 rev. 0 | page 47 of 69 data interface: standard conversion operation in standard mode operation, the ad7761 operates as the master and streams data to the dsp or fpga. the ad7761 supplies the data, the data clock (dclk), and a falling edge framing signal ( drdy ) to the slave device. all of these signals are synchronous. the data interface connections to dsp/fpga are shown in figure 73. the formatx pins determine how the data is output from the ad7761. figure 70 through figure 72 show the data interface operating in standard mode at the maximum data rate. in all instances, drdy is asserted one clock cycle before the msb of the data conversion is made available on the data pin. each drdy falling edge starts the output of the new adc conversion data. the first eight bits output after the drdy falling edge are the header bits; the last 16 bits are the adc conversion result. figure 70, figure 71, and figure 72 are distinct examples of the impact of the formatx pins on the ad7761 output operating in standard conversion operation. figure 70 to figure 72 represent running the ad7761 at maximum data rate for the three formatx options. figure 70 shows formatx = 00. each adc has its own data out pin running at the mclk/4 bit rate. in pin control mode, this is achieved by selecting mode 0xa (fast mode, dclk = mclk/4, standard conversion, see table 18) with the decimation rate set as 32. figure 71 shows formatx = 01 sharing dout1 at the maximum bit rate. in pin control mode, this is achieved by selecting mode 0x8 (fast mode, dclk = mclk/1, standard conversion) with a decimation rate of 32. if running in pin control mode, the example shown in figure 72 represents mode 0x4 (median mode, dclk = mclk/1, standard conversion) with a decimation rate of 32, giving the maximum output data capacity possible on one doutx pin. dclk drdy sample n sample n + 1 dout0 dout1 dout7 14285-080 figure 70. formatx = 00: each adc has a dedicated data output pin, maximum data rate ch0 (n)ch1 (n)ch2 (n)ch3 (n) dclk drdy dout0 sample n sample n + 1 dout1 dout7 dout2 ch0 ( n + 1 )ch1 ( n + 1 ) ch2 ( n + 1 )ch3 ( n + 1 ) ch4 (n)ch5 (n)ch6 (n)ch7 (n) ch4 ( n + 1 )ch5 ( n + 1 ) ch6 ( n + 1 )ch7 ( n + 1 ) 14285-081 figure 71. formatx = 01: channel 0 to channel 3 share dout0, and channel 4 to channel 7 share dout1, maximum data rate (dout6 and dout7 are inputs in this configuration)
ad7761 data sheet rev. 0 | page 48 of 69 dclk drdy dout0 sample n sample n + 1 sample n + 2 dout7 dout1 14285-082 figure 72. formatx = 11 or 10: channel 0 to channel 7 output on dout0 only, maximum data rate (dout6 and dout7 are inputs in th is configuration) dsp/fpga ad7761 dclk drdy dout0 to dout7 mclk 14285-083 figure 73. data interface: standard conversion operation, ad7761 = master, dsp/fpga = slave s ync_in dout0 dout1 drdy 24 dclks 24 dclks dout7 t settle settled data settled data settled data settled data settled data settled data 14285-084 figure 74. one-shot mode data interface: one-shot conversion operation one-shot mode is available in both spi and pin control modes. this conversion mode is available by selecting one of mode 0xc to mode 0xf when in pin control mode. in spi control mode, set bit 4 (one-shot mode) of register 0x06, the data control register. figure 74 shows the device operating in one-shot mode. in one-shot mode, the ad7761 is a pseudoslave. conversions occur on request by the master device, for example, the dsp or fpga. the sync_in pin initiates the conversion request. in one-shot mode, all adcs run continuously; however, the rising edge of the sync_in pin controls the point in time from which data is output.
data sheet ad7761 rev. 0 | page 49 of 69 to receive data, the master must pulse the sync_in pin to reset the filter and force drdy low. drdy subsequently goes high to indicate to the master device that the device has valid settled data available. unlike standard mode, drdy remains high for the number of clock periods of valid data before it goes low again; thus, in this conversion mode, it is an active high frame of the data. when the master pulses sync_in and the ad7761 receives the rising edge of this signal, the digital filter is reset and the full settling time of the filter elapses before the data is available. the duration of the settling time depends on the filter path and decimation rate. running one-shot mode with the sinc5 filter allows the fastest throughput, because this filter has a lower settling time than the wideband filter. as soon as settled data is available on any channel, the device outputs data from all channels. the contents of bit 6 of the channel header status bits indicates whether the data is fully settled. the period before the data is settled on all channels (t settle ) is shown in figure 74. after the data has settled on all channels, drdy is asserted high and the device outputs the required settled data on all channels before drdy is asserted low. if the user config- ures the same filter and decimation rate on each adc, the data is settled for all channels on the first drdy output frame, which avoids a period of unsettled data prior to the settled data and ensures that all data is output at the same time on all adcs. the device then waits for another sync_in signal before outputting more data. because all the adcs are sampling continuously, one-shot mode affects the sampling theory of the ad7761 . particularly, a user periodically sending a sync_in pulse to the device is a form of subsampling of the adc output. the subsampling occurs at the rate of the sync_in pulses. the sync_in pulse must be synchronous with the master clock to ensure coherent sampling and to reduce the effects of jitter on the frequency response. daisy-chaining daisy-chaining devices allows numerous devices to use the same data interface lines by cascading the outputs of multiple adcs from separate ad7761 devices. only one adc device has its data interface in direct connection with the digital host. for the ad7761 , this connection can be implemented by cascading dout0 and dout1 through a number of devices, or using only dout0; whether two data output pins or only one data output pin is enabled depends on the formatx pins. the ability to daisy-chain devices and the limit on the number of devices that can be handled by the chain is dependent on the power mode, dclk, and the decimation rate employed. the maximum usable dclk frequency allowed when daisy- chaining devices is limited by the combination of timing specifications in table 2 or table 4, as well as by the propagation delay of the data between devices and any skew between the mclk signals at each ad7761 device. the propagation delay and mclk skew are dependent on the pcb layout and trace lengths. this feature is especially useful for reducing component count and wiring connections, for example, in isolated multiconverter applications or for systems with a limited interfacing capacity. when daisy-chaining on the ad7761 , dout6 and dout7 become serial data inputs, and dout0 and dout1 remain as serial data outputs under the control of the formatx pins. start sync_in dout1 dout0 drdy dout6 mclk dout7 sync_out dsp/ fpga start sync_in dout1 dout0 drdy dout6 mclk dout7 sync_out dnc dnc iovdd iovdd start sync_in dout1 dout0 drdy dout6 mclk dout7 sync_out dnc dnc ad7761 ad7761 ad7761 synchronization logic digital filter synchronization logic digital filter synchronization logic digital filter master clock 14285-085 figure 75. daisy-chaining multiple ad7761 devices figure 75 shows an example of daisy-chaining ad7761 devices, when formatx = 01. in this case, the dout1 and dout0 pins of the ad7761 devices are cascaded to the dout6 and dout7 pins of the next device in the chain. data readback is analogous to clocking a shift register where data is clocked out on the rising edge of dclk. input data on the dout6 and dout7 pins is sampled on the falling edge of dclk. the scheme operates by passing the output data of the dout0 and dout1 pins of an ad7761 upstream device to the dout6 and dout7 inputs of the next ad7761 device downstream in the chain. the data then continues through the chain until it is clocked onto the dout0 and dout1 pins of the final down- stream device in the chain. daisy-chaining can be achieved in a similar manner on the ad7761 when using only the dout0 pin. in this case, only pin 21 of the ad7761 is used as the serial data input pin. in a daisy-chained system of ad7761 devices, two successive synchronization pulses must be applied to guarantee that all adcs are synchronized. two synchronization pulses are also required in a
ad7761 data sheet rev. 0 | page 50 of 69 system of more than one ad7761 device sharing a single mclk signal, where the drdy pin of only one device is used to detect new data. the maximum dclk frequency that can be used when daisy- chaining devices is a function of the ad7761 timing specifications (t 4 , and t 11 in table 4), the mclk duty cycle, and any timing differences between the ad7761 devices due to layout and spacing of devices on the pcb. use the following formula to aid in determining the maximum operating frequency of the interface: ?? skew p 411 max tttt f ???? ? 2 1 where: f max is the maximum useable dclk frequency. t 11 and t 4 are the ad7761 timing specifications (see table 4). t p is the maximum propagation delay of the data between successive ad7761 devices in the chain. t skew is the maximum skew in the mclk signal seen by any pair of ad7761 devices in the chain. the mclk duty cycle is 50:50, or dclk is set to mclk/2, mclk/4, or mclk/8. in the case where the mclk duty cycle is not 50:50 and the interface is configured with dclk = mclk/1, ensure that the applied mclk signal meets the minimum mclk high pulse width requirement, as calculated by the following formula: mclk minimum high pulse width = t 11 + t 4 + t p + t skew synchronization an important consideration for daisy-chaining more than two ad7761 devices is synchronization. the basic provision for synchronizing multiple devices is that each device is clocked with the same base mclk signal. the ad7761 offers three options to allow ease of system synchro- nization. choosing between the options depends on the system, but is determined by whether the user can supply a synchronization pulse that is truly synchronous with the base mclk signal. if the user cannot provide a signal that is synchronous to the base mclk signal, one of the following two methods can be employed: ? apply a start pulse to the first ad7761 device. the first ad7761 device samples the asynchronous start pulse and generates a pulse on sync_out of the first device related to the base mclk signal for distribution locally. ? use synchronization over spi (only available in spi control mode) to write a synchronization command to the first ad7761 device. similarl to the start pin method, the spi sync generates a pulse on sync_out of the first device related to the base mclk signal for distribution locally. in both cases, route the sync_out pin of the first device to the sync_in pin of that same device and to the sync_in pins of all other devices that are to be synchronized (see figure 76). the sync_out pins of the other devices must remain open circuit. tie all unused start pins to a logic 1 through pull-up resistors. 14285-086 start sync_in dout1 dout0 drdy mclk sync_out dsp/ fpga ad7761 synchronization logic digital filter start sync_in drdy mclk sync_out ad7761 synchronization logic digital filter master clock dnc dnc iovdd figure 76. synchronizing multiple ad7761 devices using sync_out if the user can provide a signal that is synchronous to the base mclk, this signal can be applied directly to the sync_in pin. route the signal from a star point and connect it directly to the sync_in pin of each ad7761 device (see figure 77). the signal is sampled on the rising mclk edge; setup and hold times are associated with the sync_in input are relative to the ad7761 mclk rising edge. in this case, tie the start pin to logic 1 through a pull-up resistor; sync_out is not used and can remain open circuit. 14285-087 sync_in dout1 dout0 drdy mclk dsp/ fpga ad7761 synchronization logic digital filter dout1 dout0 drdy sync_in mclk ad7761 synchronization logic digital filter start iovdd start iovd d figure 77. synchronizing multiple ad7761 devices using only sync_in
data sheet ad7761 rev. 0 | page 51 of 69 crc check on data interface the ad7761 delivers 24 bits per channel as standard, which by default consists of 8 status header bits and 16 bits of data. the header bits move to the default value per the description in table 29. however, there is also the option to employ a crc check on the adc conversion data. this functionality is available only when operating in spi control mode. the function is controlled by crc_select in the interface configuration register (register 0x07). when employed, the crc message is calculated internally by the ad7761 on a per channel basis. the crc then replaces the 8-bit header every four samples or every 16 samples. the following is an example of how the crc works for four- sample mode (see figure 78): 1. after a synchronization pulse is applied to the ad7761 , the crc register is cleared to 0xff. 2. the next four 16-bit conversion data samples (n to n + 3) for a given channel stream into the crc calculation. 3. for the first three samples that are output after the synchronization pulse (n to n + 2), the header contains the normal status bits. 4. for the fourth sample after the synchronization pulse (n + 3), the 8-bit crc is sent out instead of the normal header status bits, followed by the sample conversion data. this crc calculation includes the conversion data that is output immediately after the crc header. 5. the crc register is then cleared back to 0xff and the cycle begins again for the fifth to eighth samples after the synchronization pulse. it is possible to have channels outputting at different rates (for example, decimation by 32 on channel 0 and decimation by 64 on channel 1). in such cases, the crc header still appears across all channels at the same time, that is, at every fourth drdy pulse after a synchronization. for the channels operating at a relatively slower odr, the crc is still calculated and emitted every 4 or 16 drdy cycles, even if this means that the nulled data is included. therefore, a crc is calculated for only nulled samples or for a combination of nulled samples and actual conversion data. the ad7761 uses a crc polynomial to calculate the crc message. the 8-bit crc polynomial used is x 8 + x 2 + x + 1. to generate the checksum, the data is left shifted by eight bits to create a number ending in eight 1s. the polynomial is aligned such that its msb is adjacent to the leftmost logic 1 of the data. an exclusive or (xor) function is applied to the data to produce a new, shorter number. the polyno- mial is again aligned such that its msb is adjacent to the leftmost logic 1 of the new result, and the procedure is repeated. this process repeats until the original data is reduced to a value less than the polynomial. this is the 8-bit checksum. n ? 1 dout0 header n 8 bits data n header n + 1 data n + 1 header n + 2 data n + 2 crc data n + 3 16 bits 8 bits 16 bits 8 bits 16 bits 8 bits 16 bits 14285-088 figure 78. crc 4-bit stream table 31. wideband filter sync_in to settled data power mode filter type decimation factor delay from first mclk rise after sync_in rise to first drdy rise delay from first mclk rise after sync_in rise to earliest settled data, drdy rise group a group b group a group b group a group b mclk periods mclk periods mclk periods fast wideband wideband 32 unused 336 8400 not applicable wideband wideband 64 unused 620 16,748 not applicable wideband wideband 128 unused 1187 33,443 not applicable wideband wideband 256 unused 2325 66,837 not applicable wideband wideband 512 unused 4601 133,625 not applicable wideband wideband 1024 unused 9153 267,201 not applicable wideband wideband 32 32 758 8822 8822 wideband wideband 32 64 758 8822 17,014 wideband wideband 32 128 758 8822 33,526 wideband wideband 32 256 758 8822 66,934 wideband wideband 32 512 758 8822 133,622 wideband wideband 32 1024 758 8822 267,253 wideband wideband 64 32 759 17,015 8823 wideband wideband 128 32 760 33,528 8824 wideband wideband 256 32 762 66,938 8826 wideband wideband 512 32 782 133,646 8846 wideband wideband 1024 32 806 267,302 8870
ad7761 data sheet rev. 0 | page 52 of 69 power mode filter type decimation factor delay from first mclk rise after sync_in rise to first drdy rise delay from first mclk rise after sync_in rise to earliest settled data, drdy rise group a group b group a group b group a group b mclk periods mclk periods mclk periods median wideband wideband 32 un used 656 16,784 not applicable wideband wideband 64 unused 1225 33,481 not applicable wideband wideband 128 unused 2359 66,871 not applicable wideband wideband 256 unused 4635 133,659 not applicable wideband wideband 512 unused 9187 267,235 not applicable wideband wideband 1024 unused 18,291 534,387 not applicable wideband wideband 32 32 820 16,948 16,948 wideband wideband 32 64 820 16,948 33,588 wideband wideband 32 128 820 16,948 66,868 wideband wideband 32 256 820 16,948 133,684 wideband wideband 32 512 820 16,948 267,316 wideband wideband 32 1024 820 16,948 534,580 wideband wideband 64 32 822 33,590 16,950 wideband wideband 128 32 824 66,872 16,952 wideband wideband 256 32 844 133,708 16,972 wideband wideband 512 32 836 267,332 16,964 wideband wideband 1024 32 852 534,612 16,980 focus wideband wideband 32 un used 2587 67,099 not applicable wideband wideband 64 unused 4855 133,879 not applicable wideband wideband 128 unused 9391 267,439 not applicable wideband wideband 256 unused 18,495 534,591 not applicable wideband wideband 512 unused 36,703 1,068,895 not applicable wideband wideband 1024 unused 73,119 2,137, 503 not applicable wideband wideband 32 32 2587 67,099 67,099 wideband wideband 32 64 2587 67,099 134,683 wideband wideband 32 128 2587 67,099 267,803 wideband wideband 32 256 2587 67,099 535,067 wideband wideband 32 512 2587 67,099 1,069,595 wideband wideband 32 1024 2587 67,099 2,137,627 wideband wideband 64 32 2587 134,683 67,099 wideband wideband 128 32 2587 267,803 67,099 wideband wideband 256 32 2587 535,067 67,099 wideband wideband 512 32 2587 1,069,595 67,099 wideband wideband 1024 32 2587 2,137,627 67,099
data sheet ad7761 rev. 0 | page 53 of 69 table 32. sinc5 filter sync_in to settled data 1 power mode filter type decimation factor delay from first mclk rise after sync_in rise to first drdy rise delay from first mclk rise after sync_in rise to earliest settled data, drdy rise group a group b group a group b group a group b mclk periods mclk periods mclk periods fast sinc5 sinc5 32 unused 199 839 not applicable sinc5 sinc5 64 unused 327 1607 not applicable sinc5 sinc5 128 unused 583 3143 not applicable sinc5 sinc5 256 unused 1095 6215 not applicable sinc5 sinc5 512 unused 2119 12359 not applicable sinc5 sinc5 1024 unused 4167 24,647 not applicable sinc5 sinc5 32 32 199 839 839 sinc5 sinc5 32 64 199 839 1607 sinc5 sinc5 32 128 199 839 3143 sinc5 sinc5 32 256 199 839 6215 sinc5 sinc5 32 512 199 839 12,359 sinc5 sinc5 32 1024 199 839 24,647 sinc5 sinc5 64 32 199 1607 839 sinc5 sinc5 1024 32 199 24,647 839 median sinc5 sinc5 32 unused 383 1663 not applicable sinc5 sinc5 64 unused 639 3199 not applicable sinc5 sinc5 128 unused 1151 6271 not applicable sinc5 sinc5 256 unused 2175 12,415 not applicable sinc5 sinc5 512 unused 4223 24,703 not applicable sinc5 sinc5 1024 unused 8319 49,279 not applicable sinc5 sinc5 32 32 383 1663 1663 sinc5 sinc5 32 64 383 1663 3199 sinc5 sinc5 32 128 383 1663 6271 sinc5 sinc5 32 256 398 1663 12,415 sinc5 sinc5 32 512 398 1663 24,703 sinc5 sinc5 32 1024 398 1663 49,279 sinc5 sinc5 64 32 383 3199 1663 sinc5 sinc5 1024 32 398 49,279 1663 focus sinc5 sinc5 32 unused 1487 6607 not applicable sinc5 sinc5 64 unused 2511 12,751 not applicable sinc5 sinc5 128 unused 4559 25,039 not applicable sinc5 sinc5 256 unused 8655 49,615 not applicable sinc5 sinc5 512 unused 16,847 98,767 not applicable sinc5 sinc5 1024 unused 33,231 197,071 not applicable sinc5 sinc5 32 32 1487 6607 6607 sinc5 sinc5 32 64 1487 6607 12,751 sinc5 sinc5 32 128 1487 6607 25,039 sinc5 sinc5 32 256 1487 6607 49,615 sinc5 sinc5 32 512 1487 6607 98,767 sinc5 sinc5 32 1024 1487 6607 197,071 sinc5 sinc5 64 32 1487 12,751 6607 sinc5 sinc5 1024 32 1487 197,071 6607 1 this table is based on default internal clock divide settings of mclk/4 in fast mode, mclk/8 in median mode, and mclk/32 in fo cus mode.
ad7761 data sheet rev. 0 | page 54 of 69 functionality gpio functionality the ad7761 has additional gpio functionality when operated in spi control mode. this fully configurable mode allows the device to operate five gpios. the gpiox pins can be set as inputs or outputs (read or write) on a per pin basis. in write mode, these gpio pins can be used to control other circuits such as switches, multiplexers, and buffers, over the same spi interface as the ad7761 . sharing the spi interface in this way allows the user to use a lower overall number of data lines from the controller compared to a system where multiple control signals are required. this sharing is especially useful in systems where reducing the number of control lines across an isolation barrier is important. see figure 79 for details of the gpiox pin options available on the ad7761. similarly, a gpio read is a useful feature because it allows a peripheral device to send information to the input gpio and then this information can be read from the spi interface of the ad7761. 12 13 14 15 to dsp/fpga 11 17 st1/sclk 18 dec1/sdi 19 dec0/sdo 20 dout7 21 dout6 22 dout5 23 dout4 24 dout3 25 dout2 14285-089 mode0/gpio0 mode1/gpio1 mode2/gpio2 mode3/gpio3 filter/gpio4 gpio pins 16 st0/cs figure 79. gpio functionality configuration control and readback of the gpiox pins are set in register 0x0e, register 0x0f, and register 0x10 (see table 45, table 46, and table 47 for more information).
data sheet ad7761 rev. 0 | page 55 of 69 register map details (spi control) register map table 33. detailed register map reg. name bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0 reset rw 0x00 channel standby ch_7 ch_6 ch_5 ch_4 ch_3 ch_2 ch_1 ch_0 0x00 rw 0x01 channel mode a unused filter_type_a dec_rate_a 0x0d rw 0x02 channel mode b unused filt er_type_b dec_rate_b 0x0d rw 0x03 channel mode select ch_7_mode ch_6_mode ch_5_mode ch_4_mode ch_3 _mode ch_2_mode ch_1_mode ch_0_mode 0x00 rw 0x04 power_mode sleep_ mode unused power_mode lvds_e nable unused mclk_div 0x00 rw 0x05 general configuration unused reserved retime_en vcm_pd reserved unused vcm_vsel 0x08 rw 0x06 data control spi_sync unused sing le_shot_en unused spi_reset 0x80 rw 0x07 interface configuration unused crc_select dclk_div 0x0 rw 0x08 bist control unused ram_bist_ start 0x0 rw 0x09 device status unused chip_error no_clock_ error ram_bist_pass ram_bist_ running 0x0 r 0x0a revision id revision_id 0x06 r 0x0b reserved reserved 0x00 r 0x0c reserved reserved 0x00 r 0x0d reserved reserved 0x00 r 0x0e gpio control ugpio_ enable unused gpioe4_filter gpioe3_mode3 gpioe2_mode2 gpioe1_mode1 gpio0_mode0 0x00 rw 0x0f gpio write data unused gp io4_write gpio3_write gpio2_write gpio1_write gpio0_write 0x00 rw 0x10 gpio read data unused gp io4_read gpio3_read gpio2_read gpio1_read gpio0_read 0x00 r 0x11 precharge buffer 1 ch3_prebuf_ neg_en ch3_prebuf_ pos_en ch2_prebuf_ neg_en ch2_prebuf_ pos_en ch1_prebuf_ neg_en ch1_prebuf_ pos_en ch0_prebuf_ neg_en ch0_prebuf_ pos_en 0xff rw 0x12 precharge buffe r 2 ch7_prebuf_ neg_en ch7_prebuf_ pos_en ch6_prebuf_ neg_en ch6_prebuf_ pos_en ch5_prebuf_ neg_en ch5_prebuf_ pos_en ch4_prebuf_ neg_en ch4_prebuf_ pos_en 0xff rw 0x13 positive reference precharge buffer ch7_refp_ buf ch6_refp_ buf ch5_refp_ buf ch4_refp_buf ch3_refp_buf ch2_re fp_buf ch1_refp _buf ch0_refp_ buf 0x00 rw 0x14 negative reference precharge buffer ch7_refn_ buf ch6_refn_ buf ch5_refn_ buf ch4_refn_buf ch3_refn_buf ch2_refn_buf ch1_refn_buf ch0_refn_ buf 0x00 rw 0x1e channel 0 offset ch0_offset_msb 0x00 rw 0x1f ch0_offset_mid 0x20 ch0_offset_lsb 0x21 channel 1 offset ch1_offset_msb 0x00 rw 0x22 ch1_offset_mid 0x23 ch1_offset_lsb 0x24 channel 2 offset ch2_offset_msb 0x00 rw 0x25 ch2_offset_mid 0x26 ch2_offset_lsb 0x27 channel 3 offset ch3_offset_msb 0x00 rw 0x28 ch3_offset_mid 0x29 ch3_offset_lsb 0x2a channel 4 offset ch4_offset_msb 0x00 rw 0x2b ch4_offset_mid 0x2c ch4_offset_lsb 0x2d channel 5 offset ch5_offset_msb 0x00 rw 0x2e ch5_offset_mid 0x2f ch5_offset_lsb 0x30 channel 6 offset ch6_offset_msb 0x00 rw 0x31 ch6_offset_mid 0x32 ch6_offset_lsb 0x33 channel 7 offset ch7_offset_msb 0x00 rw 0x34 ch7_offset_mid 0x35 ch7_offset_lsb 0x36 channel 0 gain ch0_gain_msb 0xxx rw 0x37 ch0_gain_mid 0x38 ch0_gain_lsb
ad7761 data sheet rev. 0 | page 56 of 69 reg. name bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0 reset rw 0x39 channel 1 gain ch1_gain_msb 0xxx rw 0x3a ch1_gain_mid 0x3b ch1_gain_lsb 0x3c channel 2 gain ch2_gain_msb 0xxx rw 0x3d ch2_gain_mid 0x3e ch2_gain_lsb 0x3f channel 3 gain ch3_gain_msb 0xxx rw 0x40 ch3_gain_mid 0x41 ch3_gain_lsb 0x42 channel 4 gain ch4_gain_msb 0xxx rw 0x43 ch4_gain_mid 0x44 ch4_gain_lsb 0x45 channel 5 gain ch5_gain_msb 0xxx rw 0x46 ch5_gain_mid 0x47 ch5_gain_lsb 0x48 channel 6 gain ch6_gain_msb 0xxx rw 0x49 ch6_gain_mid 0x4a ch6_gain_lsb 0x4b channel 7 gain ch 7_gain_msb 0xxx rw 0x4c ch7_gain_mid 0x4d ch7_gain_lsb 0x4e channel 0 sync offset ch0_sync_offset 0x00 rw 0x4f channel 1 sync offset ch1_sync_offset 0x00 rw 0x50 channel 2 sync offset ch2_sync_offset 0x00 rw 0x51 channel 3 sync offset ch3_sync_offset 0x00 rw 0x52 channel 4 sync offset ch4_sync_offset 0x00 rw 0x53 channel 5 sync offset ch5_sync_offset 0x00 rw 0x54 channel 6 sync offset ch6_sync_offset 0x00 rw 0x55 channel 7 sync offset ch7_sync_offset 0x00 rw 0x56 diagnostic rx ch7_rx ch6_ rx ch5_rx ch4_rx ch3_rx ch 2_rx ch1_rx ch0_rx 0x00 rw 0x57 diagnostic mux control unused grpb_sel unused grpa_sel 0x00 rw 0x58 modulator delay control unused clk_mod_del_en reserved 0x02 rw 0x59 chop control unused gr pa_chop grpb_chop 0x0a rw
data sheet ad7761 rev. 0 | page 57 of 69 channel standby register address: 0x00, reset: 0x00, name: channel standby each of the adc channels can be put into standby mode independently by setting the appropriate bit in the channel standby regis ter. when a channel is in standby mode, its position in the data output stream is held. the 8-bit header is all zeros, as is the con version result output of 16 zeros. the vcm voltage output is associated with the channel 0 circuitr y. if channel 0 is put into standby mode, the vcm voltage outpu t is also disabled for maximum power savings. channel 0 must be enabled while vcm is being used externally to the ad7761. the crystal excitation circuitry is associated with the channel 4 circuitry. if channel 4 is put into standby mode, the crystal circuitry is also disabled for maximum power savings. channel 4 must be enabled while the external crystal is used on the ad7761. table 34. bit descriptions for channel standby bits bit name settings description reset access 7 ch_7 channel 7 0x0 rw 0 enabled 1 standby 6 ch_6 channel 6 0x0 rw 0 enabled 1 standby 5 ch_5 channel 5 0x0 rw 0 enabled 1 standby 4 ch_4 channel 4 0x0 rw 0 enabled 1 standby 3 ch_3 channel 3 0x0 rw 0 enabled 1 standby 2 ch_2 channel 2 0x0 rw 0 enabled 1 standby 1 ch_1 channel 1 0x0 rw 0 enabled 1 standby 0 ch_0 channel 0 0x0 rw 0 enabled 1 standby channel mode a register address: 0x01, reset: 0x0d, name: channel mode a two mode options are available on the ad7761 adcs. the channel modes are defined by the contents of the channel mode a and channel mode b registers. each mode is then mapped as desired to the required adc channel. channel mode a and channel mode b allow different filter types and decimation rates to be selected and mapped to any of the adc channels. when different decimation rates are selected, the ad7761 outputs a data ready signal at the fastest selected decimation rate. any channel that runs at a lower output data rate is updated only at that slower rate. in between valid result data, the data for that chan nel is set to zero and the repeated data bit is set in the header status bits to distinguish it from a real conversion result (see the adc convers ion output: header and data section).
ad7761 data sheet rev. 0 | page 58 of 69 table 35. bit descriptions for channel mode a bits bit name settings description reset access 3 filter_type_a filter selection 0x1 rw 0 wideband filter 1 sinc5 filter [2:0] dec_rate_a decimation rate selection 0x5 rw 000 32 001 64 010 128 011 256 100 512 101 1024 110 1024 111 1024 channel mode b register address: 0x02, reset: 0x0d, name: channel mode b table 36. bit descriptions for channel mode b bits bit name settings description reset access 3 filter_type_b filter selection 0x1 rw 0 wideband filter 1 sinc5 filter [2:0] dec_rate_b decimation rate selection 0x5 rw 000 32 001 64 010 128 011 256 100 512 101 1024 110 1024 111 1024 channel mode select register address: 0x03, reset: 0x00, name: channel mode select this register selects the mapping of each adc channel to either channel mode a or channel mode b. table 37. bit descriptions for channel mode select bits bit name settings description reset access 7 ch_7_mode channel 7 0x0 rw 0 mode a 1 mode b 6 ch_6_mode channel 6 0x0 rw 0 mode a 1 mode b 5 ch_5_mode channel 5 0x0 rw 0 mode a 1 mode b 4 ch_4_mode channel 4 0x0 rw 0 mode a 1 mode b 3 ch_3_mode channel 3 0x0 rw 0 mode a 1 mode b
data sheet ad7761 rev. 0 | page 59 of 69 bits bit name settings description reset access 2 ch_2_mode channel 2 0x0 rw 0 mode a 1 mode b 1 ch_1_mode channel 1 0x0 rw 0 mode a 1 mode b 0 ch_0_mode channel 0 0x0 rw 0 mode a 1 mode b power mode select register address: 0x04, reset: 0x00, name: power_mode table 38. bit descriptions for power_mode bits bit name settings description reset access 7 sleep_mode in sleep mode, many of the digital cloc ks are disabled and all of the adcs are disabled. the analog ldos are not disabled. 0x0 rw the ad7761 spi is live and is available to the user. writing to this bit brings the ad7761 out of sleep mode again. 0 normal operation. 1 sleep mode. [5:4] power_mode power mode. the power mode bits control the power mode setting for the bias currents used on all adcs on the ad7761 . the user can select the current consumption target to meet the application. the power modes of fast, median, and focus give optimum performance when mapped to the correct mclk division setting. these power mode bits do not control the mclk division of the adcs. see the mclk_div bits for control of the division of the mclk input. 0x0 rw 00 focus. 10 median. 11 fast. 3 lvds_enable lvds clock. 0x0 rw 0 lvds input clock disabled. 1 lvds input clock enabled. [1:0] mclk_div mclk division. the mclk division bits control the divided ratio between the mclk applied at the input to the ad7761 and the clock used by each of the adc modulators. the appropriate division ratio depends on the following factors: power mode, decimation rate, and the base mclk available in the system. see the clocking, sampling tree, and power scaling section for more information on setting mclk_div correctly. 0x0 rw 00 mclk/32: with a base mclk of 32.768 mhz, set to mclk/32 for focus mode. 10 mclk/8: with a base mclk of 32.768 mhz, set to mclk/8 for median mode. 11 mclk/4: with a base mclk of 32.768 mhz, set to mclk/4 for fast mode. general device configuration register address: 0x05, reset: 0x08, name: general configuration table 39. bit descriptions for general configuration bits bit name settings description reset access 5 retime_en sync_out signal retime enable bit. 0x0 rw 0 disabled: normal timing of sync_out . 1 enabled: sync_out signal derived from alternate mclk edge.
ad7761 data sheet rev. 0 | page 60 of 69 bits bit name settings description reset access 4 vcm_pd vcm buffer power-down. 0x0 rw 0 enabled: vcm buffer normal mode. 1 powered down: vcm buffer powered down. [1:0] vcm_vsel vcm voltage. these bits select the output voltage of the vcm pin. this voltage is derived from the avdd1 supply and can be output as half of that avdd1 voltage, or other fixed voltages, with respect to avss. the vcm voltage output is associated with th e channel 0 circuitry. if channel 0 is put into standby mode, the vcm voltage output is also disabled for maximum power savings. channel 0 must be enabled while vcm is being used externally to the ad7761 . 0x0 rw 00 (avdd1 ? avss)/2 v. 01 1.65 v. 10 2.5 v. 11 2.14 v. data control: soft reset, sync, and single-shot control register address: 0x06, reset: 0x80, name: data control table 40. bit descriptions for data control bits bit name settings description reset access 7 spi_sync software synchronization of the ad7761 . this command has the same effect as sending a signal pulse to the start pin. to operate the spi_sync, the user must write to this bit two separate times. first, write a zero, putting spi_sync low, and then write a 1 to set spi_sync logic high again. the spi_sync command is recogn ized after the last rising edge of sclk in the spi instruction where the spi_sync bit is changed from low to high. the spi_sync command is then output synchronous to the ad7761 mclk on the sync_out pin. the user must connect the sync_out signal to the sync_in pin on the pcb. the sync_out pin can also be routed to the sync_in pins of other ad7761 devices, allowing larger channel count simultaneous sampling systems. as per any synchronization pulse seen by the sync_in pin, the digital filters of the ad7761 are reset. the full settling time of the filters must elapse before data is output on the data interface. in a daisy-chained system of ad7761 devices, two successive synchronization pulses must be applied to guarantee that all adcs are synchronized. two synchronization pulses are also required in a system of more than one ad7761 device sharing a single mclk signal, where the drdy pin of only one device is used to detect new data. 0x1 rw 0 change to spi_sync low. 1 change to spi_sync high. 4 single_shot_en one-shot mode. enables one-shot mode. in one-shot mode, the ad7761 outputs a conversion result in response to a sync_in rising edge. 0x0 rw 0 disabled. 1 enabled. [1:0] spi_reset soft reset. these bits allow a full device reset over the spi port. two successive commands must be received in the correct order to generate a reset: first, write 0x03 to the soft reset bits, and then write 0x02 to the soft reset bits. this sequence causes the digital core to reset and all registers return to their default values. following a soft reset, if the spi master sends a command to the ad7761 , the device responds on the next frame to that command with an output of 0x0e00. 0x0 rw 00 no effect. 01 no effect. 10 second reset command. 11 first reset command.
data sheet ad7761 rev. 0 | page 61 of 69 interface configuration register address: 0x07, reset: 0x0, name: interface configuration table 41. bit descriptions for interface configuration bits bit name settings description reset access [3:2] crc_select crc select. these bits allow the user to implement a crc on the data interface. when selected, the crc replaces the header every fourth or 16 th output sample depending on the crc option chosen. there are two options for the crc; both use the same polynomial: x 8 + x 2 + x + 1. the options offer the user the ability to reduce the duty cycle of the crc calculation by performing it less often: in the case of having it every 16 th sample, or more often in the case of every fourth conversion. the crc is calculated on a per channel basis and it includes conversion data only. 0x0 rw 00 no crc. status bits with every conversion. 01 replace the header with crc message every 4 samples. 10 replace the header with crc message every 16 samples. 11 replace the header with crc message every 16 samples. [1:0] dclk_div dclk divider. these bits control division of the dclk clock used to clock out conversion data on the doutx pins. the dclk signal is derived from the mclk signal applied to the ad7761 . the dclk divide mode allows the user to optimize the dclk output to fit the application. optimizing the dclk signal per application depends on the requirements of the user. when the ad7761 uses the highest capacity output on the fewest doutx pins, for example, running in decimate by 32 using the dout0 and dout1 pins, the dclk signal must equal the mclk sign al; thus, in this case, choosing the no division setting is the only way th e user can output all the data within the conversion period. there are other cases, however, when the adc may be running in fast mode with high decimation rates, or in median or focus mode where the dclk signal does not need to run at the same speed as mclk. in these cases, the dclk divide allows the user to reduce the clock speed and makes routing and isolating such signals easier. 0x0 rw 00 divide by 8. 01 divide by 4. 10 divide by 2. 11 no division. digital filter ram built in self test (bist) register address: 0x08, reset: 0x0, name: bist control table 42. bit descriptions for bist control bits bit name settings description reset access 0 ram_bist_start ram bist. filter ram bist is a built in self test of the internal ram. normal adc conversion is disrupted when this test is run. a synchronization pulse is required after this test is comp lete to resume normal adc operation. the test can be run at intervals depending on user preference. the status and result of the ram bist is available in the device status register; see the ram_bist_pass and ram_bist_running bits in table 43. 0x0 rw 0 off. 1 begin ram bist.
ad7761 data sheet rev. 0 | page 62 of 69 status register address: 0x09, reset: 0x0, name: device status table 43. bit descriptions for device status bits bit name settings description reset access 3 chip_error chip error. chip error is a global error flag that is output within the status byte of each adc conversion output. the following bits lead to the chip error bit being set to logic high: crc check on internally hard coded settings (after power-up) does not pass; xor check on the internal memory does not pass (this check runs continuously in the background); and clock error is detected on power-up. 0x0 r 0 no error present. 1 error has occurred. 2 no_clock_error external clock check. this bit indicates whether the externally applied mclk is detected correctly. if the mclk is not applied correctly to the adc at power-up, this bit is set and the dclk frequency is approximately 16 mhz. if this bit is set, the chip error bit is set to logic high in the status bits of the data output head ers, and the conversion results are output as all zeros regardless of th e analog input voltages applied to the adc channels. 0x0 r 0 mclk detected. 1 no mclk detected. 1 ram_bist_pass bist pass/fail. ram bist result status. this bit indicates the result of the most recent ram bist. the result is latched to this register and is only cleared by a device reset. 0x0 r 0 bist failed or not run. 1 bist passed. 0 ram_bist_running bist status. reading back the value of this bit allows the user to poll when the bist test has finished. 0x0 r 0 bist not running. 1 bist running. revision identification register address: 0x0a, reset: 0x06, name: revision id table 44. bit descriptions for revision id bits bit name description reset access [7:0] revision_id asic revision. 8-bit id for revision details. 0x06 r gpio control register address: 0x0e, reset: 0x00, name: gpio control table 45. bit descriptions for gpio control bits bit name setting description reset access 7 ugpio_enable user gpio enable. the gpiox pins are dual purpose and can be operated only when the device is in spi control mode. by default, when the ad7761 is powered up in spi control mode, the gpiox pins are disabled. this bit is a universal enable/ disable for all gpiox input/outputs. the direction of each general-purpose pin is determined by bits[4:0] of this register. 0x0 rw 0 gpio disabled. 1 gpio enabled. 4 gpioe4_filter gpio4 direction. this bit assigns the direct ion of gpio4 as either an input or an output. for spi control, gpio4 maps to pin 11, which is the filter/gpio4 pin. 0x0 rw 0 input. 1 output.
data sheet ad7761 rev. 0 | page 63 of 69 bits bit name setting description reset access 3 gpioe3_mode3 gpio3 direction. this bit assigns the direct ion of gpio3 as either an input or an output. for spi control, gpio3 maps to pin 15, which is the mode3/gpio3 pin. 0x0 rw 0 input. 1 output. 2 gpioe2_mode2 gpio2 direction. this bit assigns the direct ion of gpio2 as either an input or an output. for spi control, gpio2 maps to pin 14, which is the mode2/gpio2 pin. 0x0 rw 0 input. 1 output. 1 gpioe1_mode1 gpio1 direction. this bit assigns the direct ion of gpio1 as either an input or an output. for spi control, gpio1 maps to pin 13, which is the mode1/gpio1 pin. 0x0 rw 0 input. 1 output. 0 gpio0_mode0 gpio0 direction. this bit assigns the direct ion of gpio0 as either an input or an output. for spi control, gpio0 maps to pin 12, which is the mode0/gpio0 pin. 0x0 rw 0 input. 1 output. gpio write data register address: 0x0f, reset: 0x00, name: gpio write data this register writes the values to be set on each of the general-purpose pins when selected as general-purpose outputs. each bi t, from bits[4:0], maps directly to the gpiox pins. table 46. bit descriptions for gpio write data bits bit name description reset access 4 gpio4_write filter/gpio4 0x0 rw 3 gpio3_write mode3/gpio3 0x0 rw 2 gpio2_write mode2/gpio2 0x0 rw 1 gpio1_write mode1/gpio1 0x0 rw 0 gpio0_write mode0/gpio0 0x0 rw gpio read data register address: 0x10, reset: 0x00, name: gpio read data this register reads back the value of the logic input level at the general-purpose pins when selected to operate as general-pur pose inputs. each bit, from bits[4:0], maps directly to the gpio0 to gpio4 pins. table 47. bit descriptions for gpio read data bits bit name description reset access 4 gpio4_read filter/gpio4 0x0 r 3 gpio3_read mode3/gpio3 0x0 r 2 gpio2_read mode2/gpio2 0x0 r 1 gpio1_read mode1/gpio1 0x0 r 0 gpio0_read mode0/gpio0 0x00 r
ad7761 data sheet rev. 0 | page 64 of 69 analog input precharge buffer enable register channel 0 to channel 3 address: 0x11, reset: 0xff, name: precharge buffer 1 this register turns on or off the precharge buffers on the analog inputs. when writing to these registers, the user must write the inverse of the required bit settings. for example, to clear bit 7 of this register, the user must write 0x01 to the register. this clears bit 7 and sets all other bits. if the user reads the register again after writing 0x01, the data read is 0xfe, as required. table 48. bit descriptions for precharge buffer 1 bits bit name settings description reset 7 ch3_prebuf_neg_en 0 off 0x1 1 on 6 ch3_prebuf_pos_en 0 off 0x1 1 on 5 ch2_prebuf_neg_en 0 off 0x1 1 on 4 ch2_prebuf_pos_en 0 off 0x1 1 on 3 ch1_prebuf_neg_en 0 off 0x1 1 on 2 ch1_prebuf_pos_en 0 off 0x1 1 on 1 ch0_prebuf_neg_en 0 off 0x1 1 on 0 ch0_prebuf_pos_en 0 off 0x1 1 on analog input precharge buffer enable register channel 4 to channel 7 address: 0x12, reset: 0xff, name: precharge buffer 2 this register turns on or off the precharge buffers on the analog inputs. when writing to these registers, the user must write the inverse of the required bit settings. for example, to clear bit 7 of this register, the user must write 0x01 to the register. this clears bit 7 and sets all other bits. if the user reads the register again after writing 0x01, the data read is 0xfe, as required. table 49. bit descriptions for precharge buffer 2 bits bit name settings description reset 7 ch7_prebuf_neg_en 0 off 0x1 1 on 6 ch7_prebuf_pos_en 0 off 0x1 1 on 5 ch6_prebuf_neg_en 0 off 0x1 1 on 4 ch6_prebuf_pos_en 0 off 0x1 1 on 3 ch5_prebuf_neg_en 0 off 0x1 1 on 2 ch5_prebuf_pos_en 0 off 0x1 1 on 1 ch4_prebuf_neg_en 0 off 0x1 1 on 0 ch4_prebuf_pos_en 0 off 0x1 1 on
data sheet ad7761 rev. 0 | page 65 of 69 positive reference precharg e buffer enable register address: 0x13, reset: 0x00, name: positive reference precharge buffer this register turns on or off the precharge buffers on the reference positive input to each of the adcs from channel 0 to chann el 7. table 50. bit descriptions for posi tive reference precharge buffer bits bit name settings description reset 7 ch7_refp_buf 0 off 0x0 1 on 6 ch6_refp_buf 0 off 0x0 1 on 5 ch5_refp_buf 0 off 0x0 1 on 4 ch4_refp_buf 0 off 0x0 1 on 3 ch3_refp_buf 0 off 0x0 1 on 2 ch2_refp_buf 0 off 0x0 1 on 1 ch1_refp_buf 0 off 0x0 1 on 0 ch0_refp_buf 0 off 0x0 1 on negative reference precharge buffer enable register address: 0x14, reset: 0x00, name: negative reference precharge buffer this register turns on or off the precharge buffers on the reference negative input to each of the adcs from channel 0 to chann el 7. table 51. bit descriptions for nega tive reference precharge buffer bits bit name settings description reset 7 ch7_refn_buf 0 off 0x0 1 on 6 ch6_refn_buf 0 off 0x0 1 on 5 ch5_refn_buf 0 off 0x0 1 on 4 ch4_refn_buf 0 off 0x0 1 on 3 ch3_refn_buf 0 off 0x0 1 on 2 ch2_refn_buf 0 off 0x0 1 on 1 ch1_refn_buf 0 off 0x0 1 on 0 ch0_refn_buf 0 off 0x0 1 on offset registers the chx_offset_msb, chx_offset_mid, and chx_offset_lsb registers are 24 bit, signed twos complement registers for channel offset adjustment. if the channel gain setting is at its ideal nominal value of 0x555555, an lsb of offset register adjustment changes the digital output by ?1/192 lsbs. for example, changing the offset register from 0 to 4800 changes the digital output by ?25 lsbs. because offset adjustment occurs before gain adjustment, the ratio of 1/192 changes linearly with gain adjustment via the chx_gain_x registers . after a reset or power cycle, the register values revert to the default factory setting.
ad7761 data sheet rev. 0 | page 66 of 69 table 52. per channel 24-bit offset registers, three 8-bit regi sters for each channel, split up as msb, mid, and lsb address name description reset access msb mid lsb msb mid lsb 0x1e 0x1f 0x20 channel 0 offset channel 0 offset registers: upper, middle, and lower bytes (24 bits in total) 0x00 0x00 0x00 r w 0x21 0x22 0x23 channel 1 offset channel 1 offset registers: upper, middle, and lower bytes (24 bits in total) 0x00 0x00 0x00 rw 0x24 0x25 0x26 channel 2 offset channel 2 offset registers: upper, middle, and lower bytes (24 bits in total) 0x00 0x00 0x00 rw 0x27 0x28 0x29 channel 3 offset channel 3 offset registers: upper, middle, and lower bytes (24 bits in total) 0x00 0x00 0x00 rw 0x2a 0x2b 0x2c channel 4 offset channel 4 offset registers: upper, middle, and lower bytes (24 bits in total) 0x00 0x00 0x00 rw 0x2d 0x2e 0x2f channel 5 offset channel 5 offset registers: upper, middle, and lower bytes (24 bits in total) 0x00 0x00 0x00 r w 0x30 0x31 0x32 channel 6 offset channel 6 offset registers: upper, middle, and lower bytes (24 bits in total) 0x00 0x00 0x00 r w 0x33 0x34 0x35 channel 7 offset channel 7 offset registers: upper, middle, and lower bytes (24 bits in total) 0x00 0x00 0x00 r w gain registers each adc channel has an associated gain coefficient. the coeffici ent is stored in three single-byte registers split up as msb, mid, and lsb. each of the gain registers are factory programmed. nominall y, this gain is around the value 0x555555 (for an adc channel). the user can overwrite the gain register setting; however, after a reset or power cycle, the gain register values revert to the har d coded programmed factory setting. table 53. per channel 24-bit gain registers, three 8-bit registers for each channel, split up as msb, mid, and lsb address name description reset access msb mid lsb msb mid lsb 0x36 0x37 0x38 channel 0 gain channel 0 gain registers: upper, middle, and lower bytes (24 bits in total) 0xxx 0xxx 0xxx rw 0x39 0x3a 0x3b channel 1 gain channel 1 gain registers: upper, middle, and lower bytes (24 bits in total) 0xxx 0xxx 0xxx rw 0x3c 0x3d 0x3e channel 2 gain channel 2 gain registers: upper, middle, and lower bytes (24 bits in total) 0xxx 0xxx 0xxx rw 0x3f 0x40 0x41 channel 3 gain channel 3 gain registers: upper, middle, and lower bytes (24 bits in total) 0xxx 0xxx 0xxx rw 0x42 0x43 0x44 channel 4 gain channel 4 gain registers: upper, middle, and lower bytes (24 bits in total) 0xxx 0xxx 0xxx rw 0x45 0x46 0x47 channel 5 gain channel 5 gain registers: upper, middle, and lower bytes (24 bits in total) 0xxx 0xxx 0xxx rw 0x48 0x49 0x4a channel 6 gain channel 6 gain registers: upper, middle, and lower bytes (24 bits in total) 0xxx 0xxx 0xxx rw 0x4b 0x4c 0x4d channel 7 gain channel 7 gain registers: upper, middle, and lower bytes (24 bits in total) 0xxx 0xxx 0xxx rw sync phase offset registers the ad7761 has one synchronization signal for all channels. the sync phase offset register allows the user to vary the phase delay on eac h of the channels relative to the synchronization edge received on the sync_in pin. see the sync phase offset adjustment section for details on the use of this function. table 54 . per channel 8-bit sync phase offset registers address name description reset access 0x4e channel 0 sync offset channel 0 sync phase offset register 0x00 rw 0x4f channel 1 sync offset channel 1 sync phase offset register 0x00 rw 0x50 channel 2 sync offset channel 2 sync phase offset register 0x00 rw 0x51 channel 3 sync offset channel 3 sync phase offset register 0x00 rw 0x52 channel 4 sync offset channel 4 sync phase offset register 0x00 rw 0x53 channel 5 sync offset channel 5 sync phase offset register 0x00 rw 0x54 channel 6 sync offset channel 6 sync phase offset register 0x00 rw 0x55 channel 7 sync offset channel 7 sync phase offset register 0x00 rw adc diagnostic receive select register address: 0x56, reset: 0x00, name: diagnostic rx the ad7761 adc diagnostic allows the user to select a zero-scale, positive full-scale, or negative full-scale input to the adc, which can be converted to verify the correct operation of the adc channel. this register enables the diagnostic. enable the receive (rx) for each channel and set each bit in this register to 1. the adc diagnostic feature depends on some features of the analog input precharge buffers. the user must ensure that the analog input precharge buffers are enabled on the channels that are selected to receive the diagnostic voltages internally.
data sheet ad7761 rev. 0 | page 67 of 69 table 55. bit descriptions for diagnostic rx bits bit name settings description reset access 7 ch7_rx channel 7 0x0 rw 0 not in use 1 receive 6 ch6_rx channel 6 0x0 rw 0 not in use 1 receive 5 ch5_rx channel 5 0x0 rw 0 not in use 1 receive 4 ch4_rx channel 4 0x0 rw 0 not in use 1 receive 3 ch3_rx channel 3 0x0 rw 0 not in use 1 receive 2 ch2_rx channel 2 0x0 rw 0 not in use 1 receive 1 ch1_rx channel 1 0x0 rw 0 not in use 1 receive 0 ch0_rx channel 0 0x0 rw 0 not in use 1 receive adc diagnostic control register address: 0x57, reset: 0x00, name: diagnostic mux control the ad7761 adc diagnostic allows the user to select a zero-scale, positive full-scale, or negative full-scale input to the adc, which can be converted to verify the correct operation of the adc channel. this register controls the voltage that is applied to each of the adc channels for the diagnostic. there are three input voltage options that the user can select. the voltage selected is mapped to the channels based on which mode (channel mode a or channel mode b) they belong to, which is set according to the channel mode select register (re gister 0x03). set bits[7:0] to 1 in the adc diagnostic receive select register, then select the voltage check desired for the channels on cha nnel mode a and the channels on channel mode b through bits[2:0] and bits[6:4], respectively. table 56. bit descriptions for diagnostic mux control bits bit name settings description reset access [6:4] grpb_sel mux b. 0x0 rw 000 off. 011 positive full-scale adc check. a voltage close to positive full scale is applied internally to the adc channel. 100 negative full-scale adc check. a voltage close to negative (or minus) full scale is applied internally to the adc channel. 101 zero-scale adc check. a voltage close to 0 v is applied internally to the adc channel. [2:0] grpa_sel mux a. 0x0 rw 000 off. 011 positive full-scale adc check. a voltage close to positive full scale is applied internally to the adc channel. 100 negative full-scale adc check. a voltage close to negative (or minus) full scale is applied internally to the adc channel. 101 zero-scale adc check. a voltage close to 0 v is applied internally to the adc channel.
ad7761 data sheet rev. 0 | page 68 of 69 modulator delay control register address: 0x58, reset: 0x02, name: modulator delay control table 57. bit descriptions for modulator delay control bits bit name settings description reset access [3:2] clk_mod_del_en enable delayed modulator clock. 0x0 rw 00 disabled delayed clock for all channels. 01 enable delayed clock for channel 0 to channel 3 only on the ad7761 . 10 enable delayed clock for channel 4 to channel 7 only on the ad7761 . 11 enable delayed clock for all channels. [1:0] reserved 10 not a user option. must be set to 0x2. 0x2 rw chopping control register address: 0x59, reset: 0x0a, name: chop control table 58. bit descriptions for chop control bits bit name settings description reset access [3:2] grpa_chop group a chopping 0x2 rw 01 chop at f mod /8 10 chop at f mod /32 [1:0] grpb_chop group b chopping 0x2 rw 01 chop at f mod /8 10 chop at f mod /32
data sheet ad7761 rev. 0 | page 69 of 69 outline dimensions compliant to jedec standards ms-026-bcd 051706-a top view (pins down) 1 16 17 33 32 48 49 64 0.27 0.22 0.17 0.50 bsc lead pitch 12.20 12.00 sq 11.80 pin 1 1.60 max 0.75 0.60 0.45 10.20 10.00 sq 9.80 view a 0.20 0.09 1.45 1.40 1.35 0.08 coplanarity view a rotated 90 ccw seating plane 0 . 1 5 0 . 0 5 7 3.5 0 figure 80. 64-lead low profile quad flat package [lqfp] (st-64-2) dimensions shown in millimeters ordering guide model 1 temperature range package description package option AD7761BSTZ ?40c to +105c 64-lead low prof ile quad flat package [lqfp] st-64-2 AD7761BSTZ-rl7 ?40c to +105c 64-lead low profile quad flat package [lqfp] st-64-2 eval-ad7761fmcz evaluation board eval-sdp-ch1z controller board 1 z = rohs compliant part. ?2016 analog devices, inc. all rights reserved. trademarks and registered trademarks are the prop erty of their respective owners. d14285-0-4/16(0)

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