View AD9251-20EBZ_8916637.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

14- bit , 20 msps /40 msps /65 msps /80 msps , 1.8 v dual analog - to - digital converter data sheet ad9251 rev. b document feedback 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 o r 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 t he property of their respective owners. one technology way, p.o. box 9106, norwood, ma 02062 - 9106, u.s.a. tel: 781.329.4700 ? 2009 C 2016 analog devices, inc. all rights reserved. technical support www.analog.com features 1.8 v analog supply operation 1.8 v to 3.3 v output supply snr 74.3 dbf s at 9.7 mhz input 71.5 dbf s at 200 mhz input sfdr 93 dbc at 9.7 mhz input 80 dbc at 200 mhz input low power 33 mw per ch annel at 20 msps 73 mw per ch annel at 80 msps different ial input with 700 mhz bandwidth on - chip voltage reference and sample - and - hold circuit 2 v p - p differential analog input dnl = 0. 4 5 lsb serial port control options offset binary, gray code, or twos complement data format optional clock duty cycle stabiliz er integer 1 - to - 8 input clock d ivider data output multiplex option built - in selectable digital test pattern generation energy - saving power - down modes data cl ock o ut w ith p rogrammable clock and data alignment applications communications diversity r adio sy stems multimode digital receivers gsm, edge, w - c d m a, lt e, cdma2000, wimax, td - scdma i/q demodulation systems smart antenna systems battery - powered instruments hand held scope meters portable medical i maging ultrasound radar/lidar functional block dia gram vin+ a vin? a vref sense vcm rbias vin?b vin+b or a d0 a d13 a dco a d r vdd orb d13b d0b dcob sdio gnd a vdd sclk spi programming d at a mux option pdwn dfs clk+ clk? mode controls dcs dut y cycle s t abilizer sync divide 1 t o 8 oeb csb ref select adc cmos output buffer adc cmos output buffer ad9251 07938-001 figure 1. product highlights 1. the ad9251 operates from a single 1.8 v analog power supply and features a separate digital output driver supply to a c commodate 1.8 v to 3.3 v logic families. 2. the patented sample - and - hold circuit ma intains excellent perfor m ance for input frequencies up to 200 mhz and is designed for low cost, low power , and ease of use. 3. a standard serial port interface supports various product features and functions, such as data output formatting, internal clock di vider , power - down, dco/data timing and offset adjustments, and voltage refe r ence mode s. 4. the ad9251 is packaged in a 64 - lead rohs compliant lfcsp that is pin compatible with the ad9268 16- bit adc, the ad9258 14- bit adc, the ad9231 12- bit adc, and the ad9204 10- bit adc , e nabl ing a simple migra tion path between 10 - bit and 16 - bi t converters sampling from 20 msps to 125 msp s .
ad9251* product page quick links last content update: 02/23/2017 comparable parts view a parametric search of comparable parts. evaluation kits ? ad9251 evaluation board documentation application notes ? an-1142: techniques for high speed adc pcb layout ? an-742: frequency domain response of switched- capacitor adcs ? an-807: multicarrier wcdma feasibility ? an-808: multicarrier cdma2000 feasibility ? an-812: microcontroller-based serial port interface (spi) boot circuit ? an-827: a resonant approach to interfacing amplifiers to switched-capacitor adcs ? an-878: high speed adc spi control software ? an-905: visual analog converter evaluation tool version 1.0 user manual ? an-935: designing an adc transformer-coupled front end data sheet ? ad9251: 14-bit, 20 msps/40 msps/65 msps/80 msps, 1.8 v dual analog-to-digital converter data sheet user guides ? ug-003: evaluating the ad9650/ad9268/ad9258/ ad9251/ad9231/ad9204 analog-to-digital converters tools and simulations ? visual analog ? ad9251 ibis models ? ad9204/ad9231/ad9251 s-parameter data reference materials product selection guide ? rf source booklet technical articles ? designing 1-ppm dac accuracy into instrumentation applications - part 1 ? designing 1-ppm dac accuracy into instrumentation applications - part 2 ? improve the design of your passive wideband adc front-end network ? ms-2210: designing power supplies for high speed adc design resources ? ad9251 material declaration ? pcn-pdn information ? quality and reliability ? symbols and footprints discussions view all ad9251 engineerzone discussions. sample and buy visit the product page to see pricing options. technical support submit a technical question or find your regional support number. document feedback submit feedback for this data sheet. this page is dynamically generated by analog devices, inc., and inserted into this data sheet. a dynamic change to the content on this page will not trigger a change to either the revision number or the content of the product data sheet. this dynamic page may be frequently modified.
ad9251 data sheet rev. b | page 2 of 36 table of contents features .............................................................................................. 1 applications ....................................................................................... 1 functional block diagram .............................................................. 1 product highlights ........................................................................... 1 revision history ............................................................................... 2 general description ......................................................................... 3 specifications ..................................................................................... 4 dc specifications ......................................................................... 4 ac specifications .......................................................................... 5 digital specifications ................................................................... 6 switching specifications .............................................................. 7 timing specifications .................................................................. 8 absolute maximum ratings .......................................................... 10 thermal characteristics ............................................................ 10 esd caution ................................................................................ 10 pin configuration and function descriptions ........................... 11 typical performance characteristics ........................................... 13 ad9251 - 80 .................................................................................. 13 ad9251 - 65 .................................................................................. 15 ad9251 - 40 .................................................................................. 16 ad9251 - 20 .................................................................................. 17 equivalent circuits ......................................................................... 18 theory of operation ...................................................................... 20 adc architecture ...................................................................... 20 analog input considerations .................................................... 20 voltage reference ....................................................................... 23 clock input considerations ...................................................... 24 channel/chip synchronization ................................................ 26 powe r dissipation and standby mode .................................... 26 digital outputs ........................................................................... 27 timing .......................................................................................... 27 b uilt - in self - te st (bist ) and o utput te st .................................. 28 built - in self - te st (bist ) ............................................................ 28 output test modes ..................................................................... 28 serial port interface (spi) .............................................................. 29 configuration using the spi ..................................................... 29 hardware interface ..................................................................... 30 configuration without the spi ................................................ 30 spi accessible features .............................................................. 30 memory map .................................................................................. 31 reading the memory map register table ............................... 31 open locations .......................................................................... 31 default v alues ............................................................................. 31 memory map register table ..................................................... 32 memory map register descriptions ........................................ 34 applications information .............................................................. 35 design guidelines ...................................................................... 35 outline dimensions ....................................................................... 36 ordering guide .......................................................................... 36 revision history 9 /2016 rev. a to rev. b changes to figure 3 .......................................................................... 8 10/ 20 09 rev. 0 to rev. a changes to features .......................................................................... 1 change to table 1 ............................................................................. 4 moved timing diagrams ................................................................. 8 deleted table 11; renumbered se quentially .............................. 22 changes to internal reference connection section .................. 23 moved channel/chip synchronization section ......................... 26 change to table 15 ......................................................................... 30 changes to reading the memory map register table section ................................................................................... 31 changes to table 16 ....................................................................... 32 7 / 20 09 revision 0: initial version
data sheet ad9251 rev. b | page 3 of 36 general description the ad9251 is a monolithic, dual - channel , 1.8 v supply, 14- bit, 20 msps /40 msps /65 msps /80 msps analog - to - digital converter (adc) . it featur es a high performance sample - and - hold circuit and on - chip voltage reference. the product use s multistage differe n tial pipeline architecture with output error correction logic to provide 14 - bit accuracy at 80 msps data rates and to guarantee no missing codes over the full opera t ing temperature range. the adc contains several features designed to m aximize flexibility and minimize system cost, such as programmable clock and data alignment and programmable digital test pattern generation. the available digital test patterns include built - in deterministic and pseudorandom patterns, along with custom us er - defined test patterns entered via the serial port interface (spi). a differe n tial clock input controls all internal conversion cycles. an optional duty cycle stabilizer (dcs) compensates for wide variations in the clock duty cycle while maintaining exce llent overall adc pe r formance. the digital output data is presented in offset binary, gray code, or twos complement format . a data output clock (dco) is pr o vided for each adc channel to ensure proper latch timing with receiving logic. both 1.8 v and 3.3 v cmos levels are supported and output data can be multiplexed onto a single output bus. the a d9251 is available in a 64 - lead rohs compliant lfcsp and is spec i fied over the industrial temperature range (?40c to +85c).
ad9251 data sheet rev. b | page 4 of 36 specifications dc specificatio ns avdd = 1.8 v; drvdd = 1.8 v, maximum sample rate, 2 v p - p differential input, 1.0 v internal reference; ain = ?1.0 dbfs, dcs disabled, unless ot h erwise noted. table 1 . parameter temp ad9251 - 20/ ad9251 - 40 ad9251 - 65 ad9251 - 80 uni t min typ max min typ max min typ max resolution full 14 14 14 bits accuracy no missing codes full gua r anteed guara n teed guara n teed offset error full 0.1 0.70 0.1 0.50 0. 1 0.70 % fsr gain error 1 full ?1.5 ?1.5 ?1.5 % fsr differential nonlinearity (dnl) 2 full 0.60 0.75 0.70 lsb 25c 0.3 0.45 0.45 lsb integral nonli n earity (inl) 2 full 1.75 1.75 2.50 lsb 25c 0.6 0.6 1.0 lsb matching charact eristics offset error 25c 0.0 0.65 0.0 0.65 0.0 0.65 % fsr gain error 1 25c 0.2 0.2 0.2 % fsr temperature drift offset error full 2 2 2 ppm/c internal vol t age refere nce output voltage (1 v mode) full 0.981 0.993 1.005 0.981 0.993 1.005 0.981 0.993 1.005 v load regul a tion error at 1.0 ma full 2 2 2 mv input - referred noise vref = 1.0 v 25c 0.98 0.98 0.98 lsb rms analog input input span, vref = 1.0 v full 2 2 2 v p -p input capac i tance 3 ful l 6 6 6 pf input common - mode voltage full 0.9 0.9 0.9 v input common - mode range full 0.5 1.3 0.5 1.3 0.5 1.3 v reference input resistance full 7.5 7.5 7.5 k? power supplies supply voltage avdd full 1.7 1.8 1.9 1.7 1.8 1.9 1.7 1.8 1.9 v drvdd full 1.7 3.6 1.7 3.6 1.7 3.6 v supply current iavdd 2 full 36.5/49.5 39.4/52.8 69.0 72.9 80.5 85.5 ma idrvdd 2 (1.8 v) full 3.4/5.6 8. 4 10. 3 ma idrvdd 2 (3.3 v) full 6.3/10.6 16.0 19.5 ma power co n sumption dc input full 66/89 125 145 mw si ne wave i n put 2 (drvdd = 1.8 v) full 71.8/99 77.0/105.5 139.0 146.5 163.4 173 mw sine wave i n put 2 (drvdd = 3.3 v) full 86.5/124 176.7 209 mw standby power 4 full 37 37 37 mw power - down power full 2.2 2.2 2.2 mw 1 measured with 1.0 v external reference. 2 m easured wi th a 10 mhz input frequency at rated sample rate, full - scale sine wave, with approximately 5 pf loading on each output bit. 3 input capacitance refers to the effective capacitance between one differential input pin and a gnd. 4 standby power is measured with a dc input and the clk active.
data sheet ad9251 rev. b | page 5 of 36 ac specifications avdd = 1.8 v; drvdd = 1.8 v, maximum sample rate, 2 v p - p differential input, 1.0 v internal reference; ain = ?1.0 dbfs, dcs disabled, unless ot h erwise noted. table 2 . parameter 1 temp ad9251 - 20/ ad9251 - 40 ad9251 - 65 ad9251 - 80 unit min typ max min typ max min typ max signal - to - noise ratio (snr) f in = 9.7 mhz 25c 74.7 74.5 74.3 dbf s f in = 30.5 mhz 25c 74.4 74.3 74.1 dbfs full 73.6 7 3.6 dbf s f in = 70 mhz 25c 73.7 73.7 73.6 dbf s full 72.5 dbfs f in = 200 mhz 25c 71.5 71.5 71.5 dbfs signal - to - noise - and - disto r tion (sinad) f in = 9.7 mhz 25c 74.6 74.4 74.1 dbfs f in = 30.5 mhz 25c 74.3 74.2 74.0 dbfs full 73.4 73.4 dbfs f in = 70 mhz 25c 73.6 73.6 73.5 dbfs full 72.4 dbfs f in = 200 mhz 25c 70.0 70. 0 70. 0 dbfs effective number of bits (enob) f in = 9.7 mhz 25c 12.0 12.0 12.0 bits f in = 30.5 mhz 25c 12.0 12.0 12.0 bits f in = 70 mhz 25c 11.9 11.9 11.9 bits f in = 200 mhz 25c 11.3 11.3 11.3 bits worst second or third ha r monic f in = 9.7 mhz 25c ? 95 ? 9 5 ? 93 dbc f in = 30.5 mhz 25c ? 95 ? 95 ? 93 dbc full ? 81 ? 81 dbc f in = 70 mhz 25c ? 94 ? 94 ? 92 dbc full ? 81 dbc f in = 200 mhz 25c ? 80 ? 80 ? 80 dbc spurious - free dynamic range (sfdr) f in = 9.7 mh z 25c 95 95 93 dbc f in = 30.5 mhz 25c 94 94 93 dbc full 81 81 dbc f in = 70 mhz 25c 93 93 92 dbc full 81 dbc f in = 200 mhz 25c 80 80 80 dbc worst other (harmonic or spur) f in = 9.7 mhz 25c ? 98 ? 98 ? 97 dbc f in = 30.5 mhz 25c ? 98 ? 98 ? 97 dbc full ? 90 ? 90 dbc f in = 70 mhz 25c ? 98 ? 98 ? 96 dbc full ? 89 dbc f in = 200 mhz 25c ? 95 ? 95 ? 95 dbc two - tone sfdr f in = 30. 5 mhz (?7 dbfs), 32.5 mhz ( ?7 dbfs) 25c 90 90 90 dbc crosstalk 2 full ?110 ?110 ?110 dbc analog input bandwidth 25c 700 700 700 mhz 1 see the an - 835 application note, understanding high speed adc testing and evaluation , for a complete set of definitions. 2 crosstalk is measured at 100 mhz with ? 1.0 dbfs on one channel and no input on the alternate channel.
ad9251 data s heet rev. b | page 6 of 36 digital specificatio ns avdd = 1.8 v; drvdd = 1.8 v, maximum sample rate, 2 v p - p differential input, 1.0 v internal reference; ain = ?1.0 dbfs, dcs disabled, unless ot h erwise noted. table 3 . parameter temp ad9251 - 20/ ad9251 - 40/ ad9251 - 65/ ad9251 - 80 unit min typ max differential clock inputs (clk+, clk ? ) logic compliance cmos/lvds/lvpecl int ernal co m mon - mode bias full 0.9 v differential i n put voltage full 0.2 3.6 v p - p input voltage range full gnd ? 0.3 avdd + 0.2 v high level i n put current full ?10 +10 a low level input current full ?10 +10 a input resistance full 8 10 12 k ? input capac i tance full 4 pf logic i n puts (sclk/dfs, sync, p d w n) 1 high level i n put voltage full 1.2 drvdd + 0.3 v low level input voltage full 0 0.8 v high level i n put current full ?50 ? 75 a low level input current full ?10 +10 a input r esistance full 30 k? input capac i tance full 2 pf logic i n puts ( csb ) 2 high level i n put voltage full 1.2 drvdd + 0.3 v low level input voltage full 0 0.8 v high level i n put current full ?10 +10 a low level input current full 40 135 a in put resistance full 26 k? input capac i tance full 2 pf logic i n puts (sdio/dcs) 2 high level i n put voltage full 1.2 drvdd + 0.3 v low level input voltage full 0 0.8 v high level i n put current full ?10 +10 a low level input current full 40 130 a input resistance full 26 k? input capac i tance full 5 pf digital outputs drvdd = 3.3 v high level output voltage, i oh = 50 a full 3.29 v high level output voltage , i oh = 0.5 ma full 3.25 v lo w level output voltage, i ol = 1.6 ma full 0.2 v low level output voltage , i ol = 50 a full 0.05 v drvdd = 1.8 v high level output voltage , i oh = 50 a full 1.79 v high level output voltage , i oh = 0.5 ma full 1.75 v low level output vo lt age, i ol = 1.6 ma full 0.2 v low level output voltage , i ol = 50 a full 0.05 v 1 internal 30 k ? pull - down. 2 internal 30 k ? pull - up.
data sheet ad9251 rev. a | page 7 of 36 switching specificat ions avdd = 1.8 v; drvdd = 1.8 v, maximum sample r ate, 2 v p - p differential input, 1.0 v internal reference; ain = ?1.0 dbfs, dcs disabled, unless ot h erwise noted. table 4 . parameter temp ad9251 - 20/ad9251 - 40 ad9251 - 65 ad9251 - 80 unit min typ max min typ max min typ max clock input param e ters input clock rate full 625 625 625 mhz conversion rate 1 full 3 20/40 3 65 3 80 msps clk period divide - by - 1 mode (t clk ) full 50/25 15.38 12.5 ns clk pulse width high (t ch ) 25.0/12.5 7.69 6.25 ns apertur e delay (t a ) full 1.0 1.0 1.0 ns aperture uncertainty (jitter, t j ) full 0.1 0.1 0.1 ps rms data output param e ters data propagation delay (t pd ) full 3 3 3 ns dco propagation delay (t dco ) full 3 3 3 ns dco to data ske w (t skew ) full 0.1 0.1 0.1 ns pipeline delay (latency) full 9 9 9 cycles wake - up time 2 full 350 350 350 s standby full 600/400 300 260 ns out - of - range recovery time full 2 2 2 cycles 1 conversion rate is the clock rate after the clk d ivider. 2 wake - up time is dependent on the value of the decoupling capacitors.
ad9251 data s heet rev. a | page 8 of 36 timing specification s table 5 . parameter conditions min typ max unit sync timing requirements t ssync sync to rising edge of clk setup time 0.24 ns t hsync sync to rising edge of clk hold time 0.40 ns spi timing requirements t ds setup time between the data and the rising edge of sclk 2 ns t dh hold time between the data and the rising edge of sclk 2 ns t clk period of the sclk 40 ns t s setup time between csb and sclk 2 ns t h hold time between csb and sclk 2 ns t high sclk pulse width hig h 10 ns t low sclk pulse width low 10 ns t en_sdio time required for the sdio pin to switch from an input to an output relative to the sclk falling edge 10 ns t dis_sdio time required for the sdio pin to switch from an output to an input relative to the sclk rising edge 10 ns timing diagrams t pd t skew t ch t dco t clk n ? 9 n ? 1 n + 1 n + 2 n + 3 n + 5 n + 4 n n ? 8 n ? 7 n ? 6 n ? 5 vin clk+ clk? ch a/ch b d at a dcoa/dcob t a 07938-002 figure 2 . cmos output data timing t pd t skew t ch t dco t clk ch a n ? 9 ch b n ? 9 ch a n ? 8 ch b n ? 8 ch a n ? 7 ch b n ? 7 ch a n ? 6 ch b n ? 6 ch a n ? 5 n ? 1 n + 1 n + 2 n + 3 n + 5 n + 4 n vin clk+ clk? ch a/ch b data as appears on ch a output pins dcoa/dcob t a 07938-003 figure 3 . cmos interleaved output timing , output as appears on channel a output pins
data sheet ad9251 rev. a | page 9 of 36 sync clk+ t hsync t ssync 07938-004 figure 4 . sync input timing requirements
ad9251 data s heet rev. a | page 10 of 36 absolute maximum rat ings table 6 . parameter rating avdd to agnd ? 0.3 v to +2.0 v drvdd to agnd ? 0.3 v to +3.9 v vin+a , vin+b, vin?a , vin?b to agnd ? 0.3 v to avdd + 0.2 v clk+, clk? to agnd ? 0.3 v to avdd + 0.2 v sync to agnd ? 0.3 v to drvdd + 0.3 v vref to agnd ? 0.3 v to avdd + 0.2 v sense to agnd ? 0.3 v to avdd + 0.2 v vcm to agnd ? 0.3 v to avdd + 0.2 v rbias to agnd ? 0.3 v to avdd + 0.2 v csb to agnd ? 0.3 v to drvdd + 0.3 v sclk/dfs to agnd ? 0.3 v to drvdd + 0.3 v sdio/dcs to agnd ? 0.3 v to drvdd + 0.3 v oeb to agnd ? 0.3 v to drvdd + 0.3 v pdwn to agnd ? 0 .3 v to drvdd + 0.3 v d0a/d0b th rough d13a/d13b to agnd ? 0.3 v to drvdd + 0.3 v dcoa/dcob to agnd ? 0.3 v to drvdd + 0.3 v operating temperature range (ambient) ? 40c to +85c maximum junction temperature under bias 150c storage temperature range (am bient) ? 65c to +150c 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 characterist ics the exposed paddle is the only ground connection for the chip. the exposed paddle must be soldered to the agnd plane of the us e rs circuit board. soldering the exposed paddle to the us e rs board also increases the reliability of the solder joints and maximizes the thermal capability of the package. typical ja is specified for a 4 - layer pcb with a solid ground plane. as shown in table 7 , airflow improves heat dissipation, which reduces ja . in addition, metal in direct contact with the package leads from metal traces, thro ugh holes, ground, and power planes, r e duces the ja . table 7 . thermal resistance package type airflow velocity (m/s ec ) ja 1, 2 jc 1, 3 jb 1, 4 unit 64 - lead lfcsp 0 23 2.0 c/w (cp - 64 - 4) 1.0 20 12 c/w 2.5 18 c/w 1 per jed ec 51 - 7, plus jedec 25 - 5 2s2p test board. 2 per jedec jesd51 - 2 (still air) or jedec jesd51 - 6 (moving air). 3 per mil - std 883, method 1012.1. 4 per jedec jesd51 - 8 (still air). esd caution
data sheet ad9251 rev. a | page 11 of 36 pin configuration and function descrip tions pin 1 indic a t or 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 d10b d1 1b dr vdd d12b (msb) d13b orb dcob dco a nc nc (lsb) d0 a dr vdd d1 a d2 a d3 a d4 a 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 a vdd a vdd vin+b vin?b a vdd a vdd rbias vcm sense vref a vdd a vdd vin? a vin+ a a vdd a vdd 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 clk+ clk? sync nc nc (lsb) d0b d1b d2b d3b dr vdd d4b d5b d6b d7b d8b d9b pdwn oeb csb sclk/dfs sdio/dcs or a d13 a (msb) d12 a d1 1a d10 a d9 a dr vdd d8 a d7 a d6 a d5 a 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 ad9251 t op view (not to scale) 07938-005 notes 1. nc = no connect 2. the exposed paddle must be soldered to the pcb ground to ensure proper heat dissipation, noise, and mechanical strength benefits. figure 5 . pin configuration table 8 . pin function description pin no. mnemonic description 0 gnd exposed paddle is the only ground connection for the chip. must be connected to pcb agnd. 1, 2 clk+, clk? differential encode clock. pecl, lvds , or 1.8 v cmos inputs. 3 sync digital input . sync input to clock divider. 3 0 k ? internal pull - down. 4, 5, 25, 26 nc do not connect . 6 to 9, 11 to 18, 20, 21 d0b to d13b channel b digital outputs . d13b = msb. 10, 19, 28, 37 drvdd digital output driver supply (1.8 v to 3.3 v). 22 orb channel b out - of - range digital output . 23 dcob channel b data clock digital output. 24 dcoa channel a data clock digital output. 27, 29 to 36, 38 to 42 d0a to d13a channel a digital outputs . d13a = msb . 43 ora channel a out - of - range digital output . 44 sdio/dcs spi data in put /out put (sdio) . bidirectional spi data i/o in spi mode . 3 0 k ? i nternal pull - down in spi mode. duty cycle stabilizer (dcs) . static enable input for duty cycle stabilizer in non - spi mode. 3 0 k? internal pull - up in non- spi (dcs) mode. 45 sclk/dfs spi clock (sclk) input in spi mode . 30 k ? internal pull - down. data f ormat select (dfs) . s tatic control of data output format in no n - spi mode. 30 k ? internal pull - down. dfs high = twos complement output . dfs low = offset binary output . 46 csb spi chip select. act ive low enable; 3 0 k ? internal pull - up. 47 oeb digital in put. enable channel a and channel b digital outputs if low, tristate outputs if high. 3 0 k ? internal pull - down. 48 pdwn digital input . 3 0 k ? internal pull - down . pdwn high = power - down device . pdwn low = run device , normal operation . 49, 50, 53, 54, 5 9, 60, 63, 64 avdd 1.8 v analog supply pins . 51, 52 vin + a , vin?a channel a analog inputs.
ad9251 data sheet rev. a | page 12 of 36 pin no. mnemonic description 55 vref voltage reference input/output. 56 sense reference mode selection. 57 vcm analog output voltage at midsupply to set common mode of the analog inputs. 58 rbias sets analog current bias. connect to 10 k ? (1% tolerance) resistor to ground. 61 , 62 vin?b , vin+b channel b analog inputs.
data sheet ad9251 rev. a | page 13 of 36 typical performance characteristics ad9251 - 80 avdd = 1.8 v; drvdd = 1.8 v, maximum sample rate, 2 v p - p differential inpu t, 1.0 v internal reference; ain = ?1.0 dbfs, dcs disabled, unless ot h erwise noted. 0 ?15 ?30 ?45 ?60 ?75 ?90 ?105 ?120 4 8 12 16 20 24 28 32 36 frequency (mhz) amplitude (dbfs) 07938-033 4 3 5 6 2 80msps 9.7mhz @ ?1dbfs snr = 73.4db (74.4dbfs) sfdr = 94.4dbc figure 6 . single- tone fft with f in = 9.7 mhz 0 ?15 ?30 ?45 ?60 ?75 ?90 ?105 ?120 4 4 2 3 5 6 8 12 16 20 24 28 32 36 frequency (mhz) amplitude (dbfs) 07938-062 80msps 70.3mhz @ ?1dbfs snr = 72.1db (73.1dbfs) sfdr = 93.5dbc figure 7 . single- tone fft with f in = 70.3 mhz 0 ?15 ?30 ?45 ?60 ?75 ?90 ?105 ?120 4 8 12 16 20 24 28 32 36 frequency (mhz) amplitude (dbfs) 07938-200 80msps 30.5mhz @ ?7dbfs 32.5mhz @ ?7dbfs sfdr = 89.5dbc (96.5dbfs) f 2 ? f 1 f 1 + f 2 2 f 1 ? f 2 2 f 2 + f 1 2 f 2 ? f 1 2 f 1 + f 2 figure 8 . two - tone fft with f in1 = 30.5 mhz and f in2 = 32.5 mhz 0 ?15 ?30 ?45 ?60 ?75 ?90 ?105 ?120 4 8 12 16 20 24 28 32 36 frequency (mhz) amplitude (dbfs) 07938-034 4 3 5 6 2 80msps 30.5mhz @ ?1dbfs snr = 73.2db (74.2dbfs) sfdr = 93.6dbc figure 9 . single- tone fft with f in = 30.5 mhz 0 ?15 ?30 ?45 ?60 ?75 ?90 ?105 ?120 4 4 2 3 5 6 8 12 16 20 24 28 32 36 frequency (mhz) amplitude (dbfs) 07938-036 80msps 200mhz @ ?1dbfs snr = 70.5db (71.5dbfs) sfdr = 80.2dbc figure 10 . single - tone fft with f in = 200 mhz 0 ?20 ?40 ?60 ?80 ?100 ?120 ?90 ?78 ?66 ?54 ?42 ?30 ?18 ?6 input amplitude (dbfs) sfdr/imd3 (dbc/dbfs) 07938-054 sfdr (dbc) sfdr (dbfs) imd3 (dbc) imd3 (dbfs) figure 11 . two - tone sfdr/imd3 vs. input amplitude (ain) with f in1 = 30.5 mhz and f in2 = 32.5 mhz
ad9251 data sheet rev. a | page 14 of 36 avdd = 1.8 v; drvdd = 1.8 v, maximum sample rate, 2 v p - p differential input, 1.0 v internal reference; ai n = ?1.0 dbfs, dcs disabled, unless ot h erwise noted. 100 90 80 70 60 50 40 30 20 10 0 0 50 100 150 200 input frequency (mhz) snr/sfdr (dbfs/dbc) 07938-057 sfdr (dbc) snr (dbfs) figure 12 . snr/sfdr vs. input frequency (ain) with 2 v p - p full scale 120 100 80 60 40 20 0 10 20 30 40 50 60 70 80 sample rate (msps) snr/sfdr (dbfs/dbc) 07938-055 sfdr (dbc) snr (dbfs) figure 13 . snr/sfdr vs. sample rate with ain = 9.7 mhz 0.5 0.4 0.3 0.2 0.1 0 ?0.1 ?0.2 ?0.3 ?0.4 ?0.5 0 2048 4096 6144 8192 10,240 12,288 14,336 16,384 output code dnl error (lsb) 07938-038 figure 14 . dnl error with f in = 9.7 mhz 120 snr snrfs sfdr sfdrfs 100 80 60 40 20 0 ?90 ?80 ?60 ?40 ?20 0 input amplitude (dbfs) snr/sfdr (dbfs) 07938-061 figure 15 . snr/sfdr vs. input amplitude (ain) with f in = 9.7 mhz 450,000 400,000 350,000 300,000 250,000 200,000 150,000 100,000 50,000 0 n ? 4 n ? 3 n ? 2 n ? 1 n n + 1 n + 2 n + 3 n + 4 output code number of hits 07938-048 figure 16 . grounded input histogram 0.5 2.0 1.5 1.0 0 ?0.5 ?1.0 ?1.5 ?2.0 0 2048 4096 6144 8192 10,240 12,288 14,336 16,384 output code inl error (lsb) 07938-037 figure 17 . inl with f in = 9.7 mhz
data sheet ad9251 rev. a | page 15 of 36 ad9251 - 65 avdd = 1.8 v; drvdd = 1.8 v, maximum sample rate, 2 v p - p differential input, 1.0 v internal reference; ain = ?1.0 dbfs, dcs disabled, unless ot h erwise noted. 0 ?15 ?30 ?45 ?60 ?75 ?90 ?105 ?120 3 6 9 12 15 18 21 24 27 30 frequency (mhz) amplitude (dbfs) 07938-030 4 3 5 6 2 65msps 9.7mhz @ ?1dbfs snr = 73.5db (74.5dbfs) sfdr = 97.7dbc figure 18 . single - tone fft with f in = 9.7 mhz 0 ?15 ?30 ?45 ?60 ?75 ?90 ?105 ?120 3 6 9 12 15 18 21 24 27 30 frequency (mhz) amplitude (dbfs) 07938-032 4 3 5 6 2 65msps 70.3mhz @ ?1dbfs snr = 72.6db (73.6dbfs) sfdr = 94.1dbc figure 19 . single - tone fft with f in = 70.3 mhz 0 ?15 ?30 ?45 ?60 ?75 ?90 ?105 ?120 3 6 9 12 15 18 21 24 27 30 frequency (mhz) amplitude (dbfs) 07938-031 65msps 30.5mhz @ ?1dbfs snr = 73.3db (74.3dbfs) sfdr = 99.3dbc 4 3 5 6 2 figure 20 . single - tone fft with f in = 30.5 mhz 120 snr snrfs sfdr sfdrfs 100 80 60 40 20 0 ?90 ?80 ?60 ?40 ?20 0 input amplitude (dbfs) snr/sfdr (dbfs) 07938-060 figure 21 . snr/sfdr vs. input amplitude (ain) with f in = 9.7 mhz 100 90 80 70 60 50 40 30 20 10 0 0 50 100 150 200 input frequency (mhz) snr/sfdr (dbfs/dbc) 07938-056 sfdr (dbc) snr (dbfs) figure 22 . snr/sfdr vs. input frequenc y (ain) wi th 2 v p - p full scale
ad9251 data sheet rev. a | page 16 of 36 ad9251 - 40 avdd = 1.8 v; drvdd = 1.8 v, maximum sample rate, 2 v p - p differential input, 1.0 v internal reference; ain = ?1.0 dbfs, dcs disabled, unless ot h erwise noted. 0 ?15 ?30 ?45 ?60 ?75 ?90 ?105 ?120 2 4 6 8 10 12 14 16 18 frequency (mhz) amplitude (db) 07938-028 4 3 5 6 2 40msps 9.7mhz @ ?1dbfs snr = 73.5db (74.5dbfs) sfdr = 95.4dbc figure 23 . single - tone fft with f in = 9.7 mhz 0 ?15 ?30 ?45 ?60 ?75 ?90 ?105 ?120 2 4 6 8 10 12 14 16 18 frequency (mhz) amplitude (dbfs) 07938-029 4 3 5 6 2 40msps 30.5mhz @ ?1dbfs snr = 73.2db (74.2dbfs) sfdr = 95.7dbc figure 24 . single - tone fft with f in = 30.5 mhz 120 snr snrfs sfdr sfdrfs 100 80 60 40 20 0 ?90 ?80 ?60 ?40 ?20 0 input amplitude (dbfs) snr/sfdr (dbfs) 07938-059 figure 25 . snr/sfdr vs. input amplitude (ain) with f in = 9.7 mhz
data sheet ad9251 rev. a | page 17 of 36 ad9251 - 20 avdd = 1.8 v; drvdd = 1.8 v, maximum sample rate, 2 v p - p differential input, 1.0 v internal reference; ain = ?1.0 dbfs, dcs disabled, unless ot h erwise noted. 0 ?15 ?30 ?45 ?60 ?75 ?90 ?105 ?120 1.90 950k 2.85 3.80 4.75 5.70 6.65 7.60 8.55 9.50 frequency (mhz) amplitude (dbfs) 07938-024 4 3 5 6 2 20msps 9.7mhz @ ?1dbfs snr = 73.5dbfs (74.5dbfs) sfdr = 97.2dbc figure 26 . single - tone fft with f in = 9.7 mhz 0 ?15 ?30 ?45 ?60 ?75 ?90 ?105 ?120 1.90 950k 2.85 3.80 4.75 5.70 6.65 7.60 8.55 9.50 frequency (mhz) amplitude (dbfs) 07938-026 4 3 5 6 2 20msps 30.5mhz @ ?1dbfs snr = 73.2db (74.2dbfs) sfdr = 98.1dbc figure 27 . single - tone fft with f in = 30.5 mhz 120 100 80 60 40 20 0 ?100 ?90 ?80 ?70 ?60 ?50 ?40 ?30 ?20 ?10 0 input amplitude (dbfs) snr/sfdr (dbc/dbfs) 07938-058 sfdr (dbfs) snr (dbfs) sfdr (dbc) snr (dbc) figure 28 . snr/sfdr vs. input amplitude (ain) with f in = 9.7 mhz
ad9251 data sheet rev. a | page 18 of 36 equivalent circuits 07938-039 a vdd vinx figure 29 . equivalent analog input circuit 07938-040 clk+ clk? 0.9v 15k? 5? 5? 15k? 07938-041 30k? 30k? sdio/dcs 350? a vdd dr vdd figure 31 . equivalent sdio/dcs input circuit 07938-042 d r vdd figure 32 . equivalent digit al output circuit 07938-043 350? d r vdd 30k? sclk/dfs, sync, oeb, and pdwn figure 33 . equivalent sclk/dfs, sync, oeb, and pdwn input circuit 07938-044 rbias and vcm 375? a vdd figure 34 . equivalent rbias, vcm circuit
data sheet ad9251 rev. a | page 19 of 36 07938-045 30k? csb 350? a vdd d r vdd figure 35 . equivalent csb input circuit 07938-046 sense 375? a vdd figure 36 . equivalent sense circuit 07938-047 7.5k? vref 375? a vdd figure 37 . equivalent vref circuit
ad9251 data sheet rev. a | page 20 of 36 theory of operation the ad9251 dual adc design can be used for diversity reception of signals, where the adcs are operating identically on the same carrier but from two separate antennae. the adcs can also be operated with independent analog inputs. the user can sample any f s /2 frequency segment from dc to 200 mhz, using appropriate low-pass or band-pass filtering at the adc inputs with little loss in adc performance. operation to 300 mhz analog input is permitted but occurs at the expense of increased adc noise and distortion. in nondiversity applications, the ad9251 can be used as a base- band or direct downconversion receiver, where one adc is used for i input data and the other is used for q input data. synchronization capability is provided to allow synchronized timing between multiple channels or multiple devices. programming and control of the ad9251 is accomplished using a 3-bit spi-compatible serial interface. adc architecture the ad9251 architecture consists of a multistage, pipelined adc. each stage provides sufficient overlap to correct for flash errors in the preceding stage. the quantized outputs from each stage are combined into a final 14-bit result in the digital correction logic. the pipelined architecture permits the first stage to operate with a new input sample while the remaining stages operate with preceding samples. sampling occurs on the rising edge of the clock. each stage of the pipeline, excluding the last, consists of a low resolution flash adc connected to a switched-capacitor dac and an interstage residue amplifier (for example, a multiplying digital-to-analog converter (mdac)). the residue amplifier magnifies the difference between the reconstructed dac output and the flash input for the next stage in the pipeline. one bit of redundancy is used in each stage to facilitate digital correction of flash errors. the last stage simply consists of a flash adc. the output staging block aligns the data, corrects errors, and passes the data to the cmos output buffers. the output buffers are powered from a separate (drvdd) supply, allowing adjustment of the output voltage swing. during power-down, the output buffers go into a high impedance state. analog input considerations the analog input to the ad9251 is a differential switched- capacitor circuit designed for processing differential input signals. this circuit can support a wide common-mode range while maintaining excellent performance. by using an input common-mode voltage of midsupply, users can minimize signal-dependent errors and achieve optimum performance. ss h c par c sample c sample c par vin?x h ss h v in+x h 07938-006 figure 38. switched-cap acitor input circuit the clock signal alternately switches the input circuit between sample-and-hold mode (see figure 38). when the input circuit is switched to sample mode, the signal source must be capable of charging the sample capacitors and settling within one-half of a clock cycle. a small resistor in series with each input can help reduce the peak transient current injected from the output stage of the driving source. in addition, low q inductors or ferrite beads can be placed on each leg of the input to reduce high differential capacitance at the analog inputs and, therefore, achieve the maximum bandwidth of the adc. such use of low q inductors or ferrite beads is required when driving the converter front end at high if frequencies. either a shunt capacitor or two single-ended capacitors can be placed on the inputs to provide a matching passive network. this ultimately creates a low-pass filter at the input to limit unwanted broadband noise. see the an-742 application note, the an-827 application note, and the analog dialogue article transformer-coupled front-end for wideband a/d converters (volume 39, april 2005) for more information. in general, the precise values depend on the application.
data sheet ad9251 rev. a | page 21 of 36 input common mode the analog inputs of the ad9251 are not internally dc-biased. therefore, in ac-coupled applications, the user must provide a dc bias externally. setting the device so that vcm = avdd/2 is recommended for optimum performance, but the device can function over a wider range with reasonable performance, as shown in figure 39 and figure 40. an on-board, common-mode voltage reference is included in the design and is available from the vcm pin. the vcm pin must be decoupled to ground by a 0.1 f capacitor, as described in the applications information section. 100 90 80 70 60 50 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 input common-mode voltage (v) snr/sfdr (dbfs/dbc) 07938-049 sfdr (dbc) snr (dbfs) figure 39. snr/sfdr vs. input common-mode voltage, f in = 32.1 mhz, f s = 80 msps 100 90 80 70 60 50 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 input common-mode voltage (v) snr/sfdr (dbfs/dbc) 07938-050 sfdr (dbc) snr (dbfs) figure 40. snr/sfdr vs. input common-mode voltage, f in = 10.3 mhz, f s = 20 msps differential input configurations optimum performance is achieved while driving the ad9251 in a differential input configuration. for baseband applications, the ad8138, ada4937-2 , and ada4938-2 differential drivers provide excellent performance and a flexible interface to the adc. the output common-mode voltage of the ada4938-2 is easily set with the vcm pin of the ad9251 (see figure 41), and the driver can be configured in a sallen-key filter topology to provide band limiting of the input signal. avdd vin 76.8 ? 120? 0.1f 33? 33? 10pf 200? 200? 90? ada4938 adc vin?x vin+x vcm 07938-007 figure 41. differential input configuration using the ada4938-2 for baseband applications below ~10 mhz where snr is a key parameter, differential transformer-coupling is the recommended input configuration. an example is shown in figure 42. to bias the analog input, the vcm voltage can be connected to the center tap of the secondary winding of the transformer. 2v p-p 49.9 ? 0.1f r r c 07938-008 adc vcm vin+x vin?x figure 42. differential transformer-coupled configuration the signal characteristics must be considered when selecting a transformer. most rf transformers saturate at frequencies below a few megahertz (mhz). excessive signal power can also cause core saturation, which leads to distortion. at input frequencies in the second nyquist zone and above, the noise performance of most amplifiers is not adequate to achieve the true snr performance of the ad9251. for applications above ~10 mhz where snr is a key parameter, differential double balun coupling is the recommended input configuration (see figure 44). an alternative to using a transformer-coupled input at frequencies in the second nyquist zone is to use the ad8352 differential driver. an example is shown in figure 45. see the ad8352 data sheet for more information. in any configuration, the value of shunt capacitor c is dependent on the input frequency and source impedance and may need to be reduced or removed. table 9 displays the suggested values to set the rc network. however, these values are dependent on the input signal and should be used only as a starting guide. table 9. example rc network frequency range (mhz) r series ( each) c differential (pf) 0 to 70 33 22 70 to 200 125 open
ad9251 data sheet rev. a | page 22 of 36 single - ended input configuration a single - ended input can provide adequate performance in cost - sensitive applications. in this configuration, sfdr and distortion performance degrade due to the large input common - mode swing. if the source i m pedances on each input are matched, there should be little effect on snr performance. figure 43 shows a typical single - ended input configuration. 1v p - p r r c 49. 9? 0.1 f 10 f 10 f 0.1 f a v d d 1 k? 1 k? 1 k? 1 k? a d c a v d d vin+x vin?x 07938-009 figure 43 . single - en ded input configuration adc r 0.1f 0.1f 2v p-p vcm c r 0.1f s 0.1f 25? 25? s p a p 07938-010 vin+x vin?x figure 44 . differential double balun input configuration ad8352 0? 0? c d r d r g 0.1f 0.1f 0.1f 0.1f 16 1 2 3 4 5 11 0.1f 0.1f 10 14 0.1f 8, 13 v cc 200? 200? analog input analog input r r c adc vcm 07938-011 vin+x vin?x figure 45 . differential input configuration using the ad8352
data sheet ad9251 rev. b | page 23 of 36 voltage reference a stable and accurate 1.0 v voltage reference is built into the ad9251. the vref can be configured using either the internal 1.0 v reference or an externally applied 1.0 v reference voltage. the various reference modes are summarized in the sections that follow. the reference decoupling section describes the best practices pcb layout of the reference. internal reference connection a comparator within the ad9251 detects the potential at the sense pin and configures the reference into two possible modes, which are summarized in table 10. if sense is grounded, the reference amplifier switch is connected to the internal resistor divider (see figure 46), setting vref to 1.0 v. vref sense 0.5v adc select logic 0.1f 1.0f vin?a/vin?b vin+a/vin+b adc core 07938-012 figure 46. internal reference configuration if the internal reference of the ad9251 is used to drive multiple converters to improve gain matching, the loading of the reference by the other converters must be considered. figure 47 shows how the internal reference voltage is affected by loading. 0 ?3.0 02.0 load current (ma) reference voltage error (%) ?0.5 ?1.0 ?1.5 ?2.0 ?2.5 0.2 0.4 0.6 0.8 1.0 1.4 1.6 1.8 1.2 07938-014 internal v ref = 0.993v figure 47. v ref accuracy vs. load current table 10. reference configuration summary selected mode sense voltage (v) resulting vref (v) resulting differential span (v p-p) fixed internal reference agnd to 0.2 1.0 internal 2.0 fixed external reference avdd 1.0 applied to external vref pin 2.0
ad9251 data sheet rev. b | page 24 of 36 external reference operation the use of an external reference may be necessary to enhance the gain accuracy of the adc or improve thermal drift charac - teristics. figure 48 shows the typical drift characteri s tics of the internal reference in 1.0 v mode. 4 3 2 1 0 ?1 ?2 ?3 ?4 ?5 ?6 ?40 ?20 0 20 40 60 80 temperature (c) v ref error (mv) 07938-052 v ref error (mv) figure 48 . typical v ref drift when the sen se pin is tied to avdd, the internal reference is disabled, allowing the use of an external reference. an internal reference buffer loads the external reference with an equivalent 7.5 k? load (see figure 37 ). the internal buffer generates the positive and negative full - scale references for the adc core. therefore, the external reference must be limited to a maximum of 1.0 v. clock input consider ations for optimum performance, clock the ad9251 sample clock inputs, clk+ and clk?, w ith a differential signal. the signal is typically ac - coupled into the clk+ and clk ? pins via a transformer or capacitors. these pins are biased internally (see figure 49 ) and require no exte r nal bias. 0.9v avdd 2pf 2pf clk? clk+ 07938-016 figure 49 . equivalent clock input circuit clock input options th e ad9251 has a very f lexible clock input structure. the c lock in put can be a cmos, lvds, lvpecl, or sine wave signal. regardless of the type of signal being used, clock source jitter is of the most concern, as described in the jitter considerations section. figure 50 and figure 51 show two preferred methods for clock - ing the ad9251 (at clock rates up to 62 5 mhz). a low jitter cloc k source is converted from a single - ended signal to a di f ferential signal using either an rf transformer or an rf balun. the rf balun configuration is recommended for clock frequencies between 125 mhz and 625 mhz, and the rf trans former is recom - mended for clock frequencies from 10 mhz to 200 mhz. the back - to - back schottky diodes across the transformer/balun secondary limit clock excursions into the ad9251 to approximately 0.8 v p - p differe n tial. this limit helps prevent the large voltage swings of the clock from feeding through to other portions of the ad9251 while preserving the fast rise and fall times of the signal that are critical to a low jitter performance. 0.1f 0.1f 0.1f 0.1f schottky diodes: hsms2822 clock input 50? 100? clk? clk+ adc mini-circuits ? adt1-1wt, 1:1 z xfmr 07938-017 figure 50 . transformer - coupled differen tial clock (up to 200 mhz) 0.1f 0.1f 1nf clock input 1nf 50? clk? clk+ schottky diodes: hsms2822 07938-018 adc figure 51 . balun - coupled differential clock (up to 625 mhz)
data sheet ad9251 rev. b | page 25 of 36 if a low jitter clock source is not available, another option is to ac couple a differential pecl signal to the sample clock input pins, as shown in figure 52. the ad9510/ ad9511 / ad9512 / ad9513/ ad9514 / ad9515 / ad9516/ad9517 clock drivers offer excellent jitter performance. 100 ? 0.1f 0.1f 0.1f 0.1f 240? 240? 50k? 50k? clk? clk+ clock input clock input 07938-019 adc ad951x pecl driver figure 52. differential pecl sample clock (up to 625 mhz) a third option is to ac couple a differential lvds signal to the sample clock input pins, as shown in figure 53. the ad9510/ ad9511/ad9512/ad9513/ad9514/ad9515/ad9516/ad9517 clock drivers offer excellent jitter performance. 100 ? 0.1f 0.1f 0.1f 0.1f 50k? 50k? clk? clk+ adc clock input clock input 07938-020 ad951x lvds driver figure 53. differential lvds sample clock (up to 625 mhz) in some applications, it may be acceptable to drive the sample clock inputs with a single-ended 1.8 v cmos signal. in such applications, drive the clk+ pin directly from a cmos gate, and bypass the clk? pin to ground with a 0.1 f capacitor (see figure 54). optional 100 ? 0.1f 0.1f 0.1f 50? 1 1 50? resistor is optional. clk? clk+ adc v cc 1k? 1k? c lock input 07938-021 ad951x cmos driver figure 54. single-ended 1.8 v cm os input clock (up to 200 mhz) input clock divider the ad9251 contains an input clock divider with the ability to divide the input clock by integer values between 1 and 8. optimum performance is obtained by enabling the internal duty cycle stabilizer (dcs) when using divide ratios other than 1, 2, or 4. the ad9251 clock divider can be synchronized using the external sync input. bit 1 and bit 2 of register 0x100 allow the clock divider to be resynchronized on every sync signal or only on the first sync signal after the register is written. a valid sync causes the clock divider to reset to its initial state. this synchronization feature allows multiple parts to have their clock dividers aligned to guarantee simultaneous input sampling. clock duty cycle typical high speed adcs use both clock edges to generate a variety of internal timing signals and, as a result, may be sensitive to clock duty cycle. commonly, a 5% tolerance is required on the clock duty cycle to maintain dynamic performance characteristics. the ad9251 contains a duty cycle stabilizer (dcs) that retimes the nonsampling (falling) edge, providing an internal clock signal with a nominal 50% duty cycle. this allows the user to provide a wide range of clock input duty cycles without affecting the performance of the ad9251. noise and distortion perform- ance are nearly flat for a wide range of duty cycles with the dcs on, as shown in figure 55. jitter in the rising edge of the input is still of concern and is not easily reduced by the internal stabilization circuit. the duty cycle control loop does not function for clock rates less than 20 mhz, nominally. the loop has a time constant associated with it that must be considered in applications in which the clock rate can change dynamically. a wait time of 1.5 s to 5 s is required after a dynamic clock frequency increase or decrease before the dcs loop is relocked to the input signal. 80 75 70 65 60 55 50 40 45 10 20 30 40 50 60 70 80 positive duty cycle (%) snr (dbfs) 07938-053 dcs on dcs off figure 55. snr vs. dcs on/off
ad9251 data sheet rev. b | page 26 of 36 jitter considerations high speed, high resolution adcs are sensitive to the quality of the clock input. the degradation in sn r from the low fre - quency snr (snr lf ) at a given input frequency ( f input ) due to jitter ( t jrms ) can be calc u lated by snr hf = ?10 log[(2 f input t jrms ) 2 + 10 ) 10 / ( lf snr ? 80 75 70 65 60 55 50 45 1 10 100 1k frequency (mhz) snr (dbfs) 07938-022 0.5ps 0.2ps 0.05ps 1.0ps 1.5ps 2.0ps 2.5ps 3.0ps figure 56 . snr vs. input frequency and jitter the clock input should be t reated as an analog signal in cases where aperture jitter may affect the dynamic range of the ad9251. to avoid modulating the clock signal with digital noise, keep p ower supplies for clock drivers separate from the adc output driver supplies . low jitter, c rystal - controlled oscillators make the best clock sources. if the clock is generated from another type of source (by gating, dividing, or another met h od), it should be retimed by the original clock at the last step. see the an - 501 application note and the an - 756 application note available on www.analog.com for more in formation. channel/chip synchro nization the ad9251 has a sync input that offers the user flexible synchronization options for synchronizing sample clocks across multiple adcs . the input clock divider can be enabled to synchronize on a single occurrence of the sync signal or on every occurrence. the sync input is in ternally synchronized to the sample clock; however, to ensure there is no timing uncertainty between multiple parts, the sync input signal should be externally synchronized to the input clock signal, meeting the setup and hold times shown in table 5 . drive the sync input using a single - ended cmos - type signal. power dissipation an d standby mode as shown in figure 57 , the analog core power dissipated by the ad9251 is prop ortional to its sample rate . t he digital power dissipation of the cmos outputs are determined primarily by the strength of the digital drivers and the load on each output bit. the maximum drvdd current (idrvdd) can be calculated a s idrvdd = v drvdd c load f clk n where n is th e number of output bits (30, in the case of the ad9251). this maximum current occurs when every output bit switches on every clock cycle, that is, a full - scale square wave at the nyquist frequency of f clk /2. in practice, the drvdd current is estab - lished by the average number of output bits switching, which is determined by the sample rate and the characte r istics of the analog input signal. reducing the capacitive load presented to the output drivers can minimize digital power co n sumption. the data in figure 57 was taken using the same operating conditions as those used for the typical performance characteristics , with a 5 pf load on each ou t put driver. 150 130 110 90 70 50 0 10 20 30 40 50 60 70 80 clock rate (msps) analog core power (mw) 07938-051 ad9251-80 ad9251-65 ad9251-40 ad9251-20 figure 57 . analog core power vs. clock rate the ad9251 is placed in power - down mode either by the spi port or by asserting the pdwn pin high . in this state, the adc typically dissipates 2.2 mw. during power - down, the output drivers are placed in a high impedance state. asse rting the pdwn pin low returns the ad9251 to its normal operating mode. note that pdwn is re f erenced to the digital output driver supply (drvdd) and should not exceed that supply voltage. low power dissipation in power - down mode is achieved by shutting dow n the reference, reference buffer, biasing networks, and clock. internal capacitors are discharged when entering power - down mode and then must be recharged when r e turning to normal operation. as a result, wake - up time is related to the time spent in power - down mode, and shorter power - down cycles result in proportio n ally shorter wake - up times. when using the spi port interface, the user can place the adc in power - down mode or standby mode. standby mode allows the user to keep the internal reference circuitry powered when faster wake - up times are required. see the memory map section for more details.
data sheet ad9251 rev. b | page 27 of 36 digital outputs the ad9251 output drivers can be configured to interface with 1.8 v to 3.3 v cmos logic families. o utput data can als o be multiplexed onto a single output bus to reduce the total number of traces required. the cmos output drivers are sized to provide suff i cient output current to drive a wide variety of logic families. however, large drive currents tend to cause current g litches on the supplies and may affect converter performance. applications requiring the adc to drive large capacitive loads or large fanouts may require external buffers or latches. the output data format can be selected to be either offset binary or tw os complement by setting the sclk/dfs pin when opera t ing in the external pin mode (see table 11). as detailed in the an - 877 application note, interfacing to high speed adcs via spi , the data format can be selected for offset binary, twos complement, or gray code when using the spi co n trol. table 11 . sclk/dfs mode selection (external pin mode) voltage at pin sclk/dfs sdio/dcs agnd offset binary (default) dcs d isabled drvdd twos complement dcs enabled (default) digital output enable function ( oeb ) the ad9251 has a flexible three - state ability for the digital output pins. the three - state mode is enabled using the oeb pin or through the spi interface. if the oeb pin is low, the output data drivers and dcos are enabled. if the oeb pin is high, the ou t put data drivers and dcos are placed in a high impedance state. this oeb function is not intended for rapid access to the data bus. note that oeb is re f erenced to the digital output driver supply (drvdd) and should not exceed that supply voltage. when using the spi interface, the data outputs and dco of each channel can be independently three - stated by using the output disable (oeb) bit ( bit 4 ) in register 0x14 . timing the ad9251 provides latched data with a pipeline d e lay of 9 clock cycles. data outputs are available one propagation d e lay (t pd ) after the rising edge of the clock signal. minimize t he length of the output data lines and loads placed on them to reduce tr ansients within the ad9251. these transients can degrade converter dynamic perfor m ance. the lowest typical conversion rate of the ad9251 is 3 msps. at clock rates below 3 msps, dynamic performance can d e grade. data clock output (dco) the ad9251 provides two data clock output (dco) signals intended for capturing the data in an external regi s ter. t he cmos data outputs are valid on the rising edge of the dco, unless the dco clock polarity has been changed via the spi. see figure 2 and figure 3 for a graphical timing d e scription. table 12 . output data format input (v) condition (v) offset binary output mode twos complement mode or vin+ ? vin? < ?vref ? 0.5 lsb 00 0000 0000 0 000 10 0000 0000 0000 1 vin+ ? vin? = ?vref 00 0000 0000 0000 10 0000 0000 0000 0 vin+ ? vin? = 0 10 0000 0000 0000 00 0000 0000 0000 0 vin+ ? vin? = +vref ? 1.0 lsb 11 1111 1111 1111 01 1111 1111 1111 0 vin+ ? vin? > +vref ? 0.5 lsb 11 1111 1111 1111 01 1111 1111 1111 1
ad9251 data sheet rev. b | page 28 of 36 built - in self - test (bist) and outp ut test the ad9251 includes a built - in test feature designed to enable verification of the integrity of each channel , as well as to facilitate board level debugging. a built - in self - test (bist ) feature that verifies the integrity of the digital datapath of the ad9251 is included . various output test options are also provided to place predictable values on the outputs of the ad9251. built - in self - test (bist) the bist is a thorou gh test of the digital portion of the selected ad9251 signal path. perform t he bist test after a reset to ensure the part is in a known state. during bist, data from an internal pseudorandom noise ( pn) source is driven through the digital datapath of both channels , starting at the adc block output. at the datapath output, crc logic calculates a signature from the data. the bist sequence runs for 512 cycles and then stops. once completed, the bist compares the signature results with a pre - determined value. i f the signatures match, the bist sets bit 0 of register 0x24 , signifying the test passed. if the bist test fail s , bit 0 of register 0x24 is cleared. the outputs are connected during this test, so the pn sequence can be observed as it runs. writing the valu e 0x05 to register 0x0e runs the bist. this enables the bi t 0 (bist enable ) of register 0x 0e and resets the pn sequence generator , bit 2 (bist init ) of register 0x0e. at the completion of the bist, bit 0 of register 0x24 is automatically cleared. the pn se quence can be continued fr om its last value by writing a 0 in bit 2 of register 0x0e. however, if the pn sequence is not reset, the signature cal culation does not equal the pre determined value at the end of the test. at that point, t he user need s to re ly o n verifying the output data . output test modes the output test options are described in table 16 at address 0x0d. when an output test mode is enabled, the anal og section of the adc is discon nected from the digital back - end block s and the test pattern is run through the output formatting block. some of the test patterns are subject to output formatting, and some are not. the pn generators from the pn sequence tests can be reset by setting bit 4 or bit 5 of register 0x0d. these tes ts can be performed with or without an analog signal (if present, the analog signal is ignored), but they do require an encode clock. for more information, see the an - 877 application note, interfacing to high s peed adcs via spi .
data sheet ad9251 rev. b | page 29 of 36 serial port interface (spi) the ad9251 serial port interface (spi) allows the user to configure the converter for specific functions or operations through a structured register space provided inside the adc. the spi gives the user added flexibility and customization, depending on the application. addresses are accessed via the serial port and can be written to or read from via the port. memory is organized into bytes that can be further divided into fields, which are documented in the memory map section. for detailed operational information, see an-877 application note, interfacing to high speed adcs via spi . configuration using the spi three pins define the spi of this adc: the sclk, the sdio, and the csb (see table 13). the sclk (a serial clock) is used to synchronize the read and write data presented from and to the adc. the sdio (serial data input/output) is a dual-purpose pin that allows data to be sent and read from the internal adc memory map registers. the csb (chip select bar) is an active- low control that enables or disables the read and write cycles. table 13. serial port interface pins pin function sclk serial clock. the serial shift clock input, which is used to synchronize serial interface reads and writes. sdio serial data input/output. a dual-purpose pin that typically serves as an input or an output, depending on the instruction being sent and the relative position in the timing frame. csb chip select bar. an active-low control that gates the read and write cycles. the falling edge of csb, in conjunction with the rising edge of sclk, determines the start of the framing. an example of the serial timing and its definitions can be found in figure 58 and table 5. other modes involving the csb are available. the csb can be held low indefinitely, which permanently enables the device; this is called streaming. the csb can stall high between bytes to allow for additional external timing. when csb is tied high, spi functions are placed in high impedance mode. this mode turns on any spi pin secondary functions. during an instruction phase, a 16-bit instruction is transmitted. data follows the instruction phase, and its length is determined by the w0 and w1 bits as shown in figure 58. all data is composed of 8-bit words. the first bit of the first byte in a multibyte serial data transfer frame indicates whether a read command or a write command is issued. this allows the serial data input/output (sdio) pin to change direction from an input to an output at the appropriate point in the serial frame. in addition to word length, the instruction phase determines whether the serial frame is a read or write operation, allowing the serial port to be used both to program the chip and to read the contents of the on-chip memory. if the instruction is a readback operation, performing a readback causes the serial data input/ output (sdio) pin to change direction from an input to an output at the appropriate point in the serial frame. data can be sent in msb-first mode or in lsb-first mode. msb first is the default on power-up and can be changed via the spi port configuration register. for more information about this and other features, see the an-877 application note, interfacing to high speed adcs via spi . don?t care don?t care don?t care don?t care sdio sclk csb t s t dh t clk t ds t h r/w w1 w0 a12 a11 a10 a9 a8 a7 d5 d4 d3 d2 d1 d0 t low t high 07938-023 figure 58. serial port interface timing diagram
ad9251 data sheet rev. b | page 30 of 36 hardware interface the pins described in table 13 constitute the physical interface between the programming d e vice of the user and the serial port of the ad9251. the sclk pin and th e csb pin function as inputs when using the spi interface. the sdio pin is bidirectional, functioning as an input during write phases and as an output during readback. the spi interface is flexible enough to be controlled by either fpgas or microcontroll ers. one method for spi configuration is described in detail in the an - 812 appli - cation note , microcontroller - based serial port interface (spi) boot circuit . the spi port should not be active during periods when the full dynamic performance of the converter is required. because the sclk signal, the csb signal, and the sdio signal are typically asynchronous to the adc clock, noise from these signals can degrade converter performance. if the on - board spi bus is used for other devices, it may be necessary to provide buffers between this bus and the ad9251 to prevent these signals from transi - tioning at the converter inputs during critical sampling periods. sdio/dcs and sclk/dfs serve a dual function when the spi interface is not being used. when the pins are strapped to avdd or ground during device power - on, they are associated with a specific function. the digital outputs section describes the strappable functions su p ported on the ad9 251. configuration withou t the spi in applications that do not interface to the spi control registers, the sdio/dcs pin, the sclk/dfs pin, the oeb pin, and the pdwn pin serve as standalone cmos - co m patible control pins. when the device is powered up, it is assumed that the user intends to use the pins as static control lines for the duty cycle stabilizer, output data format, output enable, and power - down feature control. in this mode, connect the csb chip select to avdd, which di s ables the serial port inter face. table 14 . mode selection pin external voltage configuration sdio/dcs avdd (default) duty cycle stabilizer enabled agnd duty cycle stabilizer di s abled sclk/dfs avdd twos compl e ment enabled agnd (default) offset binary en abled oeb avdd outputs in high impedance agnd (default) outputs enabled pdwn avdd chip in po wer - down or standby agnd (default) normal operation spi accessible featu res table 15 provides a brief description of the genera l features that are accessible via the spi. these features are d e scribed in detail in the an - 877 application note , interfacing to high speed adcs via spi . the ad925 1 part - specific features are described in detail in table 16. table 15 . features accessible using the spi feature description mode allows the user to set either power - down mode or standby mode clock allows the user to access the dcs via the spi offset allows the user to digitally adjust the converter offset test i/o allows the user to set test modes to have known data on output bits output mode allows the user to set up outputs output phase allo ws the user to set the output clock polarity output delay allows the user to vary the dco delay
data sheet ad9251 rev. b | page 31 of 36 memory map reading the memory m ap register table each row in the memory map register table (see table 16) has eight bit locati ons. the memory map is roughly divided into four sections: the chip co n figuration registers (address 0x00 to address 0x02); the device index and transfer registers (address 0x05 and address 0xff); the program registers, including setup, control, and test ( address 0x08 to address 0x2 e ); and the digital feature control register s (address 0x100 and address 0x101 ). table 16 d ocuments the default hexadecimal value for each hexadecimal a d dress shown. the column with the heading bit 7 ( msb ) is the start of the default hexadecimal value given. for example, address 0x05 , the channel index register, has a hexadecimal default value of 0x03 . this means that in address 0x05 bit [7: 2 ] = 0 , and the remaining bits[1 :0 ] = 1 . this setting is th e def ault channel index setting. the d efault value result s in both adc channels receiving the next write command . for more information on this function and others, see the an - 877 application note, interfacing to hig h speed adcs via spi . this document details the functions controlled by register 0x00 to register 0xff . the remaining registers, register 0x100 and register 0x101 , are documented in the memory map register descriptions section f ollowing table 16. open locations all address and bit locations that are not included in the spi m ap are not currently supported for this device. unused bits of a valid address location should be written with 0s. writing to the se locations is required only when part of an address location is open (for example, address 0x05 ). if the entire address location is open , it is omitted from the spi map (for e x ample, address 0x13 ) and should not be written. default values after the ad9 251 is reset, critical registers are loaded with default values. the default values for the regi s ters are given in the memory map register table (see table 16) . logic levels an explanation of logic level terminology follows: ? bi t is set is sy n onymous with bit is set to logic 1 or writing logic 1 for the bit. ? clear a bit is synonymous with bit is set to logic 0 or writing logic 0 for the bit. transfer register map address 0x08 to address 0x18 are shadowed. writes to the se addresses do not affect part operation until a transfer command is issued by writing 0x01 to address 0xff, setting the transfer bit. this allows these registers to be updated internally and simulta - neously when the transfer bit is set. the internal upda te takes place when the transfer bit is set, and then the bit autoclears. channel - specific registers some channel setup functions can be programmed differently for each channel. in these cases, channel address locations are internally duplicated for each c hannel. these registers and bits are designated in the memory map register table as local. these local registers and bits can be accessed by setting the appropriate channel a (bit 0) or channel b (bit 1) bits in register 0x05. if both bits are set, the sub sequent write affects the registers of both channels. in a read cycle, set only channel a or channel b to read one of the two registers. if both bits are set during an spi read cycle, the part returns the value for channel a. registers and bits designated as global in the memory map register table affect the entire part or the channel features for which independent settings are not allowed between channels. the settings in register 0x05 do not affect the global registers and bits.
ad9251 data sheet rev. b | page 32 of 36 memory map register tabl e all address and bit locations that are not included in table 16 are not currently supported for this device. table 16. addr ess (hex) register name bit 7 (msb) bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0 (lsb) default value (hex) comments chip configuration registers 0x00 spi port configuration ( global) 0 lsb first soft reset 1 1 soft reset lsb first 0 0x18 the nibbles are mirrored so that lsb - or msb - first mode reg ister s corr ectly, regardless of shift mode 0x01 chip id ( global ) 8 - bit chip id bits [ 7:0 ] ad9251 = 0x23 unique chip id used to diffe - rentiate devices; read only 0x02 chip grade ( global) open speed grade id 6:4 20 msps = 000 40 msps = 001 65 msps = 010 80 msps = 011 open unique speed grade id used to differentiate devices; read only device index and transfer registers 0x05 channel index open open open open open open adc b default adc a default 0x03 bits are set to determine which device on chip receives the next write command ; t he default is all devices on chip 0xff transfer open open open open open open open transfer 0x00 synchronously transfers data from the master shift register to the slave program registers (m ay or may not be indexed by device index ) 0x08 modes external power - down enable (local) external pin function 0x00 full power - down 0x01 standby (local) open open 0 0 = chip run 0 1 = full power - down 10 = sta ndby 11 = chip wide digital reset (local) 0x80 determines various generic modes of chip operation 0x09 clock ( global) open open open open open d uty cycl e stabilize 0x0 0 0x0b clock divide (global) open clock divider [ 2:0 ] clock divide ratio 000 = divide by 1 001 = divide by 2 010 = divide by 3 011 = divide by 4 100 = divide by 5 101 = divide by 6 110 = divide by 7 111 = divide by 8 0x00 the d ivide ratio is the value plus 1
data sheet ad9251 rev. b | page 33 of 36 addr ess (hex) register name bit 7 (msb) bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0 (lsb) default value (hex) comments 0x0d test mode ( local) user test mode (local) 00 = single 01 = alternate 10 = single once 11 = alternate once reset pn long gen reset pn short gen output test mode [ 3:0 ] (local) 0000 = off (default) 0001 = midscale short 0010 = positive fs 0011 = negative fs 0100 = alter nating checkerboard 0101 = pn 23 sequence 0110 = pn 9 sequence 0111 = one/zero word toggle 1000 = user input 1001 = 1 - /0 - bit toggle 1010 = 1x s ync 1011 = one bit high 1100 = mixed bit frequency 0x00 when set, the test data is placed on the output pins in place of normal data 0x0e bist enable open open bist init open bist e nable 0x00 when bit 0 is set, the bist function is initiated 0x10 offset adjust (local) 8 - bit device offset adjustment [ 7:0 ] (local) offset adjust in lsbs from +127 to ?128 (twos com plement format) 0x00 device offset trim 0x14 output mode 00 = 3.3 v cmos 10 = 1.8 v cmos output mux enable ( interleaved ) output d isable (local) open output invert (local) 00 = offset binary 01 = twos complement 10 = gray code 11 = offset binary (local) 0x00 configures the out puts and the format of the data 0x15 output_adjust 3.3 v dco drive strength 00 = 1 stripe (default) 01 = 2 stripes 10 = 3 stripes 11 = 4 stripes 1.8 v dco drive strength 00 = 1 stripe 01 = 2 s tripes 10 = 3 stripes (default) 11 = 4 stripes 3.3 v data drive strength 00 = 1 stripe (default) 01 = 2 stripes 10 = 3 stripes 11 = 4 stripes 1.8 v data drive strength 00 = 1 stripe 01 = 2 stripes 10 = 3 stripes (default) 11 = 4 stripes 0x22 determines c mos output drive strength properties 0x16 output_phase dco o utput polarity 0 = normal 1 = inverted (local) input clock phase adjust [ 2:0 ] (value is number of input clock cycles of phase delay) 000 = no delay 001 = 1 input clock cycle 010 = 2 input clock cycles 011 = 3 input clock cycles 100 = 4 input clock cycles 101 = 5 input clock cycles 110 = 6 input clock cycles 111 = 7 input clock cycles 0x00 on devices that utilize global clock divide, determines which phase of the divider output is used to supply the output clock ; i nternal latching is unaffected 0x17 output_delay enable dco delay enable data delay dco /data delay [ 2:0 ] 000 = 0.56 ns 001 = 1.1 2 ns 010 = 1.68 ns 011 = 2.24 ns 100 = 2.80 ns 101 = 3.36 ns 110 = 3 .92 ns 111 = 4.48 n s 0x00 this sets the fine output delay of the output clock but does not change internal timing 0x19 user_patt1_lsb b7 b6 b5 b4 b3 b2 b1 b0 0x00 user - d efined pattern , 1 lsb 0x1a user_patt1_msb b15 b14 b13 b12 b11 b10 b9 b8 0x00 user - defined pattern , 1 msb 0x1b user_patt2_lsb b7 b6 b5 b4 b3 b2 b1 b0 0x00 user - defined pattern , 2 lsb 0x1c user_patt 2_msb b15 b14 b13 b12 b11 b10 b9 b8 0x00 user - defined pattern , 2 msb 0x24 misr_lsb open open open open open open open b0 0x00 least significant byte of misr ; r ead only
ad9251 data sheet rev. b | page 34 of 36 addr ess (hex) register name bit 7 (msb) bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0 (lsb) default value (hex) comments 0x2a features open open open open open open open or oe (local) 0x01 disable the or pin for the indexed channel 0x2e output assign open open open open open open open 0 = adc a 1 = a dc b (local) ch a = 0x00 ch b = 0x01 assign an adc to an output cha nnel digital feature control 0x100 sync control (global) open open open open open clock divider next sy nc only clock divider sync enable master sync enable 0x01 0x101 usr2 enable oeb pin 47 (local) open open open enable gclk detect run gclk open disa ble s dio pull - down 0x88 enables internal oscillator for clock rates < 5 mhz memory map register descriptions for additional information about functions controlled in register 0x00 to register 0xff, see the a n - 877 application note, interfacing to high speed adcs via spi . sync control (register 0x100) bits reserved bit 2 clock divider ext sync only if the master sync enable bit (address 0x100, bit 0) and the clock divider sync enable bit (addr ess 0x100 , bit 1) are high, bit 2 allows the clock divider to sync to the first sync pulse it receives and to ignore the rest. the clock divider sync enable bit (address 0x100, bit 1) resets after it syncs. bit 1 clock divider sync enable bit 1 gates the sync puls e to the clock divider. the syn c signal is enabled when bit 1 and bit 0 are high and the device is operating in continuous sync mode as long as bit 2 of the sync control is low. bit 0 master sync enable bit 0 must be high to enable any of the sync functi ons. usr2 (register 0x101) bit enable oe b pin normally set high , this bit allows pin 47 to function as the output enable. if it is set low, it disables pin 47. bit 3 enable gclk detect normally set high , this bit enables a circuit that detects e ncod e rates below about 5 msps . when a low encode rate is detected , an internal oscillator, gclk , is enabled ensuring the proper operation of several circuits. if set low , the detector is disabled. bit 2 run gclk this bi t enables the gclk oscillator. for some applications with encode rates below 10 msps , it may be preferable to set this bit high to supersede the gclk detector. bit 0 disable sdio pull - down this bit can be set high to disable the internal 30 k pull - down on the sdio pin , which ca n be used to limit the loading when many devices are connected to the spi bus.
data sheet ad9251 rev. b | page 35 of 36 applications informa tion design guidelines before starting design and layout of the ad9251 as a system, it is recommended that the desig ner become familiar with these guidelines, which discuss the special circuit connections and layout requirements needed for certain pins. power and ground recommendations when connecting power to the ad9251, it is strongly recommended that two separate sup plies be used. use one 1.8 v supply for analog (avdd); use a separate 1.8 v to 3.3 v supply for the digital output supply (drvdd). if a common 1.8 v avdd and drvdd supply must be used, the avdd and drvdd domains must be isolated with a ferrite bead or filt er choke and separate decoupling capacitors. several different decoupling capacitors can be used to cover both high and low frequencies. locate t hese capacitors close to the point of entry at the pcb level and close to the pins of the part, with minimal tr ace length. a single pcb ground plane should be sufficient when using the ad9251. with proper decoupling and smart partitioning of the pcb analog, digital, and clock sections, optimum perform ance is easily achieved. exposed paddle thermal heat sink recomm endations the exposed paddle (pin 0) is the only ground connection for the ad9251 ; therefore , it must be connected to analog ground (agnd) on the pcb of the customer . to achieve the best electrical and thermal performance, mate an exposed (no solder mask) continuous copper plane on the pcb to the ad9251 exposed paddle, pin 0. the copper plane should have several vias to achieve the lowest possible resistive thermal path for heat dissipation to flow through the bottom of the pcb. fill or plug these vias with nonconductive epoxy. to maximize the coverage and adhesion between the adc and the pcb, a silkscreen should be overlaid to partition the continuous plane on the pcb into several uniform sections. this provides several tie points between the adc and th e pcb during the reflow process. using one continuous plane with no partitions guarantees only one tie point between the adc and the pcb. for detailed information about packaging and pcb layout of chip scale packages, see the an - 772 application note , a design and manufacturing guide for the lead frame chip scale package (lfcsp) , at www.analog.com . vcm the vcm pin should be decoupled to ground with a 0.1 f c a pacitor, as sho wn in figure 42. rbias the ad9251 requires that a 10 k? resistor be placed between the rbias pin and ground. this resistor sets the master current reference of the adc core and should have at least a 1% to l erance. reference deco upling the vref pin should be externally decoupled to ground with a low esr, 1.0 f capacitor in parallel with a low esr, 0.1 f ceramic capacitor. spi port the spi port should not be active during periods when the full dynamic performance of the converte r is required. because the sclk, csb , and sdio signals are typically asynchronous to the adc clock, noise from these signals can degrade converter performance. if the on - board spi bus is used for other devices, it may be necessary to provide buffers betwee n this bus and the ad9251 to keep these signals from transitioning at the converter inputs during critical sampling periods.
ad9251 data sheet rev. b | page 36 of 36 outline dimensions compliant to jedec standards mo-220-vmmd-4 0.25 min 1 64 16 17 49 48 32 33 0.50 0.40 0.30 0.50 bsc 0.20 ref 12 max 0.80 max 0.65 nom 1.00 0.85 0.80 7.50 ref 0.05 max 0.02 nom se a ting plane pin 1 indic a t or 6.35 6.20 sq 6.05 pin 1 indic a t or 0.30 0.23 0.18 0.60 0.42 0.24 0.60 0.42 0.24 for proper connection of the exposed pad, refer to the pin configuration and function descriptions section of this data sheet. t o p view exposed pad bot t om view 9.10 9.00 sq 8.90 8.85 8.75 sq 8.65 01-22-2015-d pkg- 1 184 figure 59 . 64 - lead lead frame chip scale package [lfcsp_vq] 9 mm 9 mm body, very thin quad (cp - 64 - 4) dimensions shown in millimeters ordering guide model 1 , 2 temperature range package description package option ad9251bcpz -80 C 40c to +85c 64- lead lead frame chip scale package (lfcsp_vq) cp -64-4 ad9251bcpzrl7 - 80 C 40c to +85c 64-l ead lead frame chip scale package (lfcsp_vq) cp -64-4 ad9251bcpz -65 C 40c to +85c 64- lead lead frame chip scale package (lfcsp_vq) cp -64-4 ad9251bcpzrl7 - 65 C 40c to +85c 64- lead lead frame chip scale package (lfcsp_vq) cp -64-4 ad9251bcpz -40 C 40c to +85c 64- lead lead frame chip scale package (lfcsp_vq) cp -64-4 ad9251bcpzrl7 - 40 C 40c to +85c 64- lead lead frame chip scale package (lfcsp_vq) cp -64-4 ad9251bcpz -20 C 40c to +85c 64- lead lead frame chip scale package (lfcsp_vq) cp -64-4 ad9251bcpzrl 7 - 20 C 40c to +85c 64- lead lead frame chip scale package (lfcsp_vq) cp -64-4 ad9251 - 80ebz evaluation board ad9251 - 65ebz evaluation board ad9251 - 40ebz evaluation board ad9251 - 20ebz evaluation board 1 z = r ohs compliant part. 2 the exposed paddle (pin 0) is the only gnd connection on the chip and must be connected to the pcb agnd. ? 2009 C 2016 ana log devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. d07938 - 0- 9/16(b)

▲Up To Search▲

Price & Availability of AD9251-20EBZ

	To Download AD9251-20EBZ Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .