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rev. 0 a 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. no license is granted by implication or otherwise under any patent or patent rights of analog devices. one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781/329-4700www.analog.com fax: 781/326-8703 ? analog devices, inc., 2002 AD7450 differential input, 1 msps 12-bit adc in soic-8 and so-8 spi and qspi are trademarks of motorola, inc. microwire is a trademark of national semiconductor corporation. features fast throughput rate: 1 msps specified for v dd of 3 v and 5 v low power at max throughput rate: 3.75 mw max at 833 ksps with 3 v supplies 9 mw max at 1 msps with 5 v supplies fully differential analog input wide input bandwidth: 70 db sinad at 300 khz input frequency flexible power/serial clock speed management no pipeline delays high-speed serial interfacespi tm /qspi tm microwire tm /dsp compatible power-down mode: 1 a max 8-lead soic and soic packages applications transducer interface battery-powered systems data acquisition systems portable instrumentation motor control communications functional block diagram 12-bit successive approximation adc control logic AD7450 v in+ sclk cs sdata v in v ref gnd v dd t/h general description the AD7450 is a 12-bit, high-speed, low power, successive approximation (sar) analog-to-digital converter that features a fully differential analog input. it operates from a single 3 v or 5 v power supply and features throughput rates up to 833 ksps or 1 msps, respectively. this part contains a low noise, wide bandwidth, differential track - and-hold amplifier (t/h) that can handle input frequencies in excess of 1 mhz with the ? db point typically being 20 mhz. the reference voltage for the AD7450 is applied externally to the v ref pin and can be varied from 100 mv to 3.5 v, depending on the power supply and what suits the application. the value of the reference voltage determines the common-mode voltage range of the part. with this truly differential input structure and variable reference input, the user can select a variety of input ranges and bias points. the conversion and data acquisition processes are controlled using cs and the serial clock, allowing the device to interface with microprocessors or dsps. the input signals are sampled on the falling edge of cs , and the conversion is also initiated at this point. the sar architecture of this part ensures that there are no pipeline delays. the AD7450 uses advanced design techniques to achieve low power dissipation at high throughput rates. product highlights 1. operation with either 3 v or 5 v power supplies. 2. high throughput with low power consumption. with a 3 v supply, the AD7450 offers 3.75 mw max power consump tion for 833 ksps throughput. 3. fully differential analog input. 4. flexible power/serial clock speed management. the conversion rate is determined by the serial clock, allowing the power to be reduced as the conversion time is reduced through the serial clock speed increase. this part also features a shutdown mode to maximize power efficiency at lower throughput rates. 5. variable voltage reference input. 6. no pipeline delay. 7. accurate control of the sampling instant via a cs input and once-off conversion control. 8. enob > 8 bits typically with 100 mv reference.
rev. 0 ?2? AD7450especifications 1 parameter conditions/comments a version b version unit dynamic performance signal-to-(noise + distortion) ratio v dd = 5 v 70 70 db min (sinad) 3 v dd = 3 v 68 68 db min total harmonic distortion (thd) 3 v dd = 5 v, e80 db typ e75 e75 db max v dd = 3 v, e78 db typ e73 e73 db max peak harmonic or spurious noise 3 v dd = 5 v, e82 db typ e75 e75 db max v dd = 3 v, e80 db typ e73 e73 db max intermodulation distortion (imd) 3 second order terms e85 e85 db typ third order terms e85 e85 db typ aperture delay 3 10 10 ns typ aperture jitter 3 50 50 ps typ full power bandwidth 3 @ e3 db 20 20 mhz typ @ e0.1 db 2.5 2.5 mhz typ power supply rejection ratio (psrr) 3, 4 e87 e87 db typ dc accuracy resolution 12 12 bits integral nonlinearity (inl) 3 () + = = = = = = v ref 5 v in+ e v ine v in+ e v ine v absolute input voltage v in+ v cm 2 = v ref v cm = ( ) ( ) = = = = = = ( = = = = = = = = = = = = )
rev. 0 AD7450 ?3? parameter conditions/comments a version b version unit conversion rate conversion time 888 ns with an 18 mhz sclk 16 16 sclk cycles 1.07 = = () = () = = = = () = = = = = = + + v ref . 6 the AD7450 is functional with a reference input from 100 mv and for v dd = 5 v, the reference can range up to 3.5 v (see references section). 7 the AD7450 is functional with a reference input from 100 mv and for v dd = 3 v, the reference can range up to 2.2 v (see references section). 8 sample tested @ 25
rev. 0 ?4? AD7450 limit at t min , t max parameter 3 v 5 v unit description f sclk 4 50 50 khz min 15 18 mhz max t convert 16 t sclk 16 t sclk t sclk = 1/f sclk 1.07 0.88 = ==( ) = = ( = = = = = = = = = = )
rev. 0 AD7450 ?5? caution esd (electrostatic discharge) sensitive device. electrostatic charges as high as 4000 v readily accumulate on the human body and test equipment and can discharge without detection. although the AD7450 features proprietary esd protection circuitry, permanent damage may occur on devices subjected to high-energy electrostatic discharges. therefore, proper esd precautions are recommended to avoid performance degradation or loss of functionality. warning! esd sensitive device absolute maximum ratings 1 (t a = 25 ) + + + + + + + () + + ja thermal impedance . . . . . . . . . . . . . . . . 157 () ( ) jc thermal impedance . . . . . . . . . . . . . . . . . 56 () ( ) () () ? ? () + + + + ==
rev. 0 ?6? AD7450 pin configuration top view (not to scale) 8 7 6 5 1 2 3 4 v ref v in+ v ine v dd sclk sdata cs gnd AD7450 pin function description pin number mnemonic function 1v ref reference input for the AD7450. an external reference must be applied to this input. for a 5 v power supply, the reference is 2.5 v ( ) ( ) + ( )( )
rev. 0 AD7450 ?7? terminology signal-to-(noise + distortion) ratio this is the measured ratio of signal-to-(noise + distortion) at the output of the adc. the signal is the rms amplitude of the fundamental. noise is the sum of all nonfundamental signals up to half the sampling frequency (f s /2), excluding dc. the ratio is dependent on the number of quantization levels in the digitiza- tion process; the more levels, the smaller the quantization noise. the theoretical signal-to-(noise + distortion) ratio for an ideal n-bit converter with a sine wave input is given by: signal?o?noise + distortion) = (6.02 n + 1.76) db thus, for a 12-bit converter, this is 74 db. total harmonic distortion total harmonic distortion (thd) is the ratio of the rms sum of harmonics to the fundamental. for the AD7450, it is defined as: thd db vvvvv v () = ++++ log where v 1 is the rms amplitude of the fundamental and v 2 , v 3 , v 4 , v 5 , and v 6 are the rms amplitudes of the second to the sixth harmonics. peak harmonic or spurious noise peak harmonic or spurious noise is defined as the ratio of the rms value of the next largest component in the adc output spectrum (up to f s /2 and excluding dc) to the rms value of the fundamental. normally, the value of this specification is deter- mined by the largest harmonic in the spectrum, but for adcs where the harmonics are buried in the noise floor, it will be a noise peak. intermodulation distortion with inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities will create distortion products at sum and difference frequencies of mfa = (+)() (+)()(+)() () + cmrr db pf pfs () log ( / ) = pf is the power at the frequency f in the adc output; pfs is the power at frequency fs in the adc output. integral nonlinearity (inl) this is the maximum deviation from a straight line passing through the endpoints of the adc transfer function. differential nonlinearity (dnl) this is the difference between the measured and the ideal 1 lsb change between any two adjacent codes in the adc. zero code error this is the deviation of the midscale code transition (111...111 to 000...000) from the ideal v in+ ?v in? (i.e., 0 lsb). positive gain error this is the deviation of the last code transition (011...110 to 011...111) from the ideal v in+ ?v in (i.e., +v ref ?1 lsb), after the zero code error has been adjusted out. negative gain error this is the deviation of the first code transition (100...000 to 100...001) from the ideal v in+ ?v in? (i.e., ? ref + 1 lsb), after the zero code error has been adjusted out. track and hold acquisition time the track and hold acquisition time is the minimum time re- quired for the track and hold amplifier to remain in track mode for its output to reach and settle to within 0.5 lsb of the ap- plied input signal. power supply rejection ratio (psrr) the power supply rejection ratio is defined as the ratio of the power in the adc output at full-scale frequency, f, to the power of a 200 mv p-p sine wave applied to the adc v dd supply of frequency f s . psrr db log pf /pfs () ( ) = pf is the power at frequency f in the adc output; pfs is the power at frequency fs in the adc output.
rev. 0 ?8? AD7450etypical performance characteristics frequency e khz snr e dbs 0 e60 e120 050 500 100 150 200 250 300 350 400 450 e20 e40 e80 e100 8192 point fft f sample = 1msps f in = 300khz sinad = 71.7db thd = e82.8db pk noise = e85.3db tpc 1. dynamic performance at 1 msps with v dd = 5 v code dnl error e lsb 1.0 0.4 e0.2 0.8 0.6 0.2 0 e0.4 e0.6 e0.8 e1.0 0 1024 2048 3072 4096 tpc 4. typical differential nonlinearity (dnl) v dd = 5 v code inl error e lsb 1.0 0.4 e0.2 0.8 0.6 0.2 0 e0.4 e0.6 e0.8 e1.0 0 1024 2048 3072 4096 tpc 7. typical integral nonlinearity (inl) v dd = 3 v frequency e khz snr e dbs 0 e60 e120 050 100 150 200 250 300 350 e20 e40 e80 e100 8192 point fft f sample = 833ksps f in = 300khz sinad = 70.2db thd = e82db pk noise = e87.1db tpc 2. dynamic performance at 833 ksps with v dd = 3 v code dnl error e lsb 1.0 0.4 e0.2 0.8 0.6 0.2 0 e0.4 e0.6 e0.8 e1.0 0 1024 2048 3072 4096 tpc 5. typical differential nonlinearity (dnl) v dd = 3 v 1.0 0.5 0 e0.5 e1.0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 c hange in dnl-lsb v ref posi tive dnl ne gative dnl tpc 8. change in dnl vs. reference voltage v dd = 5 v input frequency e khz sinad e db e63 e75 e69 e65 e67 e71 e73 10 100 1000 v dd = 4.75v v dd = 2.7v v dd = 3.3v v dd = 5.25v tpc 3. sinad vs. analog frequency for various supply voltages code inl error e lsb 1.0 0.4 e0.2 0.8 0.6 0.2 0 e0.4 e0.6 e0.8 e1.0 0 1024 2048 3072 4096 tpc 6. typical integral nonlinearity (inl) v dd = 5 v v ref 1.0 0.5 e1.0 0 e0.5 1.5 change in dnl-lsb 0 0.6 1.2 1.8 2.4 positive dnl negative dnl tpc 9. change in dnl vs. reference voltage v dd = 3.3 v* (default conditions: t a = 25 c)
rev. 0 AD7450 ?9? v ref zero-code error e lsb 1 0 e9 0.25 0.75 3.50 1.25 1.75 2.25 2.75 3.25 e5 e6 e7 e8 e2 e4 e3 e1 v dd = 3.3v f s = 833ksps v dd = 5v f s = 1msps tpc 12. change in zero-code error vs. reference voltage v dd = 5 v and 3.3 v* 12 11 10 9 8 7 6 e ffective number of bits 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 v ref v dd = 5v f s = 1msps v dd = 3.3v f s = 833ksps tpc 15. change in enob vs. refer- ence voltage v dd = 5 v and 3.3 v* 1.5 1.0 0.5 0 e0.5 e1.0 e1.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 c hange in inl-lsb v ref posi tive inl ne gative inl tpc 10. change in inl vs. reference voltage v dd = 5 v code 8,000 7,000 4,000 0 5,000 9,000 3,000 10,000 2,000 1,000 6,000 2044 2046 2047 2048 2049 2045 10,000 codes v in = v in e 10,000 conversions f s = 1msps tpc 13. histogram of the output codes with a dc input for v dd = 5 v frequency e khz cmrr e db 90 0 10 20 30 40 50 60 70 80 10 10,000 100 1,000 v dd = 5v v dd = 3v tpc 16. cmrr vs. input frequency for v dd = 5 v and 3 v v ref 0 0.6 1.2 1.8 2.4 1.0 0.5 e1.0 0 e0.5 1.5 e1.5 2.0 change in inl-lsb negative inl positive inl tpc 11. change in inl vs. reference voltage v dd = 3.3 v* 90 codes 71 codes code 8,000 7,000 4,000 0 5,000 9,000 3,000 10,000 2,000 1,000 6,000 2044 2046 2047 2048 2049 2045 9,839 codes v in = v in e 10,000 conversions f s = 833ksps tpc 14. histogram of the output codes with a dc input for v dd = 3 v * see references section.
rev. 0 ?10? AD7450 circuit information the AD7450 is a fast, low power, single-supply, 12-bit successive approximation analog-to-digital converter (adc). it can operate with a 5 v and 3 v power supply and is capable of throughput rates up to 1 msps and 833 ksps when supplied with an 18 mh z or 15 mhz clock, respectively. this part requires an external reference to be applied to the v ref pin, with the value of the reference chosen depending on the power supply and what suits the application. when operated with a 5 v supply, the maximum reference that can be applied to the part is 3.5 v, and when operated with a 3 v supply, the maximum reference that can be applied to the part is 2.2 v. (see the references section.) the AD7450 has an on-chip differential track-and-hold amplifier, a successive approximation (sar) adc, and a serial interface that is housed in either an 8-lead soic or () + + () + + + ( ) v ref / 4096. the ideal transfer characteristic of the AD7450 is shown in figure 5. analog input (v in+ e v ine ) 100...000 100...001 100...010 111...111 000...000 000...001 011...110 011...111 adc code 1lsb = 2 v ref /4096 ev ref + 1lsb 0lsb +v ref e 1lsb figure 5. ideal transfer characteristics typical connection diagram figure 6 shows a typical connection diagram for the AD7450 for both 5 v and 3 v supplies. in this setup, the gnd pin is connected to the analog ground plane of the system. the v ref pin is connected to either a 2.5 v or a 1.25 v decoupled reference source, depending on the power supply, to set up the analog input range. the common-mode voltage has to be set up exter- nally and is the value that the two inputs are centered on. for more details on driving the differential inputs and setting up the common mode, see the driving differential inputs section. the conversion result for the adc is output in a 16-bit word consisting of four leading zeros followed by the msb of the 12-bit result. for applications where power consumption is of concern, the power-down mode should be used between conversions, or bursts of several conversions, to improve power performance. see modes of operation section.
rev. 0 AD7450 ?11? cm * v ref p-p cm * * cm = common-mode voltage v in+ v ine v dd sclk sdata cs gnd v ref serial interface 3v/5v supply 1.25v/2.5v v ref 0.1 f 10 f AD7450 v ref p-p 0.1 f c/ p figure 6. typical connection diagram the analog input the analog input of the AD7450 is fully differential. differential signals have a number of benefits over single-ended signals, including noise immunity based on the device?s common-mode rejection, improvements in distortion performance, doubling of the device?s available dynamic range, and flexibility in input ranges and bias points. figure 7 defines the fully differential analog input of the AD7450. v in+ AD7450 v ine v ref p-p common-mode vo ltag e v ref p-p figure 7. differential input definition the amplitude of the differential signal is the difference between the signals applied to the v in+ and v ine pins (i.e., v in+ e v ine ). v in+ and v ine are simultaneously driven by two signals each of amplitude v ref that are 180 + ( v ref ). this is regardless of the common mode (cm). the common mode is the average of the two signals, i.e., (v in+ + v ine )/2, and is therefore the voltage that the two inputs are centered on. this results in the span of each input being cm + ( = ()=) ( = ()=) + =
rev. 0 ?12? AD7450 reference = 1.25v 1.25v p-p common-mode (cm) cm min = 0.625v cm max = 4.42v reference = 2.5v 2.5v p-p common-mode (cm) cm min = 1.25v cm max = 3.75v v in e v in v in e v in figure 10. examples of the analog inputs to v in+ and v in? for different values of v ref for v dd = 5 v analog input structure figure 11 shows the equivalent circuit of the analog input struc- ture of the AD7450. the four diodes provide esd protection for the analog inputs. care must be taken to ensure that the analog input signals never exceed the supply rails by more than 300 mv. this will cause these diodes to become forward biased and start conducting into the substrate. these diodes can conduct up to 10 ma without causing irreversible damage to the part. the capacitors, c1, in figure 11 are typically 4 pf and can prima- rily be attributed to pin capacitance. the resistors are lumped components made up of the on resistance of the switches. the value of these resistors is typically about 100 ? + ? () = = = = = = = = = = ? = = = = = + ()
rev. 0 AD7450 ?13? differential amplifier an ideal method of applying differential drive to the AD7450 is to use a differential amplifier, such as the ad8138. this part can be used as a single-ended-to-differential amplifier or as a differential- to-differential amplifier. in both cases, the analog input needs to be bipolar. it also provides common-mode level shifting and buffer- ing of the bipolar input signal. figure 14 shows how the ad8138 can be used as a single-ended-to-differential amplifier. the positive and negative outputs of the ad8138 are connected to the respective inputs on the adc via a pair of series resistors to minimize the effects of switched capacitance on the front end of the adc. the rc low-pass filter on each analog input is recommended in ac applications to remove the high-frequency components of the analog input. the architecture of the ad8138 results in outputs that are highly balanced over a wide frequency range without requiring tightly matched external components. if the analog input source being used has zero impedance then all four resistors (rg1, rg2, rf1, and rf2) should be the same. if the source has a 50 ? ? ? () () + + + ? () + ==== +
rev. 0 ?14? AD7450 the circuit configuration shown in figure 15b converts a unipolar, single-ended signal into a differential signal. v+ ve v+ ve 2 v ref p-p vref 27 27 390 220 10k external v ref 220 v dd v in+ v ine AD7450 220 0.1 f a v ref gnd figure 15b. dual op amp circuit to convert a single-ended unipolar input into a differential input rf transformer in systems that do not need to be dc-coupled, an rf transformer with a center tap offers a good solution for generating differential inputs. figure 16 shows how a transformer is used for single- ended-to-differential conversion. it provides the benefits of operating the adc in the differential mode without contributing additional noise and distortion. an rf transformer also has the benefit of providing electrical isolation between the signal source and the adc. a transformer can be used for most ac applications. the center tap is used to shift the differential signal to the common-mode level required. in this case, it is connected to the reference so the common-mode level is the value of the reference. c external v ref (2.5v) r r r v ref v in+ AD7450 v ine 2.5v 3.75v 1.25v 2.5v 3.75v 1.25v figure 16. using an rf transformer to generate differential inputs references section an external reference source is required to supply the reference to the AD7450. this reference input can range from 100 mv to 3.5 v. with a 5 v power supply, the specified reference is 2.5 v and the maximum reference is 3.5 v. with a 3.3 v power supp ly, the specified refer- ence is 1.25 v and the maximum reference is 2.4 v. in both cases, the reference is functional from 100 mv. it is important to ensure that, when c hoosing the reference value for a particular application, the maximum analog input range (v in max) is never greater than v dd + 0.3 v to comply with the maximum ratings of the part. the following two examples calculate the maximum v ref input that can be used when operating the AD7450 at v dd of 5 v and 3.3 v, respectively. example 1: vv in dd max . =+ vvv in ref ref max =+ if v v dd = thenv v in max . = therefore v v ref 3253 = vv ref max . = = = = vv in dd max . =+ vvv in ref ref max =+ if v v dd = thenv v in max . = therefore v v ref 3236 = vv ref max . = = = = () ( ? ) ? ? ? = =
rev. 0 AD7450 ?15? single-ended operation when supplied with a 5 v power supply, the AD7450 can handle a single-ended input. the design of this part is optimized for differential operation, so with a single-ended input, performance will degrade. linearity will typically degrade by 0.2 lsbs, zero code and full-scale errors will typically degrade by 2 lsbs, and ac performance is not guaranteed. to operate the AD7450 in single-ended mode, the v in+ input is coupled to the signal source, while the v ine input is biased to the appropriate voltage corresponding to the midscale code transi- tion. this voltage is the common mode, which is a fixed dc voltage (usually the reference). the v in+ input swings around this value and should have voltage span of 2 v ref to make use of the full dynamic range of the part. therefore, the input signal will have peak-to-peak values of common mode + () + ? () + + () () = = = 111 000 000 1 throughput s == tft sclk acq 2 12 5 1 1 + () += s therefore, if t 2 = 10 ns then: 10 12 5 1 18 1 ns mhz t s acq + () += tns acq = 2 8 . 51f sclk () ++ tt quiet where t 8 = 35 ns. this allows a value of 122 ns for t quiet , satis- fying the minimum requirement of 25 ns. timing example 2 having f sclk = 5 mhz and a throughput rate of 315 ksps gives a cycle time of: 11 315 000 3 174 throughput s == tft sclk acq 2 12 5 1 3 174 + () += s therefore if t 2 is 10 ns then: 10 12 5 1 5 3 174 ns mhz t s acq + () += tns acq = 2 8 . 51f sclk () ++ tt quiet where t 8 = 35 ns. this allows a value of 129 ns for t quiet , satis- fying the minimum requirement of 25 ns. as in this example and with other slower clock values, the signal may already be acquired before the conversion is complete, but it is still necessary to leave 25 ns minimum t quiet between conver- sions. in timing example 2, the signal should be fully acquired at approximately point c in figure 20.
rev. 0 ?16? AD7450 1 2 345 13 16 15 14 00 0 0 db11 db10 db2 db1 db0 t 2 4 leading zeros t 8 t q uiet t convert b cs sclk sdata t 6 t 7 t 4 t 5 t 3 t 1 three-state figure 19. serial interface timing diagram 1 2 345 13 16 15 14 t convert b cs t acq 12.5(1/f sclk ) 1/throughput 10ns sclk c t q uiet t 2 t 5 t 6 t 8 figure 20. serial interface timing example modes of operation the mode of operation of the AD7450 is selected by controlling the logic state of the cs signal during a conversion. there are two possible modes of operation, normal mode and power-down mode. the point at which cs is pulled high after the conversion has been initiated will determine whether or not the AD7450 will enter the power-down mode. similarly, if already in power-down, cs controls whether the device will return to normal operation or remain in power-down. these modes of operation are designed to provide flexible power management options. these options can be chosen to optimize the power dissipation/throughput rate ratio for differing application requirements. normal mode this mode is intended for the fastest throughput rate perfor- mance. the user does not have to worry about any power-up times since the AD7450 is kept fully powered up. figure 21 shows the general diagram of the operation of the AD7450 in this mode. the conversion is initiated on the falling edge of cs as described in the serial interface section. to ensure the part remains fully powered up, cs must remain low until at least 10 sclk falling edges have elapsed after the falling edge of cs . if cs is brought high any time after the 10th sclk falling edge, but before the 16th sclk falling edge, the part will remain powered up, but the conversion will be terminated and sdata will go back into three-state. sixteen serial clock cycles are required to complete the conver- sion and access the complete conversion result. cs may idle high until the next conversion or idle low until sometime prior to the next conversion. once a data transfer is complete, i.e., when sdata has returned to three-state, another conversion can be initiated after the quiet time, t quiet , has elapsed by again bringing cs low. 10 16 sclk 1 sdata cs 4 leading zeros and conversion result figure 21. normal mode operation power-down mode this mode is intended for use in applications where slower throughput rates are required; either the adc is powered down between each conversion or a series of conversions may be per formed at a high throughput rate, during which the adc is pow ered down for a relatively long duration between these bursts of several conversions. when the AD7450 is in the power-down mode, all analog circuitry is powered down. to enter power-down mode, the conversion process must be interrupted by bringing cs high anywhere after the second falling edge of s clk and before the 10th falling edge of sclk as shown in figure 22.
rev. 0 AD7450 ?17? cs three-state sdata 12 10 sclk figure 22. entering power-down mode once cs has been brought high in this window of sclks, the part will enter power-down, the conversion that was initiated by the falling edge of cs will be terminated, and sdata will go back into three-state. the time from the rising edge of cs to sdata three-state enabled will never be greater than t 8 (see timing specifications). if cs is brought high before the second sclk falling edge, the part will remain in normal mode and will not power down. this will avoid accidental power-down due to glitches on the cs line. to exit this mode of operation and power the AD7450 up again, a dummy conversion is performed. on the falling edge of cs , the device will begin to power up and continue to power up as long as cs is held low until after the falling edge of the 10th sclk. the device will be fully powered up after 1 (() 16). in one dummy cycle, 3.2 ( )
rev. 0 ?18? AD7450 power versus throughput rate by using the power-down mode on the AD7450 when not converting, the average power consumption of the adc decreases at lower throughput rates. figure 24 shows how, as the throughput rate is reduced, the device remains in its power-down state longer, and the average power consumption reduces accordingly. it shows this for both 5 v and 3 v power supplies. for example, if the AD7450 is operated in continuous sampling mode with a throughput rate of 100 ksps and an sclk of 18 mhz, and the device is placed in the power-down mode between conversions, then the power consumption is calculated as follows: power dissipation during normal operation = 9 mw max for v dd = 5 v. if the power-up time is one dummy cycle, i.e., 1 == () 9 mw = 1.8 mw for the same scenario, if v dd = 3 v, the power dissipation during normal operation is 3.75 mw max. the AD7450 can now be said to dissipate 3.75 mw for 2 () 3.75 mw = 0.75 mw this is how the power numbers in figure 24 are calculated. for throughput rates above 320 ksps, it is recommended that the serial clock frequency is reduced for optimum power performance. throughput ?ksps 100 0.01 0.1 1 10 0 350 100 150 200 50 250 300 power ?mw v dd = 3v sclk = 15mhz v dd = 5v sclk = 18mhz figure 24. power vs. throughput rate for power-down mode microprocessor and dsp interfacing the serial interface on the AD7450 allows the part to be directly connected to a range of different microprocessors. this section explains how to interface the AD7450 with some of the more common microcontroller and dsp serial interface protocols. AD7450 to adsp-21xx the adsp-21xx dsps are interfaced directly to the AD7450 without any glue logic required. the sport control register should be set up as follows: tfsw = rfsw = 1, alternate framing invrfs = invtfs = 1, active low frame signal dtype = 00, right justify data slen = 1111, 16-bit data-words isclk = 1, internal serial clock tfsr = rfsr = 1, frame every word irfs = 0 itfs = 1 to implement the power-down mode, slen should be set to 1001 to issue an 8-bit sclk burst. the connection diagram is shown in figure 25. the adsp-21xx has the tfs and rfs of the sport tied together, with tfs set as an output and rfs set as an input. the dsp operates in alternate framing mode and the sport control register is set up as described. the frame synchronization signal generated on the tfs is tied to cs and, as with all signal processing applica- tions, equidistant sampling is necessary. however, in this example, the timer interrupt is used to control the sampling rate of the adc and, under certain conditions, equidistant sampling may not be achieved. sclk sdata cs sclk dr rfs tfs * additional pins omitted for clarity AD7450 * adsp-21xx * figure 25. interfacing to the adsp-21xx the timer registers are loaded with a value that provides an interrupt at the required sample interval. when an interrupt is received, a value is transmitted with tfs/dt (adc control word). the tfs is used to control the rfs and hence the reading of data. the frequency of the serial clock is set in the sclkdiv register. when the instruction to transmit with tfs is given, (i.e., ax0 = tx0), the state of the sclk is checked. the dsp will wait until the sclk has gone high, low, and high before transmission will start. if the timer and sclk values are chosen such that the instruction to transmit occurs on or near the rising edge of sclk, then the data may be transmitted, or it may wait until the next clock edge. for example, the adsp-2111 has a master clock frequency of 16 mhz. if the sclkdiv register is loaded with the value 3, then a sclk of 2 mhz is obtained and eight master clock periods will elapse for every 1 sclk period. if the timer regis- ters are loaded with the value 803, then 100.5 sclks will occur between interrupts and subsequently between transmit instruc- tions. this situation will result in nonequidistant sampling as the transmit instruction is occurring on a sclk edge. if the number of sclks between interrupts is a whole integer figure of n, then equidistant sampling will be implemented by the dsp. * this figure assumes a very small time to enter power-down mode. this will increase as the burst of clocks used to enter the power-down mode is increased.
rev. 0 AD7450 ?19? AD7450 to tms320c5x/c54x the serial interface on the tms320c5x/c54x uses a continuous serial clock and frame synchronization signals to synchronize the data transfer operations with peripheral devices, such as the AD7450. t he cs input allows easy interfacing between the tms320c5x/c54x and the AD7450 with no glue logic re quired. the serial port of the tms320c5x/c54x is set up to operate in burst mode with internal clkx (tx serial clock) and fsx (tx frame sync). the serial port control register (spc) must have the following setup: fo = 0, fsm = 1, mcm = 1, and txm = 1. the format bit, fo, may be set to 1 to set the word length to 8 bits in order to implement the power-down mode on the AD7450. the con nection diagram is shown in figure 26. for signal pro cessing applica tions, it is imperative that the frame synchronization signal from the tms320c5x/c54x provide equi- distant sam pling. fsr cs fsx sclk clkr clkx * additional pins omitted for clarity AD7450 * tms320c5x/c54x * sdata dr figure 26. interfacing to the tms320c5x/c54x AD7450 to mc68hc16 the serial peripheral interface (spi) on the mc68hc16 is configured for master mode (mstr) = 1, clock polarity bit (cpol) = 1, and clock phase bit (cpha) = 0. the spi is configured by writing to the spi control register (spcr)?see the 68hc16 user manual. the serial transfer will take place as a 16-bit operation when the size bit in the spcr register is set to size = 1. to implement the power-down modes with an 8-bit transfer set size = 0. a connection diagram is shown in figure 27. cs ss/pmc3 sclk sclk/pmc2 * additional pins omitted for clarity AD7450 * mc68hc16 * sdata miso/pmc0 figure 27. interfacing to the mc68hc16 AD7450 to dsp56xxx the connection diagram in figure 28 shows how the AD7450 can be connected to the ssi (synchronous serial interface) of the dsp56xxx family of dsps from motorola. the ssi is operated in synchronous mode (syn bit in crb = 1) with internally generated 1-bit clock period frame sync for both tx and rx (bits fsl1 = 1 and fsl0 = 0 in crb). set the word length to 16 by setting bits wl1 = 1 and wl0 = 0 in cra. to imple- ment the power-down mode on the AD7450, the word length can be changed to 8 bits by setting its wl1 = 0 and wl0 = 0 in cra. it should be noted that for signal processing applica- tions, it is imperative that the frame synchronization signal from the dsp56xxx will provide equidistant sampling. cs sr2 sclk sclk * additional pins omitted for clarity AD7450 * dsp56xxx * sdata srd figure 28. interfacing to the dsp56xxx application hints grounding and layout the printed circuit board that houses the AD7450 should be designed so that the analog and digital sections are separated and confined to certain areas of the board. this facilitates the use of ground planes that can be easily separated. a minimum etch technique is generally best for ground planes since it gives the best shielding. digital and analog ground planes should be joined in only one place, and the connection should be a star ground point established as close to the gnd pin on the ad 7450 as possible. avoid running digital lines under the device, as this will couple noise onto the die. the analog ground plane should be allowed to run under the AD7450 to avoid noise coupling. the power supply lines to the AD7450 should use as large a trace as possible to provide low impedance paths and reduce the effects of glitches on the power supply line. fast switching signals, such as clocks, should be shielded with digital ground to avoid radiating noise to other sections of the board, and clock signals should never run near the analog inputs. avoid crossover of digital and analog signals. traces on opposite sides of the board should run at right angles to each other. this reduces the effects of feedthrough through the board. a microstrip technique is by far the best but is not always possible with a double-sided board. in this technique, the component side of the board is dedicated to ground planes, while signals are placed on the solder side. good decoupling is also important. all analog supplies should be decoupled with 10
rev. 0 c02646?0?5/02(0) printed in u.s.a. ?20? AD7450 outline dimensions dimensions shown in millimeters and (inches) 8-lead soic (r-8) 0.25 (0.0098) 0.19 (0.0075) 1.27 (0.0500) 0.41 (0.0161) 8 0 0.50 (0.0197) 0.25 (0.0098) 45 85 4 1 5.00 (0.1969) 4.80 (0.1890) 6.20 (0.2441) 5.80 (0.2283) pin 1 4.00 (0.1575) 3.80 (0.1496) 1.27 (0.0500) bsc 1.75 (0.0689) 1.35 (0.0531) seating plane 0.25 (0.0098) 0.10 (0.0039) 0.49 (0.0193) 0.35 (0.0138) coplanarity controlling dimensions are in millimeters; inch dimensions (in parentheses) are rounded-off millimeter equivalents for reference only and are not appropriate for use in design dimensions shown in inches and (mm) 8-lead soic (rm-8) 0.011 (0.28) 0.003 (0.08) 0.028 (0.71) 0.016 (0.41) 33 27 0.120 (3.05) 0.112 (2.84) 85 4 1 0.122 (3.10) 0.114 (2.90) 0.199 (5.05) 0.187 (4.75) pin 1 0.0256 (0.65) bsc 0.122 (3.10) 0.114 (2.90) seating plane 0.006 (0.15) 0.002 (0.05) 0.018 (0.46) 0.008 (0.20) 0.043 (1.09) 0.037 (0.94) 0.120 (3.05) 0.112 (2.84) evaluating the AD7450 performance the evaluation board package includes a fully assembled and tested evaluation board, documentation, and software for controlling the board from a pc via the evaluation board controller. the evaluation board controller can be used in conjunction with the AD7450 evaluation board, as well as many other analog devices evaluation boards ending with the cb designator, to demonstrate/evaluate the ac and dc performance of the AD7450. the software allows the user to perform ac (fast fourier transform) and dc (histogram of codes) tests on the AD7450. see the evaluation board technical note for more information.

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