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16-bit, 1.5 lsb inl, 500 ksps pulsar? differential adc in msop/qfn ad7688 rev. 0 in fo rmation furn is h e d by an al o g dev i ces is believed to be a ccu rate and r e liable. how e ver, no r e spons i bili ty is assumed by analog devices fo r its use, nor f o r an y i n fri n geme nt s of p a t e nt s or ot her ri g h t s o f th ird parties th at may result fro m its use . specifications subject to chan g e witho u t n o tice. no licen s e is g r an te d by implicati o n or ot he rwi s e un der a n y p a t e nt or p a t e nt r i ghts of anal og de vices. trad emarks an d registered tra d ema r ks are the prop erty o f their respective ow ners. one technolog y way, p.o . box 9106, norwood, ma 02062-9106, u.s.a. t e l: 781. 329. 4 700 www.analog.com fax: 781. 461. 31 13 ? 2005 a n alog de vices, i n c. al l r i ght s r e ser v ed . features 16-bit resolut i on with no missing codes throughput: 500 ksps inl: 0.4 lsb t y p, 1.5 lsb max (23 ppm of fsr) dynamic range : 96.5 db snr: 95.5 db @ 20 kh z thd: ?118 db @ 20 kh z true differenti a l analog input range v re f 0 v to v re f with v re f up to vdd on both inputs n o pipeline delay single-supply 5 v operation with 1.8 v/2.5 v/3 v/5 v logic interf ace serial interf ace spi?/qspi? /mi c rowire?/dsp-compatible daisy-chain m u ltiple adcs an d busy in dicat o r power dissipati o n 3.75 mw @ 5 v/ 100 ksps 3.75 w @ 5 v/ 100 sps standby curre nt: 1 na 10-lea d msop ( m sop-8 size) a n d 3 mm 3 mm qfn 1 (lfcsp) (s ot-23 size) pin-for-pin compatible with ad76 85, a d 76 86, and ad 768 7 applic ati o ns battery-powered eq uipment data acq u isitio ns instrumentation medical instr u ments process control s code in l ( l sb ) 1.5 1.0 0.5 0 ?0.5 ?1.0 ?1.5 0 16384 32768 49152 65535 02973-001 positive inl = +0.31lsb negative inl = ? 0.39lsb f i gure 1 . integr a l no nli n ea ri t y vs . c o d e applic ation diagram ad7688 ref gnd vdd in+ in? vio sdi sck sdo cnv 1.8v to vdd 3- or 4-wire interface (spi, daisy chain, cs) 0.5v to 5v 5v vref 0 02973-002 vref 0 fi g u r e 2 . table 1. msop , qfn 1 (lfcsp)/sot-23 16 -bit pulsar adc type 100 ksps 250 ksps 500 ksps true differential ad7684 ad7687 ad7688 pseudo ad7683 ad7685 ad7686 differential/uni polar ad7694 unipol ar ad7680 general description the ad7688 is a 16-b i t, c h a r g e r e dis t r i b u tion, succes s i v e ap p r ox i m at i o n , a n a l o g - t o - d i g i t a l c o n v e r t e r ( a d c ) t h at o p e r at e s f r o m a sin g le 5 v p o w e r sup p ly , vdd . i t co n t ai n s a lo w p o w e r , h i g h sp e e d, 1 6 - b i t s a m p l i ng a d c w i t h no m i s s i ng c o d e s , an i n t e rnal co n v er si o n c l oc k , a n d a v e r s a t ile ser i al in t e rfa c e po r t . t h e p a r t a l s o c o n t ai ns a l o w noi s e, w i d e b a nd w i d t h , s h or t a p er t u r e d e la y t r ack-an d - h o ld c i r c ui t. o n t h e c n v r i sin g e d ge, i t s a m p les t h e vol t a g e dif f er en c e b e tw e e n i n + a nd in? p i n s . the v o l t a g es on t h es e p i ns us ua l l y swin g in o p p o si t e phas e b e tw e e n 0 v and ref . t h e r e fer e n c e vol t a g e, r e f , is a p plie d ex ter n a l ly a n d c a n b e s e t u p to t h e su p p ly vol t a g e. it s p o w e r s c a l e s l i n e a r l y w i t h t h r o u g h p u t . t h e s p i- co m p a t i b le serial in t e r f a c e also f e a t ur es th e a b ili t y , usin g t h e s d i i n p u t, t o da isy - ch a i n s e v e ra l ad cs o n a si n g le , 3-w i r e b u s and p r o v ide s a n o p t i o n a l b u s y i ndi ca to r . i t is co m p a t i b le wi th 1.8 v , 2.5 v , 3 v , o r 5 v log i c, us in g t h e s e p a ra te su p p ly v i o . the ad7688 is h o us e d in a 10-l e ad mso p o r a 10-lead q f n 1 (lfcs p ) wi t h op era t ion sp e c if ie d f r o m ?40c t o +85c. 1 qfn package in developm ent. c o ntact s a l e s f o r s amples and avail abil ity.
ad7688 rev 0 | page 2 of 28 table of contents specifications ..................................................................................... 3 timing specifications ....................................................................... 5 absolute maximum ratings ............................................................ 6 esd caution .................................................................................. 6 pin configurations and function descriptions ........................... 7 ter mi nolo g y ...................................................................................... 8 typical performance characteristics ............................................. 9 circuit information .................................................................... 12 converter operation .................................................................. 12 typical connection diagram ................................................... 13 analog input ............................................................................... 14 driver amplifier choice ............................................................ 15 single-to-differential driver ..................................................... 15 volt age reference input ............................................................. 15 power supply ............................................................................... 15 supplying the adc from the reference .................................. 16 digital interface .......................................................................... 16 cs mode 3-wire, no busy indicator .................................. 17 cs mode 3-wire with busy indicator ................................... 18 cs mode 4-wire, no busy indicator ..................................... 19 cs mode 4-wire with busy indicator ................................... 20 chain mode, no busy indicator ............................................ 21 chain mode with busy indicator ........................................... 22 application hints ........................................................................... 23 layout .......................................................................................... 23 evaluating the ad7688s performance .................................... 23 outline dimensions ....................................................................... 24 ordering guide .......................................................................... 25 revision history 4/05revision 0: initial version
ad7688 rev 0 | page 3 of 28 specifications vdd = 4.5 v to 5.5 v, vio = 2.3 v to vdd, v ref = vdd, t a = C40c to +85c, unless otherwise noted. table 2. parameter conditions min typ max unit resolution 16 bits analog input voltage range in+ ? in? ?v ref +v ref v absolute input voltage in+, in? ?0.1 v ref + 0.1 v common-mode input range in+, in? 0 v ref /2 v ref /2 + 0.1 v analog input cmrr f in = 250 khz 65 db leakage current at 25c acquisition phase 1 na input impedance see the analog input section accuracy no missing codes 16 bits differential linearity error ?1 0.4 +1 lsb 1 integral linearity error ?1.5 0.4 +1.5 lsb transition noise ref = vdd = 5 v 0.4 lsb gain error 2 , t min to t max 2 6 lsb gain error temperature drift 0.3 ppm/c zero error 2 , t min to t max 0.1 1.6 mv zero temperature drift 0.3 ppm/c power supply sensitivity vdd = 5v 5% 0.05 lsb throughput conversion rate 0 500 ksps transient response full-scale step 400 ns ac accuracy dynamic range v ref = 5 v 95.8 96.5 db 3 signal-to-noise f in = 20 khz, v ref = 5 v 94 95.5 db f in = 20 khz, v ref = 5 v 92.5 db spurious-free dynamic range f in = 20 khz ?118 db total harmonic distortion f in = 20 khz ?118 db signal-to-(noise + distortion) f in = 20 khz, v ref = 5 v 93.5 95 db f in = 20 khz, v ref = 5 v, ?60 db input 36.5 db intermodulation distortion 4 115 db 1 lsb means least significant bit. with the 5 v input range, one lsb is 152.6 v. 2 see the section. these specific ations do include full temperature range variation but do not include the error con tribution from the external reference. terminology 3 all specifications in db are referred to a full-scale input fs. tested with an input signal at 0.5 db below full-scale, unless otherwise specified. 4 f in1 = 21.4 khz, f in2 = 18.9 khz, each tone at ?7 db below full-scale.
ad7688 rev 0 | page 4 of 28 parameter conditions min typ max unit vdd = 4.5 v to 5.5 v, vio = 2.3 v to vdd, v ref = vdd, t a = C40c to +85c, unless otherwise noted. table 3. reference voltage range 0.5 vdd + 0.3 v load current 500 ksps, ref = 5 v 100 a sampling dynamics ?3 db input bandwidth 9 mhz aperture delay vdd = 5 v 2.5 ns digital inputs logic levels v il C0.3 +0.3 vio v v ih 0.7 vio vio + 0.3 v i il ?1 +1 a i ih ?1 +1 a digital outputs data format serial 16 bits twos complement pipeline delay conversion results available immediately after completed conversion v ol i sink = +500 a 0.4 v v oh i source = ?500 a vio ? 0.3 v power supplies vdd specified performance 4.5 5.5 v vio specified performance 2.3 vdd + 0.3 v vio range 1.8 vdd + 0.3 v standby current 1 , 2 vdd and vio = 5 v, 25c 1 50 na power dissipation vdd = 5 v, 100 sps throughput 3.75 w vdd = 5 v, 100 ksps throughput 3.75 4.3 mw vdd = 5 v, 500 ksps throughput 21.5 mw temperature range 3 specified performance t min to t max ?40 +85 c 1 with all digital inputs forced to vio or gnd as required. 2 during acquisition phase. 3 contact sales for extended temperature range.
ad7688 rev 0 | page 5 of 28 timing specifications ?40c to +85c, vdd = 4.5 v to 5.5 v, vio = 2.3 v to 5.5 v or vdd + 0.3 v, whichever is the lowest, unless otherwise stated. see figure 3 and figure 4 for load conditions. table 4. parameter symbol min typ max unit conversion time: cnv rising edge to data available t conv 0.5 1.6 s acquisition time t acq 400 ns time between conversions t cyc 2 s cnv pulse width ( cs mode ) t cnvh 10 ns sck period ( cs mode ) t sck 15 ns sck period ( chain mode ) t sck vio above 4.5 v 17 ns vio above 3 v 18 ns vio above 2.7 v 19 ns vio above 2.3 v 20 ns sck low time t sckl 7 ns sck high time t sckh 7 ns sck falling edge to data remains valid t hsdo 5 ns sck falling edge to data valid delay t dsdo vio above 4.5 v 14 ns vio above 3 v 15 ns vio above 2.7 v 16 ns vio above 2.3 v 17 ns cnv or sdi low to sdo d15 msb valid ( cs mode) t en vio above 4.5 v 15 ns vio above 2.7 v 18 ns vio above 2.3 v 22 ns cnv or sdi high or last sck falling edge to sdo high impedance ( cs mode) t dis 25 ns sdi valid setup time from cnv rising edge ( cs mode) t ssdicnv 15 ns sdi valid hold time from cnv rising edge ( cs mode) t hsdicnv 0 ns sck valid setup time from cnv rising edge (chain mode) t ssckcnv 5 ns sck valid hold time from cnv rising edge (chain mode) t hsckcnv 5 ns sdi valid setup time from sck falling edge (chain mode) t ssdisck 3 ns sdi valid hold time from sck falling edge (chain mode) t hsdisck 4 ns sdi high to sdo high (chain mode with busy indicator) t dsdosdi vio above 4.5 v 15 ns vio above 2.3 v 26 ns
ad7688 rev 0 | page 6 of 28 absolute maximum ratings table 5. p a r a m e t e r r a t i n g analog inputs in+ 1 , in? 1 gnd ? 0.3 v to vdd + 0.3 v or 130 ma ref gnd ? 0.3 v to vdd + 0.3 v supply voltages vdd, vio to g n d ?0.3 v to +7 v vdd to vio 7 v digital inputs to gnd ?0.3 v to vio + 0.3 v digital outputs to gnd ?0.3 v to vio + 0.3 v storage temperature range ?65c to +150c junction tempe r ature 150c ja thermal impedance 200c/w (msop -10) jc thermal impedance 44c/w (msop- 10) lead temperature range jedec j-std-20 1 s e e the s e ctio n. a n al o g i n p u t s t r e s s es a b o v e t h os e list e d u nde r a b s o l u te m a xim u m r a t i n g s ma y ca us e p e r m a n e n t dama ge to t h e de vi ce. t h is is a st r e ss r a t i ng on ly ; f u n c t i on a l op e r at i o n of t h e d e v i c e a t t h e s e or an y o t h e r con d i t ions a b o v e t h o s e i ndic a te d i n t h e op er a t io na l s e c t io n o f t h is sp e c if ic a t io n is no t im pl ie d . e x p o sur e t o a b s o l u te max i m u m r a t i ng co ndi t i on s fo r ex tende d p e r i o d s ma y a f fe c t de vice rel i a b i l i t y . esd caution esd (electrostatic discharge) se nsiti ve device . electrostatic charges as hi g h as 4000 v readily accumulate on the human body and test eq uipment and can discharge without detection. although this product features proprietary esd protection circ uitry, permanent dama ge may occur on devices subjected to high energy electrostatic di scharge s . ther efore, pro p er esd precautions are rec o m m ended to avoid performan c e degradation or l o ss of functiona l ity. 500 ai ol 500 ai oh 1.4v to sdo c l 50pf 02973- 003 f i gure 3 . l o a d cir c ui t fo r di g i ta l inter f a c e t i mi ng 30% vio 70% vio 2v or vio ? 0.5v 1 0.8v or 0.5v 2 0.8v or 0.5v 2 2v or vio ? 0.5v 1 t delay t delay 02973- 004 1 2v if vio above 2.5v, vio ? 0.5v if vio below 2.5v. 2 0.8v if vio above 2.5v, 0.5v if vio below 2.5v. f i g u re 4. v o lt ag e l e vels f o r ti ming
ad7688 rev 0 | page 7 of 28 pin conf igurations and f u ncti on descriptions 02973- 005 ref 1 vdd 2 in+ 3 in? 4 gnd 5 vio 10 sdi 9 sck 8 sdo 7 cnv 6 ad7688 top view (not to scale) f i g u re 5. 10-l e ad m s op pin conf ig u r at io n 02973- 006 1 ref 2 vdd 3 in+ 4 in? 5 gnd 10 vio 9 sdi 8 sck 7 sdo 6 cnv top view (not to scale) ad7688 f i g u re 6. 10-l e ad q f n 1 ( l f c s p ) p i n c o nfi g u r a t io n pin no. mnemonic type 1 function 1 qfn package in developm ent. c o ntact s a l e s f o r s amples and avail abil ity. ta ble 6. pi n f u nct i on d e s c ri pt i o ns 1 r e f a i reference input voltage. the ref range is from 0.5 v to vdd. it i s referre d to the gnd pin. t h is pin should be decoupled closely to the pin with a 10 f capacitor. 2 v d d p p o w e r s u p p l y . 3 in+ ai differential positive analog input. 4 in? ai differential negative analog input. 5 gnd p power supply g r ound. 6 c n v d i convert input. t h is input has multip le functions. on its leading edge, it initiates the conversi ons and selects the inter f ace mod e , chai n or cs . in cs mod e , it enables the sd o pin whe n lo w. in chain mod e , the d a ta should be read when cnv is high. 7 sdo do serial data output. the conversion result is output on this pin. it is synchro n ized to sck. 8 sck di serial data clock input. when the part is se lect ed , the conversi on result is shift e d out by this cl ock. 9 sdi di serial data inpu t. this input provid es multiple f e atures. it selects th e interface mode of the ad c as follows: chain mode is selected if sdi is low during the cnv risi ng edge. in this mode, sdi is used as a data input to daisy-chain the conver sion re sults of two or mor e adcs onto a si ngle sdo line. t h e digital data level on sdi is output on sdo with a delay of 16 sck cycles. cs mode is selecte d if sdi is high during the cnv ri si ng edge. in this mode, either sdi or cnv can enable the serial outpu t signals w h en l o w, and if sdi or c nv is low whe n the conversi o n is comp lete, the busy indicator feature is enabled. 1 0 v i o p input/ output in terface digital power. no min ally at the same supply as the host i n terface ( 1 .8 v, 2.5 v, 3 v, or 5 v). 1 a i = a n al og i n put, di = digital i n put, d o = digital o u tput, and p = p o w e r .
ad7688 rev 0 | page 8 of 28 terminology integral nonlinearity error (inl) it refers to the deviation of each individual code from a line drawn from negative full scale through positive full scale. the point used as negative full scale occurs ? lsb before the first code transition. positive full scale is defined as a level 1? lsb beyond the last code transition. the deviation is measured from the middle of each code to the true straight line (figure 25). differential nonlinearity error (dnl) in an ideal adc, code transitions are 1 lsb apart. dnl is the maximum deviation from this ideal value. it is often specified in terms of resolution for which no missing codes are guaranteed. zero error it is the difference between the ideal midscale voltage, that is, 0 v, from the actual voltage producing the midscale output code, that is, 0 lsb. gain error the first transition (from 100 . . . 00 to 100 . . . 01) should occur at a level ? lsb above nominal negative full scale (?4.999924 v for the 5 v range). the last transition (from 01110 to 01111) should occur for an analog voltage 1? lsb below the nominal full scale (+4.999771 v for the 5 v range.) the gain error is the deviation of the difference between the actual level of the last transition and the actual level of the first transition from the difference between the ideal levels. spurious-free dynamic range (sfdr) sfdr is the difference, in decibels (db), between the rms amplitude of the input signal and the peak spurious signal. effective number of bits (enob) enob is a measurement of the resolution with a sine wave input. it is related to s/(n+d) by the following formula enob = ( s/[ n + d ] db ? 1.76)/6.02 and is expressed in bits. total harmonic distortion (thd) thd is the ratio of the rms sum of the first five harmonic components to the rms value of a full-scale input signal and is expressed in db. dynamic range it is the ratio of the rms value of the full scale to the total rms noise measured with the inputs shorted together. the value for dynamic range is expressed in db. signal-to-noise ratio (snr) snr is the ratio of the rms value of the actual input signal to the rms sum of all other spectral components below the nyquist frequency, excluding harmonics and dc. the value for snr is expressed in db. signal-to-(noise + distortion) ratio (s/[n+d]) s/(n+d) is the ratio of the rms value of the actual input signal to the rms sum of all other spectral components below the nyquist frequency, including harmonics but excluding dc. the value for s/(n+d) is expressed in db. aperture delay aperature delay is the measure of the acquisition performance. it is the time between the rising edge of the cnv input and when the input signal is held for a conversion. transient resp onse it is the time required for the adc to accurately acquire its input after a full-scale step function was applied.
ad7688 rev 0 | page 9 of 28 typical perf orm ance cha r acte ristics code inl (lsb) 1.5 1.0 0.5 0 ? 0.5 ? 1.0 ? 1.5 0 16384 32768 49152 65535 02973- 001 positive inl = +0.31lsb negative inl = ? 0.39lsb f i gure 7 . integr a l no nli n ea ri t y vs . c o d e 02973- 007 counts 300000 250000 150000 200000 100000 50000 0 code in hex 73 74 75 6f 70 71 72 0 0 2930 0 0 256159 2031 vdd = ref = 5v f i g u re 8. his t og r a m of a dc input at t h e cod e ce nte r frequency (khz) amplitude (db of full scale) 0 ?20 ?40 ?60 ?80 ?100 ?120 ?160 ?140 ?180 0 2 5 5 0 7 5 100 125 150 200 175 225 250 02973- 008 16384 point fft vdd = ref = 5v f s = 500ksps f in = 2khz snr = 95.6db thd = ? 117.7db sfdr = ? 117.9db 2nd harm = ? 125db 3rd harm = ? 119db f i g u re 9. fft plot code dnl (lsb) 1.5 1.0 0.5 0 ? 0.5 ? 1.0 ? 1.5 0 16384 32768 49152 65535 02973- 009 positive dnl = +0.37lsb negative dnl = ? 0.21lsb f i g u re 10. d i f f e r e nt ia l n o nl in ea rit y v s . code 02973- 010 counts 160000 120000 140000 100000 80000 40000 20000 60000 0 code in hex 74 76 71 73 0 72 0 75 00 136187 124933 vdd = ref = 5v f i g u re 11. h i s t og r a m of a dc input at t h e cod e t r ans i t i o n 02973-011 input level (db) 0 ? 1 0 ? 8? 6? 4? 2 snr (db) 100 99 98 97 96 95 94 93 92 91 90 f i g u re 12. snr v s . input l e vel
ad7688 rev 0 | page 10 of 2 8 02973- 012 reference voltage (v) 5.5 2.3 2.7 3.5 4.3 5.1 3.1 3.9 4.7 snr, s/(n + d) (d b) 100 95 85 90 70 snr enob enob (bits) 17.0 15.0 16.0 14.0 13.0 s/[n + d] f i gure 13. snr , s / ( n + d), a n d e n ob v s . r e fer e nce v o ltag e 02973- 013 temperature ( c) 125 ?55 ? 35 ?15 5 25 45 65 85 105 s nr ( d b ) 100 95 90 85 80 vref = 5v f i g u re 14. snr v s . t e mper at ur e 02973- 014 frequency (khz) 200 0 5 0 100 150 s/(n + d ) (db ) 100 95 85 90 80 75 70 vref = 5v, ?10db vref = 5v, ?1db fi g u r e 1 5 . s / ( n + d ) v s . fr e q u e n c y 02973- 015 reference voltage (v) 5.5 2.3 2.7 3.5 4.3 5.1 3.1 3.9 4.7 t hd, s f dr ( d b) ?100 ?110 ?105 ?115 ?120 ?125 ?130 sfdr thd f i gure 16. thd , sf dr v s . r e fer e nce v o ltage 02973- 016 temperature ( c) 125 ?55 ? 35 ?15 5 25 45 65 85 105 t hd ( d b) ?90 ?100 ?110 ?120 ?130 vref = 5v f i g u re 17. th d v s . t e mper at ur e 02973- 017 frequency (khz) 200 0 5 0 100 150 t hd ( d b) ?60 ?70 ?90 ?80 ?100 ?110 ?120 vref = 5v, ?10db vref = 5v, ?1db f i g u re 18. th d v s . f r equ e nc y
ad7688 rev 0 | page 11 of 2 8 supply (v) o perating current ( a) 1000 750 500 250 0 4.50 4.75 5.00 5.25 5.5 02973- 018 vio vdd f s = 100ksps f i gure 19. o p er atin g currents v s . sup p l y temperature ( c) po wer-do wn current (na) 1000 750 500 250 0 ?55 ? 35 ?15 5 25 45 65 85 105 125 02973- 019 vdd + vio f i gure 20. p o wer - d o wn cur r ents vs. t e mp e r atu r e temperature ( c) o perating current ( a) 1000 750 500 250 0 ?55 ? 35 ?15 5 25 45 65 85 105 125 02973- 020 vio vdd f s = 100ksps f i gure 21. o p er atin g currents v s . t e mper atu r e 02973- 021 temperature ( c) 125 ?55 ? 35 ?15 5 25 45 65 85 105 o ffset, g a in er r o r (lsb ) 6 4 2 0 ?2 ?4 ?6 gain error offset error f i gure 22. o ffs et a n d g a in e r r o r v s . t e mpe r atu r e 02973-022 sdo capacitive load (pf) 120 0 2 04 06 0 8 0 1 0 0 t dsdo delay (ns) 25 20 15 10 5 0 vdd = 5v, 85 c vdd = 5v, 25 c f i g u re 23. t dsdo d e l a y v s . capa cit a nc e l oad and su p p ly
ad7688 rev 0 | page 12 of 2 8 sw+ msb 16,384c in+ lsb comp control logic switches control busy output code cnv ref gnd in? 4c 2c c c 32,768c sw? msb 16,384c lsb 4c 2c c c 32,768c 02973- 023 f i gur e 2 4 . adc simpl i f ie d s c hema ti c circuit information the ad7688 is a fas t , lo w p o w e r , sin g le-su p p l y , p r ecis e 16-b i t ad c usin g a su ccessi v e a p p r o x i m a t io n a r chi t e c t u r e . the ad7688 is ca p a b l e o f con v er tin g 500,000 s a m p les p e r s e co nd (500 ks ps) a nd p o w e rs do wn betw een c o n v ersio n s. w h en op era t ing a t 100 s p s, f o r exa m p l e , i t co ns um es 3 . 75 w t y p i c a l l y , i d e a l f o r b a t t e r y - p o w e r e d ap p l i c at i o n s . the ad7688 p r o v ides t h e us er wi th an o n -c hi p trac k-and-h o ld an d d o e s not e x h i b i t a n y pi p e l i ne d e l a y or l a te nc y , m a k i ng i t ide a l f o r m u l t i p l e m u l t i p lexe d c h a nne l a p p l ic a t io n s . the ad7688 is s p ecif ie d f r o m 4.5 v t o 5.5 v and can b e i n te r f a c e d to an y of t h e 1 . 8 v to 5 v di g i t a l l o g i c f a mi ly . i t i s h o us e d i n a 10-l e ad msop o r a t i n y 10-l e ad qf n 1 (lfcs p ) tha t co m b i n es s p ac e s a v i n g s and al lo ws f l exi b le co nf i g ura t io n s . i t is p i n-f o r - p i n-co m p a t i b l e wi t h t h e ad7685 , ad7686 , an d ad7687 . 1. qfn pack age in dev e lopmen t. con t ac t s a les f o r s amples and a v ail abil it y . converter operation the ad7688 is a s u cces s i v e a p p r o x ima t ion ad c bas e d o n a ch ar ge re d i st r i but i on d a c . fi g u re 2 4 show s t h e s i m p l i f i e d s c h e ma t i c o f t h e ad c. th e c a p a ci t i v e d a c con s is ts o f tw o iden t i c a l a r ra y s o f 16 b i na r y w e ig h t e d c a p a ci t o rs, w h ich a r e co nn e c te d to t h e tw o co m p a r a t o r in p u ts. d u r i n g t h e ac quisi t io n phas e , ter m ina l s o f t h e a r ra y t i e d t o t h e c o m p ar a t or s i n put are c o n n e c t e d to g n d v i a s w + a n d s w ? . al l i ndep e nden t sw i t ch es a r e c o nn e c te d to t h e a n a l o g in p u ts. t h u s , t h e c a p a c i tor ar r a y s are u s e d a s s a m p l i ng c a p a c i tors an d a c q u i r e th e a n a l o g s i gn a l o n th e i n + a n d in ? i n p u t s . w h e n th e acq u isi t ion pha s e is co m p le t e and t h e c n v i n pu t go es hig h , a co n v ersio n phas e is ini t i a t e d . w h e n t h e con v ers i o n phas e beg i n s , sw+ and sw? a r e o p ened f i rst. th e two ca p a ci t o r a r r a y s a r e t h e n dis c o n n e c t e d f r o m t h e i n p u ts and co n n e c te d to t h e gnd i n p u t. ther efo r e , t h e dif f er en t i al v o l t a g e b e tw e e n t h e in p u ts in+ and in? ca p t ur e d a t th e end o f t h e acq u isi t ion p has e is a p plie d t o t h e co m p a r a t o r i n pu ts, c a usin g t h e co m p a r a t o r t o becom e u n b a lan c e d . b y swi t c h in g each e l emen t o f th e ca p a c i t o r ar r a y b e t w e e n g n d a n d r e f , t h e c o m p ar a t or i n put v a r i e s b y b i na r y w e ig h t e d v o l t a g e s t eps (v ref /2, v ref /4 . . . v ref /65536). the co n t r o l log i c t o g g l es t h es e s w i t ch es, s t a r t i ng wi t h t h e ms b , in o r der t o b r ing th e co m p a r a t or bac k in t o a b a l a n c e d co n d i ti o n . a f t e r th e co m p le ti o n o f th i s p r oce s s, th e pa r t r e t u rn s t o t h e acq u isi t ion phas e and t h e co n t r o l log i c g e n e ra t e s t h e ad c o u t p u t c o de an d a b u s y sig n a l i ndic a to r . b e ca us e t h e ad7688 has a n o n -bo a r d con v ersio n c l o c k, t h e s e r i al clo c k, sc k, is n o t r e q u ir e d fo r t h e con v ersio n p r o c es s.
ad7688 rev 0 | page 13 of 2 8 tra n sfer f u nctions the ideal tra n sf er c h a r ac t e r i s t ic f o r th e ad7688 is s h o w n in f i gur e 25 an d t a b l e 7. 100...000 100...001 100...010 011...101 011...110 011...111 adc co de (two s co m p lem e nt) analog input +fsr ? 1.5 lsb + fsr ? 1 lsb ?fsr + 1 lsb ?fsr ?fsr + 0.5 lsb 02973- 024 f i g u re 25. a d c ide a l t r ans f er f u nc t i o n ta ble 7. out p ut codes a n d i d ea l input volt a g es description analo g input v re f = 5 v digital o u tput co de hexa fsr C 1 lsb +4.999847 v 7fff 1 midscale + 1 lsb +152.6 v 0001 midscale 0 v 0000 midscale C 1 lsb ?152.6 v ffff Cfsr + 1 lsb ?4.999847 v 8001 Cfsr ?5 v 8000 2 typical connection diagram f i gur e 26 sh o w s a n exa m ple o f t h e r e co mm e nde d co nn e c t i on dia g ram f o r th e ad7688 w h en m u l t i p le s u p p lies a r e a v a i la b l e . 1. t h is is al so the c o d e f o r an ov er ranged anal og input ( v in + ? v in ? abov e v re f ? v gnd ). 2. t h is is al so the c o d e f o r an und e r r anged anal og input ( v in + ? v in ? belo w ?v re f + v gnd ). ad7688 ref gnd vdd in? in+ vio sdi sck sdo cnv 3- or 4-wire interface 5 100nf 100nf 5v 10 f 2 7v 7v ?2v 1.8v to vdd ref 1 0 to vref 33 ? 2.7nf 3 4 7v ?2v vref to 0 33 ? 2.7nf 3 4 02973- 025 1 see reference section for reference selection. 2 c ref is usually a 10 f ceramic capacitor (x5r). 3 see driver amplifier choice section. 4 optional filter. see analog input section. 5 see digital interface for most convenient interface mode. f i g u re 26. t y pic a l a p pli c at ion d i ag r a m w i t h m u lt ipl e su p p li es
ad7688 rev 0 | page 14 of 2 8 analog input f i gur e 27 sh o w s a n eq uivalen t c i r c ui t o f t h e in p u t s t r u c t ur e o f th e ad7688. the t w o dio d es, d1 a nd d2, p r o v ide es d p r o t e c t i o n fo r t h e a n alog in p u ts i n + a nd i n ?. c a r e m u s t be taken t o en sur e tha t t h e analog in p u t sig n al n e v e r exce e d s t h e s u p p l y ra i l s b y m o r e t h a n 0.3 v b e ca us e t h is c a us es t h es e di o d es t o b e g i n t o fo r w a r d- b i a s an d st a r t co nd uc t i ng c u r r en t. t h es e dio d e s ca n hand le a f o r w a r d-b i as ed c u r r en t o f 130 ma maxim u m. f o r in s t an ce , t h es e con d i t io ns co u l d e v e n t u al l y o c c u r w h e n t h e in pu t b u f f er s (u1) s u p p lies a r e dif f er en t f r o m vd d . i n s u ch a cas e , an i n p u t b u f f er wi t h a sho r t-cir c ui t c u r r en t li mi t a t i on can b e us e d t o prote c t t h e p a r t . c in r in d1 d2 c pin in+ or in? gnd vdd 02973-026 f i g u re 27. equiv a le nt a n al og input c i rcuit the a n alog i n pu t s t r u c t ur e al lo ws t h e s a m p lin g o f t h e t r ue dif f er en t i al sig n al b e tw e e n i n + a nd in?. by usin g t h es e dif f er en t i al i n p u ts, sig n als co mm o n t o b o t h i n p u ts a r e r e je c t e d , as s h o w n in f i g u r e 28, w h ich r e p r es en ts t h e typical cm rr o v er fr e q u e n c y . 02973- 027 frequency (khz) 10000 1 1 0 100 1000 cmrr ( d b) 80 70 60 vdd = 5v fi g u r e 2 8 . a n a l o g i n p u t c m r r v s . fr e q u e n c y d u r i n g t h e ac quisi t io n phas e , t h e im p e dan c e of t h e a n alog in p u ts (in+ o r in?) ca n b e mo dele d as a p a r a l l e l com b ina t io n o f ca p a ci t o r , c pin , a n d th e n e t w o r k f o rm e d b y th e s e ri e s c o n n e c t i on of r in a nd c in . c pi n is p r ima r i l y t h e p i n c a p a ci t a n c e . r in is typ i cal l y 6 00 ? a n d is a l u m p e d co m p o n en t made u p o f s o me s e r i al r e sis t o r s a nd t h e on r e sis t a n ce o f t h e swi t ch es. c in is typ i c a l l y 30 pf a nd is ma inl y the ad c s a m p ling ca p a ci t o r . d u r i n g t h e con v ersio n phas e , w h er e t h e s w i t ch es a r e o p e n e d , t h e in p u t im p e da n c e is l i mi t e d to c pi n . r in and c in ma k e a 1-p o le , lo w-p a ss f i l t er tha t r e d u ces undesira b l e alias i n g ef fe c t s a nd lim i ts t h e no is e. w h en t h e s o ur c e im p e dan c e o f t h e dr ivin g cir c ui t is lo w , t h e ad7688 can be dr i v en dir e c t l y . l a rg e s o ur ce im p e dan c es sig n if ican t l y a f f e c t t h e ac p e r f o r ma n c e , e s p e c i a l ly t o t a l h a r m on i c d i stor t i on ( t h d ) . t h e d c p e r f or m a n c e s are l e ss s e n s i t i v e t o t h e i n p u t im p e dan c e . th e maxim u m s o ur ce i m ped a n c e d e pe n d s o n th e a m o u n t o f t h d t h a t c a n b e tol e r a te d. t h e t h d de g r a d e s a s a f u nc t i on of t h e s o u r c e im p e dan c e and t h e max i m u m i n p u t f r e q ue n c y , as sho w n in f i gur e 29. frequency (khz) thd (db) ?60 ?70 ?80 ?90 ?100 ?110 ?120 0 2 5 5 0 7 5 100 02973- 028 r s = 250 ? r s = 100 ? r s = 50 ? r s = 33 ? f i g u re 29. th d v s . a n al og input f r equ e nc y and s o urc e r e s i s t anc e
ad7688 rev 0 | page 15 of 2 8 driver am plifier ch o i ce al t h o u g h t h e ad7688 is easy t o dr i v e , t h e dr i v er a m p l if ier n e e d s t o m e et t h e fol l o w in g r e quir em e n ts : ? the n o is e g e n e r a t e d b y t h e dr i v er a m plif ier n e e d s t o b e k e p t as lo w as p o s s i b le in o r der t o p r es er v e th e s n r and tra n si tio n n o is e p e r f o r ma n c e o f th e ad7688. n o t e tha t t h e ad7688 has a no is e m u ch lo w e r tha n m o s t o f t h e o t h e r 16-b i t ad cs and , t h er efo r e , ca n b e dr i v en b y a n o isier o p a m p w h i l e p r es er vin g t h e s a m e o r b e t t e r sys t e m p e r f o r m- a n c e . th e n o is e co min g f r o m t h e dr i v er is f i l t ere d b y t h e ad7688 a n alog in p u t cir c ui t 1-p o le , lo w-p a s s f i l t er made by r in and c in or b y t h e ext e r n a l f i l t er , if o n e is us e d . b e ca us e t h e typ i cal n o is e o f t h e ad7688 is 53 v r m s, t h e s n r deg r a d a t io n d u e t o t h e a m plif ier is ? ? ? ? ? ? ? ? ? ? ? ? ? ? + = ? 2 2 ) ( 2 5 53 20log n 3db loss ne f snr 3 w h er e: f C3db is th e in p u t ba ndwid th in mh z o f t h e ad7688 (9 mh z) o r t h e c u t o f f f r e q uen c y o f t h e i n p u t f i l t er , if o n e is us e d . n is t h e n o is e ga in o f t h e am pli f ier (fo r exa m ple , +1 in b u f f er co nf igura t io n). e n is t h e e q uival e n t i n p u t n o is e v o l t a g e o f t h e op a m p , in nv/ h z . ? fo r a c ap p l i c at i o n s , t h e d r i v e r s h o u l d h a v e a t h d p e r f o r ma n c e commen s ura t e wi th t h e ad7688. f i gur e 18 s h o w s t h e th d vs. f r e q uen c y t h a t t h e dr i v er sh o u ld e x ceed . ? f o r m u l t icha n n e l m u l t i p lexe d a p plica t io n s , t h e dr i v er a m p l if ier an d t h e ad7688 a n alo g in p u t cir c ui t m u s t s e t t l e f o r a f u l l -s cale st ep o n t o t h e ca p a ci t o r a r ra y a t a 16-b i t leve l (0.0015%, 15 p p m). i n t h e am p l if ier s da t a sh eet, s e t t ling a t 0.1% t o 0.01 % is m o r e co mm onl y s p ecif ie d . this co u l d d i f f e r si gn i f i c a n tl y f r o m th e set t li n g tim e a t a 16- b i t lev e l a nd sh o u ld b e ver if ie d p r io r to dr i v er s e le c t io n. table 8. r e com m ende d dri v er amplifiers amplifier typical applica t ion ad8021 very low noise a n d high frequency ad8022 low noise and high frequency op184 low pow er, lo w noise, a n d low fr equency ad8605 , ad8615 5 v single-supp l y, low pow er ad8519 small, l o w p o we r and low freque ncy ad8031 high frequency and low p o wer single-to-differ ential driver f o r a p plica t ion s usin g a sin g le -e n d e d a n a l og sig n a l , ei t h er b i p o la r o r uni p ola r , a sin g le -e nde d- to - d if fer e n t ia l dr i v er al lo ws fo r a dif f er en t i al i n p u t in t o t h e p a r t . the s c h e ma t i c is sho w n i n f i gur e 30. w h en p r o v i d e d a sin g le -e nde d i n p u t sig n a l , t h is co nf igura t i o n p r o d uces a dif f er en t i al v ref wi t h m i ds ca le at v ref /2. u2 10k ? 590 ? ad7688 in+ in? ref u1 analog input ( 10v, 5v, ..) 590 ? 10 f 100nf 10k ? vref vref 590 ? 100nf vref 02973-029 f i gure 30. sing le-ended-t o - d i f f e rent i a l d r ive r c i rcuit voltage r e ference input the ad7688 v o l t a g e r e f e r e n c e in p u t, ref , has a d y na mic in p u t im p e dan c e and sh o u l d t h er efo r e b e dr i v e n b y a lo w i m p e dan c e s o ur ce w i t h ef f i cien t de co u p li ng b e tw e e n t h e r e f a n d g n d p i n s , as ex plai ne d i n t h e l a yo ut s e c t ion. w h en ref is dr i v en b y a v e r y lo w im p e dan c e s o ur ce , fo r exa m ple , a r e fer e n c e b u f f er usin g t h e ad8031 or t h e ad8605 , a 10 f (x5r , 08 05 size) cera mic c h i p c a p a ci t o r is a p p r o p r i a t e f o r opt i m u m p e r f or m a nc e. i f a n u n b u f f er ed r e fer e n c e v o l t a g e is us e d , t h e de co u p ling val u e dep e n d s on t h e r e f e r e n c e us ed . f o r in s t an ce , a 22 f (x5r , 1206 size) ceramic c h i p ca p a c i t o r is a p p r o p r i a t e f o r o p tim u m p e r f o r ma n c e usin g a lo w t e m p era t ur e dr if t adr43x re f e re nc e. i f d e s i re d, s m a l l e r re f e re nc e d e c o up l i ng c a p a c i t o r v a lu e s d o w n t o 2.2 f ca n be us ed wi t h a minimal im p a c t o n p e r f o r ma n c e, es p e c i al l y d n l. rega r d les s , t h er e is n o n e e d fo r a n addi t i o n al lo w e r val u e cera mic deco u p lin g c a p a ci t o r (f o r exa m p l e , 100 nf) between t h e ref a nd gn d p i n s . power supply the ad7688 is s p ecif ie d a t 4.5 v t o 5.5 v . i t has, unlik e o t h e r lo w v o l t a g e con v er t e rs, a lo w e n o u g h n o is e t o desig n a 16- b i t re s o l u t i on s y ste m wi t h l o w su pply and re sp e c t a bl e p e r f or manc e. i t u s e s two p o we r su p p ly p i ns : a c o re su p p ly v d d and a dig i t a l in p u t/o u t p ut i n ter f ace su p p ly v i o . vi o a l lo ws dir e c t in ter f ace wi th an y log i c b e tw een 1.8 v and vd d . t o r e d u ce t h e s u p p l i es n e e d ed , the vi o a nd vd d can b e tie d t o g e t h er . the ad7688 is inde p e n d e n t o f p o w e r su p p ly s e q u en c i n g b e tw e e n vi o and v d d . a d di t i ona l ly , i t is ve r y ins e ns i t ive to p o we r su p p ly
ad7688 rev 0 | page 16 of 2 8 v a r i a t i o ns o v e r a w i d e f r e q u e nc y r a nge, a s sh ow n i n f i g u re 3 1 , w h ich r e p r es en t s ps rr o v er f r e q uen c y . 02973- 030 frequency (khz) 10000 1 1000 10 100 p s rr ( d b) 95 90 85 80 75 70 65 60 fi g u r e 3 1 . p s r r v s . fr e q u e n c y the ad7688 p o w e rs do wn a u t o ma tic a l l y a t t h e end o f eac h co n v ersio n phas e a n d , t h er efo r e , t h e p o w e r s c ale s line a r l y w i t h th e s a m p l i n g ra te , as sh o w n in f i gur e 32. this mak e s t h e p a r t ide a l f o r lo w s a m p lin g r a t e ( e ven a f e w h z ) a nd lo w b a t t e r y - p o w e r e d ap p l i c at i o n s . sampling rate (sps) operati n g current ( a) 1000 10 0.1 0.001 10 100 1000 10000 100000 1000000 02973- 031 vio vdd f i gure 3 2 . o p er a t i n g c u rrents vs . s a mpl i n g r a te supplying the adc fr om the reference f o r sim p lif i ed a p p l ica t io n s , t h e ad7688, wi t h i t s lo w o p era t in g c u r r e n t , c a n b e supp l i e d d i re c t l y u s i n g t h e re f e re nc e c i rc u i t s h o w n i n f i gur e 33. the r e fer e nce li n e c a n b e dr i v en b y ei t h er : ? the s y ste m p o we r su p p ly dire c t l y . ? a re f e re nc e vo lt age w i t h e n ou g h c u r r e n t output c a p a bi l i t y , s u c h as t h e adr43x. ? a r e f e r e n c e b u f f er , s u c h as t h e ad8031, w h ic h ca n als o f i l t e r t h e s y ste m p o we r su p p ly , a s shown in fig u re 3 3 . ad8031 ad7688 vio ref vdd 10 f 1 f 10 ? 10k ? 5v 5v 5v 1 f 1 02973- 032 1 optional reference buffer and filter. f i g u re 33. e x a m pl e of a p pl ic at io n ci r c uit digi tal in t e rface th oug h t h e ad7688 has a r e d u ced n u m b er o f p i n s , i t o f f e rs f l e x ib i l it y i n i t s s e r i a l i n te r f a c e mo d e s . the ad7688 , w h en in cs m o de, is co m p a t i b le wi t h s p i, qs p i , dig i t a l h o sts, and ds p s , e. g., bla c k f in? adsp - b f 53x o r ads p - 219x. this in t e r f ace can us e ei t h er 3-wir e o r 4-wir e . a 3-wir e in t e r f ace usin g t h e c n v , sck, and s d o sig n als minimizes wir i n g co n n e c t i o n s us ef u l , fo r in st an ce , in i s ola t e d a p plic a t io n s . a 4- wir e i n t e r f ace usin g t h e s d i, cnv , sck, and s d o sig n als al lo ws c n v , whic h ini t ia t e s t h e co n v ersio n s, t o be inde p e n d e n t o f th e r e ad bac k timin g (s d i ). this is us ef u l in l o w ji t t e r sa m p l i n g o r s i m u l t a n eo u s sa m p l i n g a p p l i c a t i o n s . the ad7688 , w h en in c h a i n mo de , p r o v ides a da isy c h a i n f e a t ur e usin g t h e s d i in p u t f o r cas c adin g m u l t i p le ad cs o n a s i ng l e d a t a l i ne s i m i l a r to a sh i f t re g i ste r . the m o de i n w h ich t h e p a r t o p era t es de p e n d s o n t h e s d i le ve l w h en t h e cnv r i sin g e d g e o c c u rs. the cs m o d e is s e le c t e d if s d i is hig h and th e c h a i n m o de is s e lec t e d if s d i is lo w . the s d i h o ld t i m e is s u c h tha t w h en s d i an d c n v a r e co nn ec t e d t o g e t h er , t h e cha i n m o de is alw a ys s e le c t e d . i n ei t h er m o de , th e ad7688 o f f e rs th e f l exib il i t y t o o p tio n al l y fo r c e a st a r t b i t i n f r o n t o f t h e da t a b i ts. this st a r t b i t can b e us e d as a b u s y sig n a l i ndic a to r to in ter r u p t t h e dig i t a l h o st and t r ig ger t h e d a t a r e ading. o t h e r w is e , wi t h o u t a b u s y i n d i c a t o r , t h e us er m u s t t i m e o u t t h e maxim u m con v ersi o n t i m e p r io r t o re a d b a c k . the b u s y i n di c a t o r fe a t ur e is e n a b le d as: ? in t h e cs mo de , if cnv o r s d i is l o w w h en t h e ad c co n v ersio n ends (f igur e 37 a nd f i gur e 41). ? i n t h e c h a i n m o de , if sc k is hig h d u r i n g t h e c n v r i sin g e d g e (f igur e 45).
ad7688 rev 0 | page 17 of 2 8 cs mode 3-wire, no busy indicator this m o de is usual l y us ed w h en a sin g le ad768 8 is co nn ec t e d t o a n s p i-co m p a t ib le dig i t a l h o s t . the co nn ec tio n dia g ra m is sho w n i n f i gur e 34 a nd t h e co r r esp o nd in g t i mi n g is g i ven in f i gur e 35. w i t h s d i tie d to vi o , a r i sin g edg e o n c n v ini t ia t e s a co n v ersio n , s e le c t s t h e cs m o de , and fo r c es s d o to hig h im p e dan c e . o n ce a con v ersio n is ini t i a t e d , i t con t i n ues t o co m p le ti o n i r r e s p ecti v e o f th e sta t e o f cn v . f o r i n s t a n ce , i t co u l d b e us ef u l t o b r in g c n v l o w t o s e lec t o t her s p i de vices , s u c h a s a n a l o g mu l t i p l e x e r s , b u t c n v mu s t b e r e t u r n e d h i g h be f o r e th e m i n i m u m co n v e r si o n tim e a n d h e ld h i gh un til t h e m a x i m u m c o n v e r s i on t i me to avoi d t h e ge ne r a t i on of t h e bu s y sig n al indic a t o r . w h en t h e con v ersio n is co m p l e t e , t h e ad7688 en t e rs t h e acq u i s i t io n phas e and p o w e rs do wn. w h en cnv g o es lo w , t h e m s b is o u t p ut o n to s d o . the r e ma ini n g da t a b i ts a r e th en c l oc k e d b y s u b s eq ue n t sc k falli n g e d g e s . th e d a ta i s v a l i d on b o t h s c k e d ge s . a l t h ou g h t h e r i s i ng e d ge c a n b e u s e d t o c a p t u r e th e d a t a , a d i gi ta l h o s t u s i n g th e s c k f a ll i n g ed g e a l lo ws a fa st er r e adi n g ra te p r o v id e d i t has an a c cep t ab le h o ld t i m e . af t e r t h e 16t h sc k fal l i n g e d g e o r w h en cnv g o es hig h , w h iche v e r is e a rlier , s d o r e t u r n s t o hig h i m p e d a n c e . cnv sck sdo sdi data in clk convert v io digital host ad7688 02973-033 f i g u re 34. cs mode 3-w i r e , n o b u sy ind i c a tor c o nne c t ion d i agr a m (sdi high) sdo d15 d14 d13 d1 d0 t dis sck 1 2 3 14 15 16 t sck t sckl t sckh t hsdo t dsdo cnv conversion acquisition t conv t cyc acquisition sdi = 1 t cnvh t acq t en 02973- 034 f i g u re 35. cs mode 3-w i r e , n o b u sy ind i c a tor s e ri al inte r f a c e ti ming (s di hig h )
ad7688 rev 0 | page 18 of 2 8 cs mode 3-wire with busy indicator this m o de is usual l y us ed w h en a sin g le ad768 8 is co nn ec t e d t o a n s p i-co m p a t ib le dig i t a l h o s t ha vin g an in ter r u p t in p u t. the co n n e c t i on di a g r a m is sh o w n i n f i gur e 36 a nd t h e co r r es p o n d in g t i min g is g i v e n in f i gur e 37. w i t h s d i tie d to vi o , a r i sin g edg e o n c n v ini t ia t e s a co n v ersio n , s e le c t s t h e cs m o de , and fo r c es s d o to hig h im p e dan c e . sd o is ma i n t a i n e d in h i g h im p e dan c e u n t i l t h e co m p let i o n o f t h e con v ersion ir r e s p e c t i v e o f t h e s t a t e o f cnv . pr io r to t h e mi n i m u m con v ersi o n t i m e , cn v c o u l d b e us e d to s e lec t other s p i de vices, s u c h as a n alog m u l t i p lexers, b u t c n v m u s t be r e t u rn e d lo w be f o r e th e mini m u m con v ersio n t i m e and he ld lo w un ti l t h e maxim u m c o n v ersio n t i m e t o gua r a n t e e t h e g e n e ra t i o n o f t h e b u s y sig n al i ndic a t o r . w h en t h e co n v ersion is co m p let e , s d o g o es f r o m hig h im p e dan c e t o lo w . w i t h a p u l l -u p o n t h e s d o lin e , t h is tr a n si tion can b e us ed as a n i n te r r upt s i g n a l to i n i t i a te t h e d a t a re a d i n g c o n t rol l e d b y t h e dig i tal h o s t . the ad7688 t h en en t e rs t h e acq u isi t io n p h as e and p o w e rs do w n . th e da t a b i ts a r e t h e n clo c k e d o u t, ms b f i rst, b y s u b s e q ue n t sck fall in g ed g e s . t h e d a t a i s val i d o n bo t h sc k e d g e s . a l th o u gh th e ri s i n g ed g e c a n b e u s ed t o c a p t u r e th e d a ta , a dig i tal h o st usin g t h e s c k fal l i n g edge al lo ws a fas t er r e adin g r a te prov i d e d it h a s an a c c e pt a b l e ho l d t i me. a f te r t h e opt i on a l 17th sck fall in g ed g e , o r w h en c n v g o e s hi g h , wh i c h e v e r i s e a rl i e r , sd o re tu r n s to h i g h i m p e da nc e. i f m u l t i p le ad7 688s a r e s e lec t e d a t t h e s a m e t i m e , t h e s d o o u t p ut p i n hand les t h is co n t en t i o n wi t h ou t dama ge o r i n d u ce d la tch-up . m e an w h i l e, i t is r e commende d to k e ep t h is co n t en t i o n as sh o r t as p o ss ib le t o lim i t ext r a p o w e r dissi p a t io n. data in irq clk convert vio digital host 02973-035 47k ? cnv sck sdo sdi v io ad7688 f i g u re 36. cs mode 3-w i r e with b u sy i n d i c a to r c o nne c t ion d i agr a m (sdi high) sdo d15 d14 d1 d0 t dis sck 12 3 1 5 1 6 1 7 t sck t sckl t sckh t hsdo t dsdo cnv conversion acquisition t conv t cyc t cnvh t acq acquisition sdi = 1 02973- 036 f i g u re 37. cs m o d e 3 - wi re w i t h bus y i n d i ca to r s e r i a l i n te r f a c e t i m i n g ( s d i h i g h )
ad7688 rev 0 | page 19 of 2 8 cs mode 4-wire, no busy indicator this m o de is usual l y us ed w h en m u l t i p le ad76 88s a r e co nn e c te d to a n s p i-co m p a t i b le dig i t a l h o st. a co nn ec tion dia g ra m exam p l e usin g tw o ad7 688s is s h o w n in f i gur e 38 an d t h e co r r es p o ndin g timin g is g i ven in f i gur e 39. w i t h s d i hig h , a r i sin g edg e on cnv ini t ia t e s a co n v ersio n , se l e ct s t h e cs m o d e , a nd fo r c es s d o to hig h i m p e dan c e. i n t h is m o de , c n v m u s t b e h e l d hig h d u r i n g t h e con v ersio n phas e and th e s u bseq uen t da ta r e ad ba c k (if s d i a n d c n v a r e lo w , s d o i s dr i v en lo w). p r io r t o t h e mini m u m con v ersio n t i m e , s d i co u l d be us e d t o s e lec t o t her s p i devices, s u c h as a n alog m u l t i p lexers, b u t s d i m u s t b e r e t u r n ed hig h bef o r e the minim u m con v ersion tim e a n d h e ld hi gh un til t h e m a xi m u m co n v e r si o n tim e t o a v o i d t h e g e n e r a t i o n o f t h e b u s y sig n al indi ca t o r . w h en t h e co n v ersio n is com p let e , t h e ad7688 en t e rs t h e acq u isi t ion phas e an d p o w e rs do w n . e a ch ad c r e su l t c a n b e r e a d b y b r in g i n g lo w i t s s d i in p u t w h ich co n s e q uen t ly o u t p uts t h e msb o n t o s d o . the r e ma ini n g da t a b i ts a r e t h en clo c k e d b y s u b s e q ue n t sck fall in g ed g e s . t h e d a t a i s val i d o n bo t h sc k e d g e s . a l th o u gh th e ri s i n g ed g e c a n b e u s ed t o c a p t u r e th e d a ta , a dig i tal h o st usin g t h e s c k fal l i n g edge al lo ws a fas t er r e adin g r a te prov i d e d it h a s an a c c e pt a b l e ho l d t i me. a f te r t h e 1 6 t h s c k fa l l in g e d ge , o r w h en s d i go es hig h , w h i c he v e r is e a rlier , s d o r e t u r n s to hi g h i m p e dan c e an d a n o t h e r a d 768 8 ca n b e r e a d . data in clk cs1 convert cs2 digital host 02973- 037 cnv sck sdo sdi ad7688 cnv sck sdo sdi ad7688 f i g u re 38. cs mo de 4 - w i r e , no b u sy ind i c a to r c o nnectio n di a g r a m sdo d15 d14 d13 d1 d0 t dis sck 12 3 3 0 3 1 3 2 t hsdo t dsdo t en conversion acquisition t conv t cyc t acq acquisition sdi(cs1) cnv t ssdicnv t hsdicnv d1 14 15 t sck t sckl t sckh d0 d15 d14 17 18 16 sdi(cs2) 02973- 038 f i g u re 39. cs mode 4-w i r e , n o b u sy ind i c a tor s e ri al inte r f a c e ti ming
ad7688 rev 0 | page 20 of 2 8 cs mode 4-wire with busy indicator this m o de is usual l y us ed w h en a sin g le ad768 8 is co nn ec t e d t o a n s p i-co m p a t ib le dig i t a l h o s t , w h ic h has a n in t e r r u p t in p u t, a nd i t is desir e d t o k e ep cnv , which is us ed t o s a m p le t h e a n a l o g in p u t, i ndep e nden t o f t h e sig n a l us e d to s e le c t t h e da t a r e ading. this r e q u ir e m en t is p a r t ic u l a r ly im p o r t a n t in a p plic a t io n s w h er e lo w j i t t e r o n cnv is desir e d. the co n n e c t i on di a g r a m is sh o w n i n f i gur e 40 a nd t h e co r r es p o n d in g t i min g is g i v e n in f i gur e 41. w i t h s d i hig h , a r i sin g e d g e on cnv ini t ia t e s a co n v ersio n , se l e ct s t h e cs m o d e , a nd fo r c es s d o to hig h i m p e dan c e. i n t h is m o de , c n v m u s t b e h e l d hig h d u r i n g t h e con v ersio n phas e and th e s u bs eq uen t da ta r e adbac k (if s d i a n d cnv a r e lo w , s d o is dr i v en lo w). p r io r t o t h e mini m u m con v ersio n t i m e , s d i co u l d be us e d t o s e lec t o t her s p i devices, s u c h as a n alog m u l t i p lexers, b u t s d i m u s t b e r e t u r n ed lo w bef o r e the minim u m con v ersion tim e an d he ld l o w un t i l t h e maxim u m co n v ersio n tim e t o g u ar an te e t h e g e ne r a t i on of t h e b u s y s i g n a l i n d i c a tor . whe n t h e co n v ersion i s co m p let e , s d o g o es f r o m hig h i m p e dan c e t o lo w . w i th a p u ll- u p o n t h e s d o li n e , th i s tra n si t i o n ca n be used a s an i n te r r upt s i g n a l to i n i t i a te t h e d a t a re a d b a ck c o n t rol l e d b y th e dig i tal h o st. the ad7688 t h en en t e rs t h e ac q u isi t io n p h as e a nd p o w e rs do w n . t h e d a t a b i ts a r e t h en clo c k e d o u t, m s b f i rst, b y s u bs eq uen t s c k fal l i n g e d g e s. th e da ta is va lid o n bo t h sc k e d g e s . a l th o u gh th e ri s i n g ed g e c a n b e u s ed t o c a p t u r e th e d a ta , a dig i tal h o st usin g t h e s c k fal l i n g edge al lo ws a fas t er r e adin g r a te prov i d e d it h a s an a c c e pt a b l e ho l d t i me. a f te r t h e opt i on a l 17t h sc k fa l l i n g e d ge , o r s d i go in g hi g h , w h ich e v e r is e a rlier , th e s d o r e t u rn s t o h i gh im pe da n c e . data in irq clk convert cs1 vio digital host 02973- 039 47k ? cnv sck sdo sdi ad7688 f i g u re 40. cs mode 4- w i r e with b u sy i n d i c a tor conne c t ion d i agr a m sdo d15 d14 d1 d0 t dis sck 12 3 1 5 1 6 1 7 t sck t sckl t sckh t hsdo t dsdo t en conversion a cquisition t conv t cyc t acq acquisition sdi cnv t ssdicnv t hsdicnv 02973- 040 f i g u re 41. cs mode 4-w i r e with b u sy i n d i c a to r s e r i a l in ter f a c e ti ming
ad7688 rev 0 | page 21 of 2 8 chain mo de, no busy i n dic a t o r this m o de can be us e d t o da isy-c h a i n m u l t i p le ad7688s o n a 3 - wir e s e r i a l i n t e r f ace . t h is fe a t ur e is us ef u l fo r r e d u cin g co m p on en t co u n t an d wir i n g c o nn ec t i o n s, f o r exa m p l e , in is ola t e d m u l t icon v e r t er a p pli c a t io n s o r fo r sy st em s w i t h a l i mite d i n te r f a c i n g c a p a c i t y . d a t a re a d b a ck i s a n a l o g ou s to cl o c k i ng a sh if t re g i ste r . a co nn ec tion dia g ra m exam p l e usin g tw o ad7 688s is s h o w n in f i gur e 42 an d t h e co r r es p o ndin g timin g is g i ven in f i gur e 43. w h en s d i a nd cnv a r e lo w , s d o is dr i v en lo w . w i t h sc k lo w , a r i sin g e d g e on cnv ini t ia t e s a co n v ersio n , s e l e c t s t h e c h a i n m o de, and dis a b l es t h e b u s y i ndic a to r . i n t h is m o de, c n v is held h i g h d u r i ng t h e co n v ersio n phas e and t h e subs e q uen t da t a r e ad back. w h en t h e con v ersion is co m p let e , t h e ms b is ou t p u t o n t o s d o and t h e ad7688 en t e rs th e acq u isi t io n p has e and po w e r s d o wn . th e r e m a i n in g da ta b i t s s t o r ed in t h e in t e rnal s h i f t r e gi s t e r a r e th en c l oc k e d b y s u b s eq uen t sc k falli n g e d g e s . f o r ea c h a d c , s d i f eed s th e i n p u t o f th e in t e r n al s h i f t r e gi s t e r a nd is clo c k e d b y t h e sc k fa l l i ng e d ge. e a ch a d c in t h e cha i n o u t p u t s i t s da t a ms b f i rs t, an d 1 6 n c l o c ks a r e r e q u ir e d t o r e ad b a ck t h e n ad cs. the da t a is valid on b o t h sck e d g e s. al t h o u g h t h e r i sin g e d g e ca n b e us e d t o c a pt ur e t h e da t a , a dig i t a l h o st using t h e s c k f a l l ing e d ge a l lo ws a f a st er r e ad in g ra t e an d, co n s e q uen t l y m o r e ad7688s in t h e cha i n, p r o v ided th e dig i tal h o s t has a n acce p t able h o ld t i m e . the maxim u m co n v ersio n r a t e ma y b e r e d u ce d d u e t o t h e t o t a l r e ad b a ck t i me . f o r i n s t a n ce , wi th a 3 n s di gi ta l h o s t s e t - u p tim e a n d 3 v in t e r f ace , u p t o f o ur ad7688s r u nnin g a t a con v ersio n ra t e o f 360 ks ps can be da isy-c h ain e d o n a 3-wir e p o r t . clk convert data in digital host 02973- 041 cnv sck sdo sdi ad7688 b cnv sck sdo sdi ad7688 a f i gure 42. chain m o de , n o b u sy ind i c a tor conne c t ion d i agr a m s do a = sdi b d a 15 d a 14 d a 13 sck 1 2 3 30 31 32 t ssdisck t hsdisc t en conversion acquisition t conv t cyc t acq acquisition cnv d a 1 14 15 t sck t sckl t sckh d a 0 17 18 16 sdi a = 0 sdo b d b 15 d b 14 d b 13 d a 1 d b 1d b 0d a 15 d a 14 t hsdo t dsdo t ssckcnv t hsckcnv d a 0 02973- 042 f i gure 43. cha i n m o de , n o b u sy ind i c a to r s e r i a l in ter f ace tim i ng
ad7688 rev 0 | page 22 of 2 8 chain mode with bus y indicator this m o de can als o be us e d t o da isy-c h ain m u l t i p le ad7688s o n a 3-wi r e se r i al i n t e rfa c e wh ile p r o v id in g a b u s y in d i ca t o r . this f e a t ur e is us ef u l f o r r e d u cin g co m p on en t c o un t and wir i n g co nn ec t i o n s, f o r exa m p l e , in is ol a t e d m u l t ico n ver t er a p plic a t io n s o r fo r sy st em s w i t h a lim i t e d i n t e r f a c in g c a p a ci ty . da t a r e ad ba ck i s a n alog o u s t o c l oc ki n g a s h i f t r e gi s t er . a co nn ec tion dia g ra m exam p l e usin g thr e e ad7688s is s h o w n in f i gur e 44 and t h e co r r esp o ndin g t i m i n g is g i ven i n f i gur e 4 5 . w h en s d i a nd cnv a r e lo w , s d o is dr i v en lo w . w i t h sc k hig h , a r i sin g e d g e o n cnv ini t ia t e s a con v ersion, s e le c t s t h e cha i n m o de, and ena b les t h e b u s y i ndi ca to r fe a t ur e. i n t h is m o de , c n v is he ld hig h d u r i n g t h e co n v ersion phas e an d t h e s u bs e q uen t da t a r e ad back. w h en al l ad cs in t h e c h ain ha v e co m p let e d t h eir co n v ersio n s, t h e n e a r end ad c (ad c c i n f i gur e 44) s d o is dr i v en hig h . this tra n si t i o n o n s d o can be used a s a b u s y i n d i ca t o r t o tri g g e r th e da ta r e ad ba ck co n t r o lled b y t h e dig i t a l hos t . th e a d 768 8 t h e n en t e rs t h e acq u isi t io n phas e an d p o w e rs do w n . the da t a b i ts sto r e d i n t h e in ter n a l shif t r e g i st er a r e t h e n clo c k e d ou t, ms b f i rst, b y subs e q uen t sc k falli n g e d ge s . f o r ea c h a d c , s d i f eed s th e i n p u t o f th e in t e r n a l sh if t r e g i st er a nd is clo c k e d b y t h e sc k fa l l i n g e d ge . e a ch ad c i n t h e cha i n ou t p u t s i t s da t a ms b f i r s t, an d 16 n + 1 c l oc k s a r e r e q u i r e d t o r e a d b a c k th e n a d c s . a l th o u gh th e r i sin g edg e can be us e d t o c a p t ur e th e da t a , a dig i tal h o s t usin g t h e s c k f a l l in g e d g e al lo ws a fas t er r e adin g ra t e a nd co n s eq uen t l y mo r e ad7688s in th e c h ain, p r o v ided the dig i t a l h o st has an acc e p t a b le h o ld t i me. f o r in st an ce, wi t h a 3 n s dig i tal h o s t s e t u p time an d 3 v in t e r f ac e , u p t o f o ur ad7688s r u nnin g a t a con v ersio n r a t e o f 360 ks ps can be da isy-c h ain e d to a s i ng l e 3 - w i re p o r t . clk convert data in irq digital host 02973- 043 cnv sck sdo sdi ad7688 c cnv sck sdo sdi ad7688 a cnv sck sdo sdi ad7688 b f i gure 44. chain m o de with b u sy i n dic a tor c o nn ec tion d i agr a m sdo a = sdi b d a 15 d a 14 d a 13 sck 123 35 47 48 t en conversion a cquisition t conv t cyc t acq acquisition cnv = sdi a d a 1 41 5 t sck t sckh t sckl d a 0 17 34 16 sdo b = sdi c d b 15 d b 14 d b 13 d a 1 d b 1d b 0d a 15 d a 14 49 t ssdisck t hsdisc t hsdo t dsdo sdo c d c 15 d c 14 d c 13 d a 1d a 0 d c 1d c 0d a 14 19 31 32 18 33 d b 1d b 0d a 15 d b 15 d b 14 t dsdosdi t ssckcnv t hsckcnv 02973-044 d a 0 t dsdosdi t dsdosdi t dsdosdi t dsdosdi f i gure 45. cha i n m o de with b u sy i n di c a tor s e ri al inte r f a c e ti ming
ad7688 rev 0 | page 23 of 2 8 appli c ation hints layout the p r in t e d c i r c ui t b o a r d t h a t ho us es t h e ad76 88 s h o u l d b e desig n e d s o t h a t t h e ana l o g a nd dig i t a l s e c t io n s a r e s e p a r a te d a nd co nf i n e d to cer t a i n a r e a s o f t h e b o a r d . t h e p i n o u t o f t h e ad7688, wi t h a l l i t s a n alog sig n als o n the lef t side an d al l i t s dig i tal sig n als on t h e r i g h t side , e a s e s this tas k . a v o i d r u nnin g dig i t a l li n e s und e r t h e de vic e b e ca us e t h e s e co u p le n o is e o n to t h e di e, un less a g r o u nd plane under t h e ad7688 is us ed as a s h ie l d . f a s t swi t c h in g sig n a l s, s u c h as cnv o r c l o c ks, sh o u ld n e v e r r u n n e ar a n alog sig n al p a th s. cr os s o ve r of d i g i t a l a n d a n a l o g s i g n a l s sh ou l d b e a v oi d e d a t l e a s t o n e g r ou n d pl a n e s h ou l d b e u s e d . i t c o u l d b e c o m m on o r s p li t b e tw ee n th e d i g i tal a n d a n alog secti o n s . i n t h e la t t e r cas e , t h e p l an es s h o u l d be jo in e d u n der n e a th t h e ad7688s. the ad7688 v o l t a g e r e f e r e n c e in p u t ref has a d y na mic in p u t im p e dan c e and sh o u l d b e de cou p le d w i t h mi ni ma l p a r a si t i c ind u c t an ce s. th is is do ne b y pl a c in g t h e r e fer e nce de co u p l i n g cera mic c a p a ci to r clos e t o , a nd ide a l l y r i g h t u p a g a i nst, t h e re f a nd gn d p i n s a nd co n n e c t i n g i t wi t h wi de, lo w im p e dan c e tra c e s . f i nal l y , t h e p o wer s u p p lies vdd a nd vi o o f th e ad7688 s h o u l d be decou p led wi t h ceramic ca p a c i t o rs ( t yp ic al l y 100 nf ) p l aced c l os e t o t h e ad7688 an d co nn ec t e d using s h o r t an d wide t r aces t o p r o v i d e lo w i m p e danc e p a t h s and r e d u ce t h e ef fe c t o f g l i t c h e s on t h e p o we r su p p ly l i ne s . an exa m ple o f l a yo u t fol l o w in g t h es e r u les is s h o w n i n f i gur e 46 an d f i gur e 47. evaluating the ad7688s performance o t h e r r e co mm en de d la yo u t s f o r th e ad7688 a r e o u tlin e d in t h e do c u m e n t a t ion o f the eva l u a t io n bo a r d f o r th e ad7688 ( e v al -ad7688 ). the e v a l u a t ion b o a r d p a cka g e i n cl udes a f u l l y ass e m b le d an d t e st e d e v a l ua t i o n b o a r d , d o c u m e n t a t ion, a nd s o f t wa r e fo r co n t r o l l in g t h e b o a r d f r o m a pc v i a t h e ev a l -c on tr ol br d 3 . 02973-045 f i g u re 46. e x a m pl e of layo ut of t h e a d 76 88 ( t op l a yer) 02973-046 f i g u re 47. e x a m pl e of layo ut of t h e a d 76 88 (bot t o m laye r)
ad7688 rev 0 | page 24 of 2 8 outline dimensions 0.23 0.08 0.80 0.60 0.40 8 0 0.15 0.00 0.27 0.17 0.95 0.85 0.75 seating plane 1.10 max 10 6 5 1 0.50 bsc 3.00 bsc 3.00 bsc 4.90 bsc pin 1 coplanarity 0.10 compliant to jedec standards mo-187-ba f i g u re 48. 10-l e ad m i ni s m al l o u t l ine p a ck ag e [m sop ] (r m - 10) di me nsio ns sho w n i n mi ll im e t e r s 3.00 bsc sq index area top view 1.50 bcs sq exposed pad (bottom view) 1.74 1.64 1.49 2.48 2.38 2.23 1 6 10 0.50 bsc 0.50 0.40 0.30 5 pin 1 indicator 0.80 0.75 0.70 0.05 max 0.02 nom seating plane 0.30 0.23 0.18 0.20 ref 0.80 max 0.55 typ side view paddle connected to gnd. this connection is not required to meet the electrical performances f i gure 49. 1 0 -l ead l e ad f r a m e ch ip s c a l e p a ck ag e [qfn 1 (lf c sp_ w d)] 3 mm 3 m m b o d y , v e r y v e r y th in, d u a l l e ad (c p - 1 0 - 9 ) di me nsio ns sho w n i n mi ll im e t e r s 1 qfn package in developm ent. c o ntact s a l e s f o r s amples and avail abil ity.
ad7688 rev 0 | page 25 of 28 ordering guide model integral nonlinearity temperature range transport media, quantity package description package option branding ad7688brm 1.5 lsb max C40c to +85c tube, 50 10-lead msop rm-10 c04 AD7688BRMRL7 1.5 lsb max C40c to +85c reel, 1,000 10-lead msop rm-10 c04 eval-ad7688cb 1 evaluation board eval-control brd2 2 controller board eval-control brd3 2 controller board 1 this board can be used as a standalone evaluation board or in conjunction with the eval-control brdx for evaluation/demonstrat ion purposes. 2 these boards allow a pc to control and communicate with all analog devices evaluation boards ending in the cb designators.
ad7688 rev 0 | page 26 of 28 notes
ad7688 rev 0 | page 27 of 28 notes
ad7688 rev 0 | page 28 of 2 8 notes ?2005 a n alo g d e vi ces, inc. all rights reserve d . tra d emar ks an d registered tra d emar ks are the prop erty of their respective owners . d02973C0C 4/05(0)

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