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16-bit, 2.5 msps, pulsar 15.5 mw adc in lfcsp data sheet ad7985 rev. c document feedback information furnished by analog devices is believed to be accurate and reliable. however, no responsibility is assumed by analog devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. specifications subject to change without notice. no license is granted by implication or otherwise under any patent or patent rights of analog devices. trademarks and registered trademarks are the property of their respective owners. one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781.329.4700 ?2009C2016 analog devices, inc. all rights reserved. technical support www.analog.com features 16-bit resolution with no missing codes throughput: 2.5 msps (turbo hi gh), 2.0 msps (turbo low) low power dissipation 15.5 mw at 2.5 msps, with external reference 28 mw at 2.5 msps, with internal reference inl: 0.7 lsb typical, 1.5 lsb maximum snr 88.5 db, with on-chip reference 90 db, with external reference 4.096 v internal reference: typical drift of 10 ppm/c pseudo differential analog input voltage range 0 v to v ref with v ref up to 5.0 v allows use of any input range no pipeline delay logic interface: 1.8 v/2.5 v/2.7 v proprietary serial interface spi-/qspi?-/microwire?-/dsp- compatible 1 ability to daisy-chain multiple adcs with busy indicator 20-lead, 4 mm 4 mm lfcsp applications battery-powered equipment communications ate data acquisition systems medical instruments application diagram 0v to v ref notes 1. gnd refers to refgnd, agnd, and dgnd. ad7985 gnd ref avdd, dvdd vio bvdd 5v 2.5v 1.8v to 2.7v vio sdi sck sdo cnv 3- or 4-wire interface: spi, cs, daisy chain (turbo = low) turbo 10f in+ in? 07947-001 figure 1. general description the ad7985 1 is a 16-bit, 2.5 msps successive approximation analog-to-digital converter (sar adc). it contains a low power, high speed, 16-bit sampling adc, an internal conversion clock, an internal reference (and buffer), error correction circuits, and a versatile serial interface port. on the rising edge of cnv, the ad7985 samples an analog input, in+, between 0 v and ref with respect to a ground sense, in?. the ad7985 features a very high sampling rate turbo mode (turbo is high) and a reduced power normal mode (turbo is low) for low power applications where the power is scaled with the throughput. in normal mode (turbo is low), the spi-compatible serial inter- face also features the ability, using the sdi input, to daisy-chain several adcs on a single 3-wire bus and provide an optional busy indicator. it is compatible with 1.8 v, 2.5 v, and 2.7 v supplies using the separate vio supply. the ad7985 is available in a 20-lead lfcsp with operation specified from ?40c to +85c. 1 protected by u. s. patent 6,703,961. table 1. msop, lfcsp, 14-/16-/18-bit pulsar? adcs 1 type 100 ksps 250 ksps 400 ksps to 500 ksps 1000 ksps adc driver 14-bit ad7940 ad7942 2 ad7946 2 16-bit ad7680 ad7685 2 ad7686 2 ad7980 2 ada4941-1 ad7683 ad7687 2 ad7688 2 ad7983 2 ada4841-1 ad7684 ad7694 ad7693 2 ad7985 3 ad8021 18-bit ad7691 2 ad7690 2 ad7982 2 ada4941-1 ad7984 2 ada4841-1 ad7986 3 ad8021 1 see www.analog.com for the latest selection of pulsar adcs and adc drivers. 2 pin for pin compatible with all other devices marked with this endnote. 3 the ad7985 and ad7986 are pin for pin compatible.
ad7985* product page quick links last content update: 04/14/2017 comparable parts view a parametric search of comparable parts. evaluation kits ? ad7985 evaluation board documentation application notes ? an-742: frequency domain response of switched- capacitor adcs data sheet ? ad7985: 16-bit, 2.5 msps, pulsar 15.5 mw adc in lfcsp data sheet technical books ? the data conversion handbook, 2005 user guides ? ug-727: evaluating the ad7944/ad7985/ad7986 14-/16-/ 18-bit, pulsar adcs reference designs ? cn0260 reference materials product selection guide ? sar adc & driver quick-match guide technical articles ? ms-1779: nine often overlooked adc specifications ? ms-2210: designing power supplies for high speed adc ? ms-2689: isolating spi for high bandwidth sensors tutorials ? mt-002: what the nyquist criterion means to your sampled data system design ? mt-031: grounding data converters and solving the mystery of "agnd" and "dgnd" design resources ? ad7985 material declaration ? pcn-pdn information ? quality and reliability ? symbols and footprints discussions view all ad7985 engineerzone discussions. sample and buy visit the product page to see pricing options. technical support submit a technical question or find your regional support number. document feedback submit feedback for this data sheet. this page is dynamically generated by analog devices, inc., and inserted into this data sheet. a dynamic change to the content on this page will not trigger a change to either the revision number or the content of the product data sheet. this dynamic page may be frequently modified.
ad7985 data sheet rev. c | page 2 of 28 table of contents features .............................................................................................. 1 ? applications ....................................................................................... 1 ? application diagram ........................................................................ 1 ? general description ......................................................................... 1 ? revision history ............................................................................... 2 ? specifications ..................................................................................... 3 ? timing specifications .................................................................. 5 ? absolute maximum ratings ............................................................ 6 ? esd caution .................................................................................. 6 ? pin configuration and function descriptions ............................. 7 ? typical performance characteristics ............................................. 9 ? terminology .................................................................................... 12 ? theory of operation ...................................................................... 13 ? circuit information .................................................................... 13 ? converter operation .................................................................. 13 ? conversion modes of operation .............................................. 13 ? typical connecti on diagram ................................................... 14 ? analog inputs ............................................................................. 15 ? driver amplifier choice ........................................................... 15 ? voltage reference input ............................................................ 16 ? power supply ............................................................................... 16 ? digital interface .............................................................................. 17 ? data reading options ............................................................... 18 ? cs mode, 3-wire without busy indicator ............................. 19 ? cs mode, 3-wire with busy indicator .................................... 20 ? cs mode, 4-wire without busy indicator ............................. 21 ? cs mode, 4-wire with busy indicator .................................... 22 ? chain mode without busy indicator ...................................... 23 ? chain mode with busy indicator ............................................. 24 ? applications information .............................................................. 25 ? layout .......................................................................................... 25 ? evaluating the ad7985 performance ...................................... 25 ? outline dimensions ....................................................................... 27 ? ordering guide .......................................................................... 27 ? revision history 3/16rev. b to rev. c changes to table 1 ............................................................................ 1 change to endnote 3, table 1 ......................................................... 3 change to signal-to-noise-and-distortion ratio parameter, table 2 ................................................................................................ 3 deleted endnote 4, table 2 .............................................................. 3 changes to figure 4 .......................................................................... 7 changes to figure 23 ...................................................................... 14 changes to driver amplifier choice section ............................. 15 change to reference decoupling section ................................... 16 changes to reading during conversion, fast host (turbo or normal mode) section and split-reading, any speed host (turbo or normal mode) section ................................................ 18 changes to figure 31 ...................................................................... 21 updated outline dimensions ....................................................... 27 changes to ordering guide .......................................................... 27 7/14rev. a to rev. b added patent endnote ...................................................................... 1 changes to figure 21 ...................................................................... 13 changes to data reading options section ................................. 18 updated outline dimensions ....................................................... 27 8/10rev. 0 to rev. a change to table 4, conversion time: cnv rising edge to data available ................................................................................ 5 9/09revision 0: initial version
data sheet ad7985 rev. c | page 3 of 28 specifications av dd = dvdd = 2.5 v, bvdd = 5 v, vio = 1.8 v to 2.7 v, v ref = 4.096 v, t a = ? 40c to +85c, unless otherwise noted. table 2 . parameter test conditions /comments min typ max unit resolution 16 bits analog input voltage range ( in+ ) ? ( in? ) 0 v ref v absolute input voltage in+ ?0.1 v ref + 0.1 v in? ?0.1 + 0.1 v leakage current at 25c acquisition phase 250 na input impedance see the analog inputs section accuracy no missing codes 16 bits differential nonl inearity error , dnl ?0.9 9 0. 5 0 + 0.9 9 lsb 1 integral nonl inearity error , inl ? 1 .50 0. 7 + 1 .50 lsb 1 transition noise 0.8 lsb 1 gain error, t min to t max 2 ? 15 2 + 15 lsb 1 gain error temperature drift 0. 8 ppm/c zero error, t min to t max 2 ?0.9 9 0. 08 +0.9 9 mv zero temperature drift 0.55 ppm/c power supply sensitivity avdd = 2.5 v 5% 90 db 3 throughput conversion rate 0 2. 5 msps transient response full - scale step 100 ns ac accuracy dynamic range v ref = 4.096 v, internal reference 87. 5 89 db 3 v ref = 5.0 v, external reference 89. 0 9 0 db 3 signal - to - noise ratio, snr f in = 20 khz, v ref = 4.096 v, internal reference 87 . 0 88 . 5 db 3 f in = 20 khz, v ref = 5.0 v, external reference 89. 0 9 0 . 0 db 3 spurious - free dynamic range, sfdr f in = 20 khz 103 db 3 total harmonic distortion, thd f in = 20 khz, v ref = 4.096 v, internal reference ?1 00 db 3 signal -to - noise - and - distortion ratio , sinad f in = 20 khz, v ref = 4.096 v 90 db 3 sampling dynamics ?3 db input bandwidth 19 mhz aperture delay 0.7 ns 1 lsb means least significant bit. wi th the 4.096 v input ra nge, one lsb is 62.5 v. 2 see the terminology section. these specifications include full temperature range variation but not the error contribut ion from the external reference. 3 all specifications expressed in decibels are referred to a full - scale input fs r and t ested with an input signal at 0.5 db below full scale, unless otherwise specified .
ad7985 data sheet rev. c | page 4 of 28 table 3 . parameter test conditions /comments min typ max unit internal reference pdref is low output voltage t a = 25c 4.081 4.096 4.111 v temperature drift ?40c to +85c 10 ppm/c line regulation avdd = 2.5 v 5% 5 0 ppm/v turn - on settling time c ref = 10 f, c refin = 0.1 f 4 0 ms refin output voltage refin at 25c 1.2 v refin output resistance 6 k? external reference pdref is high , refin is low voltage range 2.4 5.1 v current drain 500 a reference buffer refin input voltage range 1.2 v refin input current 160 a digital inputs logic levels v il ?0.3 0.1 vio v v ih 0.9 vio vio + 0.3 v i il ? 1 + 1 a i ih ? 1 + 1 a digital outputs data format serial 16 bits, straight binary 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 avdd, dvdd 2.375 2.5 2.625 v bvdd 4.75 5.0 5.25 v vio specified performance 1.8 2.5 2.7 v standby current 1 , 2 avdd = dvdd = vio = 2.5 v 1.0 a power dissipation with internal reference 2.5 msps throughput 28 33 mw 2.0 msps throughput 25 30 mw with ex ternal reference 2 .5 msps throughput 1 5.5 17 mw 2.0 msps throughput 12 13 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 req uired. 2 during acquisition phase. 3 contact an analog devices, inc., sales representative for the extended temperature range.
data sheet ad7985 rev. c | page 5 of 28 timing specification s avdd = dvdd = 2.5 v, bvdd = 5 v, vio = 1.8 v to 2.7 v, v ref = 4.096 v, t a = ?40c to +85c, unless otherwise noted. 1 table 4 . parameter symbol test conditions/comments min typ max unit conversion time: cnv rising edge to data available t conv turbo m ode/ n ormal m ode 320/ 42 0 ns acquisition time t acq 8 0 ns time between conversions t cyc turbo m ode/ n ormal m ode 4 00/ 50 0 ns cnv pulse width t cnvh cs m ode 10 ns data read during conversion t data turbo m ode/ n ormal m ode 190/29 0 ns quiet time during acquisition from last sck falling edge to cnv rising edge t quiet 20 ns sck period t sck cs m ode 9 ns t sck chain m ode 11 ns sck low time t sckl 3.5 ns sck high time t sckh 3.5 ns sck falling edge to data remains valid t hsdo 2 ns sck falling edge to data valid delay t dsdo 4 ns cnv or sdi low to sdo d1 5 msb valid t en 5 ns cnv or sdi high or last sck falling edge to sdo high impedance t dis cs m ode 8 ns sdi valid setup time from cnv rising edge t ssdicnv 4 ns sdi valid hold time from cnv rising edge t hsdicnv cs m ode 0 ns t hsdicnv chain m ode 0 ns sck valid setup time from cnv rising edge t ssckcnv chain m ode 5 ns sck valid hold time from cnv rising edge t hsckcnv chain m ode 5 ns sdi valid setup time from sck falling edge t ssdisck chain m ode 2 ns sdi valid hold time from sck falling edge t hsdisck chain m ode 3 ns sdi high to sdo high t dsdosdi chain m ode with b usy i ndicator 1 5 ns 1 see figure 2 and figure 3 for load conditions. 500a i ol 500a i oh 1.4v to sdo c l 20pf 07947-002 figure 2 . load circuit for digital interface timing 90% vio 10% vio v ih 1 v il 1 v il 1 v ih 1 t delay t delay 1 minimum v ih and maximum v il used. see digital inputs specifications in table 3. 07947-003 figure 3 . voltage levels for timing
ad7985 data sheet rev. c | page 6 of 28 absolute maximum rat ings table 5 . parameter rating analog inputs in+, in? to gnd 1 ?0.3 v to v ref + 0.3 v or 130 ma supply voltage ref, bvdd to gnd , refgnd ?0.3 v to +6.0 v a vdd , dvdd , vio to gnd ?0.3 v to +2.7 v a vdd , dvdd to vio ?6 v to +3 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 temperature 150c ja thermal impedance 2 0 - lead lfcsp 30.4 c/w lead temperatures vapor phase (60 sec) 215c infrared (1 5 sec) 220c 1 see the analog inputs section for an explanation of in+ and in?. stresses at or above those listed under absolute maximum ratings may cause permanent damage to the product. this is a stress rating only; functional operation of the product at these or any other conditions above those indicated in the operational section of this specification is not implied. operation beyond the maximum operating conditions for extended periods may affect product reliabili ty. esd caution
data sheet ad7985 rev. c | page 7 of 28 pin configuration an d function descripti ons 07947-004 notes 1. the exposed pad is not connected internally. for increased reliability of the solder joints, it is recommended that the pad be soldered to the system ground plane. 14 13 12 1 3 4 sdi 15 turbo cnv sck 11 dvdd ref refgnd 2 ref refgnd 5 in? 7 pdref 6 in+ 8 vio 9 sdo 10 dgnd 19 bvdd 20 refin 18 agnd 17 agnd 16 avdd ad7985 top view (not to scale) figure 4 . pin configuration table 6 . pin function descriptions pin no. mnemonic type 1 description 1, 2 ref ai reference output/ input voltage. when pdref is low , the internal reference and buffer are enabled , producing 4.096 v on this pin. when pdref is high , the internal reference and buffer are disabled, allowing an externally supplied voltage reference up to 5.0 v. decoupling is required with or without the internal reference and buffer. this pin is referred to the refgnd pins and must be decoupled cl osely to the refgnd pins with a 10 f capacitor. 3, 4 refgnd ai reference input analog ground. 5 in ? ai analog input ground sense. connect this pin to the analog ground plane or to a remote ground sense . 6 in + ai analog input. this pin is referred to in?. the voltage range, that is, the difference between in+ and in?, is 0 v to v ref . 7 pdref di internal reference power - down input. when this pin is low, the internal reference is enabled. when this pin is high, the internal refe rence is powered down and an external reference must be used. 8 vio p input/output interface digital power. nominally at the same supply voltage as the host interface (1.8 v, 2.5 v, or 2.7 v). 9 sdo do serial data output. the conversion result is output on this pin. it is synchronized to sck. 10 dgnd p digital power ground. 11 dvdd p digital power. nominally at 2.5 v. 12 sck di serial data clock input. when the device is selected, the conversion result is shifted out by this clock. 13 cnv di conver t input. this input has multiple functions. on the leading edge, it initiates the conversions and selects the interface mode of the device : chain mode or cs mode. in cs mode, the sdo pin is enabled when cnv is low. in chain mode, the data must be read when cnv is high. 14 sdi di serial data input. this input has multiple functions. it selects the inter face mode of the adc as follows. chain mode is selected if sdi is low during the cnv rising edge. in chain mode, sdi is used as a data input to daisy - chain the conversion results of two or more adcs onto a single sdo line. the digital data level on sdi is output on sdo with a delay of 1 6 sck cycles. cs mode is selected if sdi is high during the cnv rising edge. in cs mode, either sdi or cnv can enable the serial output signals when low. if sdi or cnv is low when the conversion is complete, the busy indicator feature is enabled. 1 5 turbo di conversion mode selection. when turbo is high , the maximum throughput ( 2.5 msps ) is achieved , and t he adc does not power down between conversions. when turbo is low , the maximum throughput is lower ( 2.0 msps) , and t he adc powers down between con versions. 16 avdd p input analog power. nominally at 2.5 v. 17, 18 agnd p analog power ground.
ad7985 data sheet rev. c | page 8 of 28 pin no. mnemonic type 1 description 19 bvdd p reference b uffer p ower. nominally at 5 .0 v . if an external reference buffer is used to achieve the maximum snr performance with a 5 v reference, the reference buffer must be powered down by connecting the refin pin to ground. the external reference buffer must be connected to the bvdd pin . 20 refin ai/o internal reference output/reference buffer input. when pdref is low , the internal band gap reference produces a 1.2 v (typical) voltage on this pin, which needs external decoupling (0.1 f typical). when pdref is high , use an external reference to provide 1.2 v (typical) to this pin. when pdref is high and refin is low , the on - chip reference buffer and the band gap reference are powered down. an external reference must be connected to ref and bvdd. 21 exposed pad ep the exposed pad is not connected internally. for increased reliability of the solder joints, it is recommended that the pad be soldered to the system ground plane. 1 ai = analog input , ai/o = bidirectional analog, di = digital input, do = digital output, and p = power .
data sheet ad7985 rev. c | page 9 of 28 typical performance characteristics avdd = dvdd = vio = 2.5 v, bvdd = 5.0 v, v ref = 5.0 v , e xternal r eference (pdref is h igh, refin is low), unless otherwise noted. 1.25 ?1.25 ?1.00 ?0.75 ?0.50 ?0.25 0 0.25 0.50 0.75 1.00 0 16,384 32,768 49,152 65,536 inl (lsb) code positive inl = +0.38lsb negative inl = ?0.46lsb 07947-105 figure 5 . integral nonlinearity vs. code 60,000 50,000 40,000 30,000 20,000 10,000 0 counts code in hex 7ffc 7ffd 7ffe 7fff 8000 8001 8002 8003 8004 0 122 6150 38,843 57,138 26,249 2537 33 0 07947-106 figure 6. histogram of dc input at code center (external reference) 60,000 50,000 40,000 30,000 20,000 10,000 0 counts code in hex 7ffd 7ffc 7ffb 7ffe 7fff 8000 8001 8002 8003 8004 8005 0 11 565 33,064 49,585 31,957 7024 403 8 0 8455 07947-107 figure 7. histogram of dc input at code center (internal reference) 1.00 ?1.00 ?0.75 ?0.50 ?0.25 0 0.25 0.50 0.75 0 16,384 32,768 49,152 65,536 dnl (lsb) code positive dnl = +0.19lsb negative dnl = ?0.20lsb 07947-108 figure 8 . differential nonlinearity vs. code 60,000 50,000 40,000 30,000 20,000 10,000 0 counts code in hex 7ffd 7ffc 7ffb 7ffe 7fff 8000 8001 8002 8003 8004 0 2 479 11,888 48,730 53,522 15,645 800 6 0 07947-109 figure 9. histogram of dc input at code transition (external reference) 50,000 45,000 40,000 35,000 30,000 25,000 20,000 15,000 10,000 5000 0 counts code in hex 7ffe 7ffd 7ffc 7fff 8000 8001 8002 8003 8004 8005 0 90 2947 20,474 46,649 43,622 15,598 1662 30 0 07947-110 figure 10 . histogram of dc input at code transition (internal reference)
ad7985 data sheet rev. c | page 10 of 28 0 ?180 ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 1250 1000 750 500 250 amplitude (db) frequency (khz) f s = 2.5msps f in = 20khz snr = 89.87db thd = ?102.76db sinad = 89.66db 07947-111 figure 11 . fft plot (external reference) 100 95 90 85 80 16 15 14 13 12 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 4.75 5.00 snr, sinad (db) enob (bits) reference voltage (v) snr sinad enob 07947-112 figure 12 . snr, sinad, and enob vs. reference voltage 95 90 85 80 75 70 65 1 1000 100 10 sinad (db) frequency (khz) 07947-113 figure 13 . sinad vs. frequency 0 ?180 ?160 ?140 ?120 ?100 ?80 ?60 ?40 ?20 0 1250 1000 750 500 250 amplitude (db) frequency (khz) f s = 2.5msps f in = 20khz snr = 88.45db thd = ?103.42db sinad = 88.32db 07947-114 figure 14 . fft plot (internal reference) ?90 ?95 ?100 ?105 ?110 100 95 90 85 80 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 4.75 5.00 thd (db) sfdr (db) reference voltage (v) sfdr thd 07947-115 figure 15 . thd and sfdr vs. reference voltage ?85 ?90 ?95 ?100 ?105 ?110 1 1000 100 10 thd (db) frequency (khz) 07947-116 figure 16 . thd vs. frequency
data sheet ad7985 rev. c | page 11 of 28 95 94 93 92 91 90 89 88 87 86 85 ?10 ?9 ?8 ?7 ?6 ?5 ?4 ?3 ?2 ?1 0 snr (db) input level (dbfs) 07947-117 figure 17 . snr v s. input level 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 2.375 2.625 i avdd i dvdd i vio i bvdd i ref 2.575 2.525 2.475 2.425 operating current (ma) avdd and dvdd voltage (v) 07947-118 figure 18 . operating current vs. supply voltage 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 ?55 ?35 ?15 5 25 45 65 85 105 125 i avdd i bvdd i ref operating current (ma) temperature (c) 07947-119 figure 19 . operating current v s. temperature 14 12 10 8 6 4 2 0 ?55 ?35 ?15 5 25 45 65 85 105 125 supply current (a) temperature (c) i avdd + i dvdd + i vio 07947-120 figure 20 . power - down current vs. temperature
ad7985 data sheet rev. c | page 12 of 28 terminology aperture delay aperture delay is the measure of the acquisition performance and is the time between the rising edge of the cnv input and when the input signal is held for a conversion. 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. dynamic range dynamic range 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 deci bels. it is mea - sured with a signal at ?60 dbf s so that it includes all noise sources and dnl artifacts. effective number of bits (enob) enob is a measurement of the resolution with a sine wave input. it is expressed in bits and is related to sinad as follows: enob = ( sinad db ? 1.76)/6.02 effective resolution effective resolution is expressed in bits and is calculated as follows: effective resolution = log 2 (2 n / rms input noise ) gain error the last transition (from 111 10 to 111 1 1) must occur for an analog voltage 1? lsb below the nominal full scale (4.999886 v for the 0 v to 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 bet ween the ideal levels. integral nonlinearity error (inl) inl 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 tr ansition. 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 (see figure 22). noise - free code resolution noise - free code resolution is the number of bits beyond which it is impossible to distinctly resolve individual codes. it is expressed in bits and is calculated as follows: noise - free code resolution = log 2 (2 n / peak - to - peak noise ) signal -to - noise ratio (snr) snr is the ratio of the rms value of the actual input signal to the rms sum of all other spectral components below the nyquist frequency, excluding harmonics and dc. the value for snr is expressed in decibels. signal -to - noise - and - distortion ratio (sinad) sinad is the ratio of the rms value of the actual input signal to the rms sum of all other spectral components below the nyquist frequency, including harmonics but excluding dc. the value for sinad is expressed in decibels. spurious - free dynamic range (sfdr) sfdr is the difference, in decibels (db), between the rms amplitude of the input signal and the peak spurious sig nal. total harmonic distortion (thd) thd is the ratio of the rms sum of the first five harmonic components to the rms value of a full - scale input signal and is expressed in decibels. transient response transient response is the time required for the adc to accurately acquire the input after a full - scale step function is applied. zero error zero error 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.
data sheet ad7985 rev. c | page 13 of 28 th e ory of operation 07947-005 com p switches contro l bus y output code cnv contro l logic sw+ lsb sw+ lsb in+ ref gnd in? msb msb c c 4c 2c 16,384c 32,768c c c 4c 2c 16,384c 32,768c figure 21 . adc simplified schematic circuit information the ad7985 is a fast, low power, single - supply, precise , 16- bit adc using a successive approximation architecture . the ad7985 features differe nt modes to optimize performance according to the application. in turbo mode, the ad7985 is capable of convert ing 2,5 00,000 samples per second ( 2.5 msps ). the ad7985 provides the user with an on - chip track - and - hold and does not exhibit any pipeline delay or latency, making it ideal for multiple multiplexed channel applications. the ad7985 can be interfaced to any 1.8 v to 2.7 v digital logic family. it is available in a space - saving 2 0 - lead lfcsp that allows flexible configurations. it is pin for pin compatible with the 18- bit ad7986. converter operation the ad7985 is a successive appr oximation adc based on a charge redistribution dac. figure 21 shows the simplified schematic of the adc. the capacitive dac consists of two identical arrays of 1 6 binary weighted capacitors that are connected to the two comparator inputs. during the acqu isition phase, t he t erminals of the array tied to the input of the comparator are connected to a gnd via sw+ and sw ? . all independent switches are connected to the analog inputs. th erefore , the capacitor arrays are used as sampling capacitors and acquire th e analog signal on the in+ and in? inputs. when the acquisition phase is complete d and the cnv input goes high, a conversion phase is initiated. when the conversion phase begins, sw+ and sw? are opened first. the two capacitor arrays are then disconnected from the analog inputs and connected to the ref gnd input. therefore, the differential voltage between the in+ and in? inputs captured at the end of the acquisition phase is applied to the comparator inputs, causing the comparator to become unbalanced. by switch - ing each element of the capacitor array between ref gnd and ref, the comparator input varies by binary weighted voltage steps (v ref /2, v ref /4 , v ref / 65, 536 ). the control logic toggles these switches, starting with the msb, to bring the comparator ba ck into a balanced condition. after the completion of this process, the device returns to the acquisition phase , and the control logic generates the adc output code and a busy signal indicator. because the ad7985 has an on - board conversion clock, the serial clock, sck, is not required for the conversion process. c onversion modes of operation the ad7985 features two conversion modes of operation : turbo and normal. turbo conversion mode (turbo is high ) allow s the fastest conversion rate of up to 2.5 msps and does not power down between conversions. the first conversion in t urbo mode must be ignored because it contai ns meaningless data. for applications that require lower power and slightly slower sampling rates, the n ormal mode ( turbo is low ) allows a maximum conversion rate of 2 .0 msps and powers down between conversion s. the first conversion in normal mode contain s m eaningful data.
ad7985 data sheet rev. c | page 14 of 28 transfer functions the ideal transfer function for the ad7985 is shown in figure 22 and table 7. 000 ... 000 000 ... 001 000 ... 010 adc code (straight binary) analog input +fsr ? 1.5 lsb +fsr ? 1 lsb ?fsr + 1 lsb ?fsr ?fsr + 0.5 lsb 07947-006 111 ... 101 111 ... 110 111 ... 111 figure 22. adc ideal transfer function table 7. output codes and ideal input voltages description analog input, v ref = 4.096 v digital output code (hex) fsr C 1 lsb 4.095938 v 0xffff 1 midscale + 1 lsb 2.048063 v 0x8001 midscale 2.048 v 0x8000 midscale C 1 lsb 2.047938 v 0x7fff Cfsr + 1 lsb 62.5 v 0x0001 Cfsr 0 v 0x0000 2 1 this is also the code for an overranged analog input (v in+ ? v in? above v ref ? refgnd). 2 this is also the code for an underranged analog input (v in+ ? v in? below refgnd). typical connection diagram figure 23 shows an example of the recommended connection diagram for the ad7985 when multiple supplies are available. 07947-007 notes 1. gnd refers to refgnd, agnd, and dgnd. ad7985 gnd ref avdd, dvdd vio bvdd 5v 2.5v 1.8v to 2.7v vio sdi sck sdo cnv 3- or 4-wire interface: spi, cs, daisy chain (turbo = low) turbo 10f in+ in? 1.5nf 10 ? v? 0v to v ref v+ figure 23. typical application diagram with multiple supplies
data sheet ad7985 rev. c | page 15 of 28 analog input s figure 24 shows an equivalent circuit of the input structure of the ad7985 . the two diodes, d1 and d2, provide esd protection for the analog i nputs , in+ and in?. take care to ensure the analog input signal does not exceed the reference input voltage (ref) by more than 0.3 v . if the analog input signal exceeds this level, the diodes be come forward - bias ed and start conducting current. these diodes can handle a forward - biased current of 130 ma maximum. however, if the supplies of the input buffer (for example, the v+ and v ? supplies of the buffer amplifier in figure 23) are different from those of ref , the analog input signal may eventually exceed the supply rails by more than 0.3 v. in such a case (for example, an input buffer with a short circuit ) , the current limitation can protect the device . c pin ref r in c in d1 d2 in+ or in? refgnd 07947-008 figure 24 . equivalent analog input circuit the analog input structure allows the sampling of the true differential signal between in+ and in?. by using these differential inputs, signal s common to both inputs are rejected. during the acquisition phase, the impedance of the analog inputs (in+ and in?) can be modeled as a parallel combination of capacitor c pin and the network formed by the series connection of resistor r in and capacitor c in . c pin is primarily the pin capacitance. r in is typically 400 ? and is a lump ed component composed of serial resistors and the on resistance of the switches. c in is typically 30 pf and is mainly the adc sampling capacitor. during the sampling phase, whe re the switches are close d, the input impedance is limited to c pin . r in and c in make a one - pole, low - pass filter that reduces undesirable aliasing effects and limits noise. when the source impedance of the driving circuit is low, the ad7985 can be driven directly. large source impedances significantly affect the ac performance, especially thd. the dc performances are less sensitive to the input impedance. the maximum source impedance depends on the amount of thd that can be tolerated. the thd degrades as a function of the source impedance and the maximum input frequency. driver amplifier cho ice although the ad7985 is easy to drive, the driver amplifier must meet the following requirements: ? the noise generated by the driver amplifier must be kept as low as possible to preserve the snr and transition noise performance of the ad7985 . the noise from the driver is filtered by the ad7985 analog input circuit one - pole, low - pass filter , made by r in and c in , or by the external filter, if one is used. because the typical noise of the ad7985 is 5 0 v rms, the snr degradation due to the amplifier is ? ? ? ? ? ? ? ? ? ? ? ? + = ? 2 2 ) ( 2 50 50 log 20 n 3db loss ne f snr ere f C 3db is the input bandwidth , in megahertz, of the ad7985 ( 19 mhz) or the cutoff frequency of the input filter, if one is used. n is the noise gain of the amplifier (for examp le, 1 in buffer configuration). e n is the equivalent input noise voltage of the op erational amp lifier in nv/hz. ? for ac applications, the driver must have a thd perfor - mance commensurate with that of the ad7985 . ? for multichannel multiplexed applications, the driver amp lifier and the ad7985 analog input circuit must settle for a full - scale step onto the capacitor array at a 16 - bit level (0.0015 %, 15 ppm). in the data sheet of the driver amplifier , settling at 0.1% to 0.01% is more commonly specified. this value may differ significantly from the settling time at a 16 - bit level and must be verified prior to driver selection. table 8 . recommended driver amplifiers amplifier ty pical application ad8021 very low noise and high frequency ad8022 low noise and high frequency ada4899 -1 ultralow noise and high frequency ad8014 low power and high frequency
ad7985 data sheet rev. c | page 16 of 28 voltage reference input the ad7985 allows the choice of a very low temperature drift internal voltage reference, an external reference, or an external buffered reference. the internal reference of the ad7985 provides excellent performance and can be used in almost all applications. internal reference, ref = 4.096 v (pdref low) to use the internal reference, the pdref input must be low. this enables the on-chip band gap reference and buffer, result- ing in a 4.096 v reference on the ref pin (1.2 v on refin). the internal reference is temperature compensated to 4.096 v 15 mv. the reference is trimmed to provide a typical drift of 10 ppm/c. the output resistance of refin is 6 k when the internal reference is enabled. it is necessary to decouple this pin with a ceramic capacitor of at least 100 nf. the output resistance of refin and the decoupling capacitor form an rc filter, which helps to reduce noise. because the output impedance of refin is typically 6 k, relative humidity (among other industrial contaminants) can directly affect the drift characteristics of the reference. a guard ring typically reduces the effects of drift under such circumstances. however, the fine pitch of the ad7985 makes this difficult to implement. one solution, in these industrial and other types of applications, is to use a conformal coating, such as dow corning? 1-2577 or humiseal? 1b73. external 1.2 v reference and internal buffer (pdref high) to use an external reference along with the internal buffer, pdref must be high. this powers down the internal reference and allows the 1.2 v reference to be applied to refin, producing 4.096 v (typically) on the ref pin. external reference (pdref high, refin low) to apply an external reference voltage directly to the ref pin, pdref must be tied high and refin must be tied low. bvdd must also be driven to the same potential as ref. for example, if ref = 2.5 v, bvdd must be tied to 2.5 v. the advantages of directly using an external voltage reference are as follows: ? snr and dynamic range improvement (about 1.7 db) resulting from the use of a larger reference voltage (5 v) instead of a typical 4.096 v reference when the internal reference is used. this is calculated by ? ? ? ? ? ? ? 0.5 096.4 log20 snr ? power savings when the internal reference is powered down (pdref high). reference decoupling the ad7985 voltage reference input, ref, has a dynamic input impedance that requires careful decoupling between the ref and refgnd pins. the layout section describes how this can be done. when using an external reference, a very low impedance source (for example, a reference buffer using the ad8031 or the ad8605 ) and a 10 f (x5r, 0805 size) ceramic chip capacitor are appro- priate for optimum performance. if an unbuffered reference voltage is used, the decoupling value depends on the reference used. for example, a 22 f (x5r, 1206 size) ceramic chip capacitor is appropriate for optimum performance using a low temperature drift adr434 reference. if desired, a reference decoupling capacitor with a value as small as 2.2 f can be used with minimal impact on performance, especially dnl. in any case, there is no need for an additional lower value ceramic decoupling capacitor (for example, 100 nf) between the ref and refgnd pins. power supply the ad7985 has four power supply pins: an analog supply (avdd), a buffer supply (bvdd), a digital supply (dvdd), and a digital input/output interface supply (vio). vio allows direct interface with any logic from 1.8 v to 2.7 v. to reduce the number of supplies needed, vio, dvdd, and avdd can be tied together. the power supplies do not need to be started in a par- ticular sequence. in addition, the ad7985 is very insensitive to power supply variations over a wide frequency range. in normal mode, the ad7985 powers down automatically at the end of each conversion phase and, therefore, the power scales linearly with the sampling rate. this makes the device ideal for low sampling rates (even a few sps) and battery- powered applications. 10 1 0.1 0.01 0.1 1 operating current (ma) sampling rate (msps) 07947-121 i bvdd i avdd i dvdd i vio i vref figure 25. operating current vs. sampling rate in normal mode
data sheet ad7985 rev. c | page 17 of 28 digital interface although the ad7985 has a reduced number of pins, it offers flexibility in the serial interface modes. i n cs mode, the ad7985 is compatible with spi, microwire, qspi, and digital hosts. in cs mode, the ad7985 can use either a 3 - wire or a 4 - wire interface. a 3 - wire interface that uses t h e c n v, sck, and sdo signals minimizes wiring connections , which is useful, for example , in isolated applications. a 4 - wire interface that uses the sdi, cnv, sck, and sdo signals allows cnv, which initiates conversions, to be independent of the readback timing (sdi). this is useful in low jitter sampling or simultaneous sampling applications. in chain mode, the ad7985 provides a daisy - chain feature that uses the sdi input for cascading multiple adcs on a single data line similar to a shift register. chain mode is available only in n ormal mode ( turbo is low ). the mode in which the device operates depends on the sdi level when the cnv rising edge occurs. cs mode is selected if sdi is high, and chain mode is selected if sdi is low. the sdi hold time is such that when sdi and cnv are connect ed together, chain mode is always selected. in n ormal mode operation, the ad7985 offers the option of forcing a start bit in front of the data bits. this start bit can be used as a busy signal ind icator to interrupt the digital host and trigger the data reading. otherwise, without a busy indicator, the user must time out the maximum conversion time prior to readback. the busy indicator feature is enabled i n cs mode if cnv or sdi is low when the adc conversion ends (see figure 29 and figure 33). turbo must be kept low for bot h digital interfaces. when cnv is low, read back can occur during conversion or acquisition, or it can be split across acquisition and conversion , as described i n the following sections. a discontinuous sck is recommended because the device is selected with cnv low, and sck a ctivity begins to clock out data. note that in the following sections, the timing diagrams indicate digital activity (sck, cnv, sdi, and sdo ) during the conversion. however, due to the possibility of performance degradation, digi - tal activity must occur only prior to the safe data reading time, t data , because the ad7985 provide s error correction circuitry that can correct for an incorrect bit decision during this time. from t data to t conv , there is no error correction , and conve rsion results may be corrupted. similarly, t quiet , the time from the last falling edge of sck to the ri sing edge of cnv, must remain free of digital activity. the user must configure the ad7985 and initiate the busy indicator (if desired in normal mode ) prior to t data . it is also possible to corrupt the sample by having sck near the sampling instant. therefore, it is recommended that the digital pins be kept quiet for approximately 2 0 ns before and 10 ns after the rising edge of cnv, using a discontinuous sck whenever po ssible to avoid any potential performance degradation.
ad7985 data sheet rev. c | page 18 of 28 d ata reading options there are three different data reading options for the ad7985 . there is the option to read during conversion, to split the read across acquisition and conversion (see figure 26 and figure 27) , and , i n normal mode, to read during acquisition. the desired sck frequency largely determine s which reading option to use . reading during conversion, fast host (turbo or normal mode) when reading during conversion (n), conversion results are for the previous (n ? 1) conversion. reading must occur only up to t data and, because this time is limited, the host must use a fast sck. the required sck frequency is calculated by en cnvh data sck t t t edges sck number f ? ? _ _ to d e termine the minimum sck frequency, follow these examples to read data from conversion (n ? 1). for t urbo m ode ( 2.5 msps ) , number_sck_edges = 1 6 ; t data = 190 ns ; t cnvh = 10 ns; t en = 5 ns f sck = 1 6 / ( 190 ns C 10 ns C 5 ns) = 91. 5 mhz for n ormal m ode ( 2.0 msps ) , number_sck_edges = 1 6 ; t data = 290 ns ; t cnvh = 10 ns; t en = 5 ns f sck = 1 6 / ( 290 ns ? 10 ns ? 5 ns) = 58.2 mhz the time between t data and t conv i s an input/output quiet time during which digital activity must n ot occur, or sensitive bit decisions may be corrupt ed . split - reading , any speed host (turbo or normal mode) to allow for a slower sck , there is the option of a split read , where data access starts at the current acquisition (n) and spans into the conversion (n). conversion results are for the previous (n ? 1) conversion. similar to reading during conversion, split - reading must occur only up to t data . for the maximum throughput, the onl y time restriction is that split - reading take place during the t acq (min imum ) + ( t data ? t quiet ) time. the time between the falling edge of sck and cnv rising is an acquisition quiet time, t quiet . to determine how to split the read for a particular sck frequency, follow these examples to read data from conversion (n ? 1) . for t urbo m ode ( 2.5 msps ) , f sck = 75 mhz; t data = 190 ns ; t cnvh = 10 ns; t en = 5 ns number_sck_edges = 75 mhz ( 190 ns ? 10 ns ? 5 ns) = 13.1 thirteen bits are read during conversion (n) , and three bits are read during acquisition (n). for n ormal m ode (2 .0 msps ) , f sck = 50 mhz; t data = 290 ns ; t cnvh = 10 ns; t en = 5 ns number_sck_edges = 50 mhz ( 290 ns C 10 ns C 5 ns) = 13.7 5 thirteen bits are read during conversion (n) , and three bits are read during acquisition (n). for slow throughputs, the time restriction is dictated by the throughput required by the user; the host is free to run at any speed. similar to reading during acquisition, data access for slow hosts must take place du ring the acquisition phase with additional time into the conversion. note that data access spanning conversion requires the cnv pin to be driven high to initiate a new conversion, and data access is not allowed when cnv is high. thus, the host must perform two bursts of data access when using this method. reading during acquisition, any speed host ( turbo or normal mode ) when reading during acquisition (n), conversion results are for the previous (n ? 1) conversion. maximum th r oughput is achievable in n ormal mode ( 2.0 msps ) ; however , in t urbo mode, 2.5 msps throughput is not achievable. for the maximum throughput, the only time restriction is that reading take place during the t acq (min imum ) time. for slow throughputs, the time restriction is dictated by th e throughput required by the user; the host is free to run at any speed. thus , for slow hosts, data access must take place during the acquisi - tion phase.
data sheet ad7985 rev. c | page 19 of 28 cs mode, 3 - wire without busy in di cator this mode is usually used when a single ad7985 is connected to an spi - compatible digital host. the connection diagram is shown in figure 26 , and the corresponding timing is given in figure 27. with sdi tied to vio, a rising edge on cnv initiates a con - version, selects cs mode, and forces sdo to high impedance. when a conversion is initiated, it continues until completion , irrespective of the state of cnv. this can be useful, for example , to bring cnv low to selec t other spi devices, such as analog multiplexers; however, cnv must be returned high before the minimum conversion time elapses and then held high for the maximum possible conversion time to avoid the generation of the busy signal indicator. when the conv ersion is complete, the ad7985 enters the acquisition phase and powers down. when cnv goes low, the msb is output onto sdo. the remaining data bits are clocked by subsequent sck falling edges. the data is valid on both sck edges. although the rising edge can capture the data, a digital host using the sck falling edge allows a faster reading rate, provided that it has an acceptable hold time. after the 1 6 th sck falling edge or when cnv goes high (whichever occurs first), sdo returns to high impedance. ad7985 sdi sdo cnv sck convert data in clk digital host vio 07947-009 figure 26 . cs mode, 3 - wire without busy indicator connection diagram (sdi high) 07947-010 acquisition (n) acquisition (n + 1) acquisition (n ? 1) 1 2 begin data (n ? 1) conversion (n) end data (n ? 1) sck cnv sdo 14 15 conversion (n ? 1) end data (n ? 2) t conv t data 0 (i/o quiet time) (i/o quiet time) 16 14 15 16 1 15 14 13 2 0 1 2 sdi = 1 > t conv (i/o quiet time) t cyc t acq t cnvh t quiet t sck t dis t dis t dis t dis t en t en t dsdo t hsdo t data t conv figu re 27 . cs mode, 3 - wire without busy indicator serial interface timing (sdi high)
ad7985 data sheet rev. c | page 20 of 28 cs mode, 3 - wire with busy indic ator this mode is usually used when a sing le ad7985 is connected to an spi - compatible digital host that has an interrupt input. it is available only in n ormal conversion mode ( turbo is low). the connection diagram is shown in figure 28, and the corre - sponding timing is given in figure 29. w ith sdi tied to vio, a rising edge on cnv initiates a conversion, selects cs mode, and forces sdo to high impedance. sdo is maintained in high impedance until the completion of the conversion , irrespective of the state of cnv. prior to the minimum conversion time, cnv can select other spi devices, such as analog multiplexers, but cnv must be returned low before the minimum conversion time elapses and then held low for the maximum possible conversion time to guarantee the generation of the busy signal indicator. when the conversion is complete, sdo goes from high imped - ance to low impedance. with a pull - up on the sdo line, this transition can be used as an interrupt signal to initiate the data read back controlled by the digital host. the ad7985 then enters the acquisition phase and powers down. the data bits are then clocked out, msb first, by subsequent sck falling edges. the data is valid on both sck edges. although the rising edge can capture the data, a digital host using the sck falling edg e allows a faster reading rate, provided that it has an acceptable hold time. after the optional 1 7 th sck falling edge , sdo returns to high impedance. if multiple ad7985 device s are selected at the same time, the sdo output pin handles this contention without damage or induced latch - up. meanwhile, it is recommended that this contention be kept as short as possible to limit extra power dissipation. ad7985 sdi sdo cnv sck convert data in clk digital host vio irq vio 47k? turbo 07947-011 figure 28 . cs mode, 3 - wire with busy indicator connection diagram (sdi high) sdo d15 d14 d1 d0 t dis sck 1 2 3 15 16 17 t sck t sckl t sckh t hsdo t dsdo cnv conversion acquisition t conv t cyc acquisition turbo = 0 sdi = 1 t cnvh t acq t quiet (i/o quiet time) 07947-012 figure 29 . cs mode, 3 - wire with busy i ndicator serial interface timing (sdi high)
data sheet ad7985 rev. c | page 21 of 28 cs mode, 4 - wire without busy in dicator this mode is usually used when multiple ad7985 device s are connected to an spi - compatible digital host. a connection dia - gram example using two ad7985 device s is shown in figure 30, and the corresponding timing is given in figure 31. with sdi high, a rising edge on cnv initiates a conversion, selects cs mode, and forces sdo to high impedance. in this mode, cnv must be held high during the conversion phase and the subsequent data readback. (if sdi and cnv are low, sdo is driven low.) prior to the minimum conversion time, sdi can select other spi devices, such as analog multi - plexers, but sdi must be returned high before the minimum conversion time elapses and then held high for the maximum possible conversion time to avoid the generatio n of the busy signal indicator. when the conversion is complete, the ad7985 enters the acquisition phase and powers down. each adc result can be read by bringing the sdi input low, which consequently outputs the msb onto sdo. th e remaining data bits are then clocked by subsequent sck falling edges. the data is valid on both sck edges. although the rising edge can be used to capture the data, a digital host using the sck falling edge allows a faster reading rate, provided that it has an acceptable hold time. after the 1 6 th sck falling edge, sdo returns to high impedance and another ad7985 can be read. ad7985 sdi sdo cnv sck convert data in clk digital host cs1 cs2 ad7985 sdi sdo cnv sck 07947-013 figure 30 . cs mode, 4 - wire without busy indicator connection diagram 07947-014 t conv aquisit i on aquisition t sckh t sckl t sck convers i on sck c nv t ssdicnv t hsdicnv sdo d15 d13 d14 d1 d0 d15 d14 d1 d0 t hsdo t dsdo t en 1 2 3 14 15 16 17 18 30 31 32 t dis sdi (cs1) sdi (cs2) t quiet t acq t cyc figure 31 . cs mode, 4 - wire without busy indicator serial interface timing
ad7985 data sheet rev. c | page 22 of 28 cs mode, 4 - wire with busy indic ator this mode is usually used when a single ad7985 is connected to an spi - compatible digital host with an interrupt input and when it is desired to keep cnv , which sample s the analog input, independent of the signal that select s the data reading. this independence is particularly important in applications where low jitter on cnv is desired. this mode is available only in n ormal conversion mode ( turbo is low ). the connection diagram is shown in figure 32, and the corresponding timing is given in figure 33. with sdi high, a rising edge on cnv initiates a conversion, selects cs mode, and forces sdo to high impedance. in this mode, cnv must be held high during the conversion phase and the subsequent data readback. (if sdi and cnv are low, sdo is driven low.) prior to the minimum conversion time, sdi can select other spi devices, such as analog multiplexers, but sdi must be returned low before the minimum conversion time elapses and then held low for the maximum possible conversion time to guarantee the generation of the busy signal indicator. when the conversion is complete, sdo goes from high imped - ance to low impedance. with a pull - up on the sdo line, this transition can be used as an interrupt signal to initiate the data readback controlled by the digital host. the ad 7985 then enters the acquisition phase and powers down. the data bits are then clocked out, msb first, by subsequent sck falling edges. the data is valid on both sck edges. although the rising edge can capture the data, a digital host using the sck falling edge allows a faster reading rate, provided that it has an accept - able hold time. after the optional 1 7 th sck falling edge or when sdi go es high (whichever occurs first), sdo returns to high impedance. ad7985 sdi sdo cnv sck convert data in clk digital host irq vio 47k? cs1 turbo 07947-015 figure 32 . cs mode, 4 - wire with busy indicator connection diagram (i/o quiet time) sdo d15 d14 d1 d0 t dis t quiet sck 1 2 3 15 16 17 t sck t sckl t sckh t hsdo t dsdo t en conversion acquisition t conv t cyc t acq acquisition sdi cnv t ssdicnv t hsdicnv turbo = 0 07947-016 figure 33 . cs mode, 4 - wire with busy indicator serial interface timing
data sheet ad7985 rev. c | page 23 of 28 chain mode without busy indicator this mode can daisy-chain multiple ad7985 devices on a 3-wire serial interface. it is available only in normal conversion mode (turbo is low). this feature is useful for reducing component count and wiring connections, for example, in isolated multi- converter applications or for systems with a limited interfacing capacity. data readback is analogous to clocking a shift register. a connection diagram example using two ad7985 devices is shown in figure 34, and the corresponding timing is given in figure 35. when sdi and cnv are low, sdo is driven low. with sck low, a rising edge on cnv initiates a conversion, selects chain mode, and disables the busy indicator. in this mode, cnv is held high during the conversion phase and the subsequent data readback. when the conversion is complete, the msb is output onto sdo, and the ad7985 enters the acquisition phase and powers down. the remaining data bits stored in the internal shift register are clocked by subsequent sck falling edges. for each adc, sdi feeds the input of the internal shift register and is clocked by the sck falling edge. each adc in the chain outputs the data msb first, and 16 n clocks are required to read back the n adcs. the data is valid on both sck edges. although the rising edge can capture the data, a digital host using the sck falling edge allows a faster reading rate and consequently more ad7985 devices in the chain, provided that the digital host has an acceptable hold time. the maximum conversion rate may be reduced due to the total readback time. convert data in clk digital host ad7985 sdi sdo cnv b sck ad7985 sdi sdo cnv a sck turbo turbo 07947-017 figure 34. chain mode without busy indicator connection diagram turbo = 0 sdo a = sdi b d a 15 d a 14 d a 13 sck 123 303132 t ssdisck t hsdisck 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 quiet t hsckcnv d a 0 07947-018 figure 35. chain mode without busy indicator serial interface timing
ad7985 data sheet rev. c | page 24 of 28 chain mode with busy indicator this mode can daisy - chain multiple ad7985 device s on a 3 - wire serial interface while providing a busy indicator. it is available only in normal conversion mode (t urbo is low). this feature is useful for reducing component count and wiring connections, for example, in isolated multiconverter applications or for systems with a limited interfacing capacity. data readback is analogous to clocking a shift register. a co nnection diagram example using three ad7985 device s is shown in figure 36 , and the cor - responding timing is given in figure 37. when sdi and cnv are low, sdo is driven low. with sck high, a rising edge on cnv initiates a conversion, selects chain mode, and enables the busy indicator feature. in this mode, cnv is held high during the conversion phase and the subsequent data readback. when all adcs in the chain have completed their conversions, the sdo pin of the adc closest to the digital host (see the ad7985 adc labeled c in figure 36 ) is driven high. this transition on sdo can be used as a busy indicator to trigger the data read - back controlled by the digital host. the ad7985 then enters the acquisition phase and powers down. the data bits stored in the internal shift register are clocked out, msb first, by subsequent sck falling edges. for each adc, sdi feeds the input of the internal shift register and is clocke d by the sck falling edge. each adc in the chain outputs the data msb first, and 1 6 n + 1 clocks are required to read back the n adcs. although the rising edge can capture the data, a digital host using the sck falling edge allows a faster reading rate a nd consequently more ad7985 device s in the chain, provided that the digital hos t has an acceptable hold time. convert data in clk digital host ad7985 sdi sdo cnv c sck ad7985 sdi sdo cnv a sck irq ad7985 sdi sdo cnv b sck turbo turbo turbo 07947-019 figure 36 . chain mode with busy indicator connection diagram sdo a = sdi b d a 15 d a 14 d a 13 sck 1 2 3 35 47 48 t en conversion acquisition t conv t cyc t acq acquisition cnv = sdi a d a 1 4 15 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 1 d b 0 d a 15 d a 14 49 t ssdisck t hsdisck t hsdo t dsdo sdo c d c 15 d c 14 d c 13 d a 1 d a 0 d c 1 d c 0 d a 14 19 31 32 18 33 d b 1 d b 0 d a 15 d b 15 d b 14 t dsdosdi t hsckcnv d a 0 t dsdosdi t dsdosdi t dsdosdi t dsdosdi turbo = 0 t ssckcnv 07947-020 figure 37 . chain mode with busy indicator serial interface timing
data sheet ad7985 rev. c | page 25 of 28 application s information layout design t he printed circuit board (pcb) that houses the ad7985 so the analog and digital sections are separated and confined to certain areas of the board. the pinout of the ad7985 , with the analog signals on the left side and the digital signals on t he right side, eases this task. avoid running digital lines under the device because the y couple noise onto the die, unle ss a ground plane under the ad7985 is used as a shield. fast switching signals, such as cnv or clocks, must not run near analog signal paths. crossover of digital and analog signals must be avoide d. at least one ground plane must be used. it can be common or split between the digital and analog sections. in the latter case, the planes must be joined underneath the ad7985 device s. the ad7985 voltage reference input s ( ref ) ha ve a dynamic input impedance and must be decoupled with minimal parasitic inductances. this is done by placing the reference decoupling ceramic capacitor close t o, ideally right against, the ref and ref gnd pins and connecting them with wide, low impedance traces. finally, the power supplies , vdd and vio of the ad7985 , must be decoupled with ceramic capacitors, typically 100 nf, placed close to the ad7985 and connected using short, wide traces to provide low impedance paths and to reduce the effect of glitches on th e power supply lines. evaluating the ad7985 performance other recommended layouts for the ad7985 are outlined in the documentation for the ad7985 evaluation board ( e va l - ad7985fmcz ). the evaluation board package includes a fully assembled and tested evaluation bo ard, documentation, and softw are for controlling the board from a pc via the e va l - sdp - ch1z board .
ad7985 data sheet rev. c | page 26 of 28 5 4 paddle 3 1 2 6 bvdd avdd dvdd vio gnd gnd gnd gnd gnd gnd gnd ref ref ref 07947-030 figure 38 . example layout of the ad7985 (top layer) 5v external reference (adr435 or adr445) gnd vio c ref bvdd avdd dvdd vio gnd gnd gnd gnd gnd gnd gnd ref ref ref c bvdd c avdd c vio c dvdd 07947-031 figure 39 . example layout of the ad7985 (bottom layer)
data sheet ad7985 rev. c | page 27 of 28 outline dimensions 0.50 bsc 0.50 0.40 0.30 0.30 0.25 0.20 compliant t o jedec standards mo-220-wggd. 061609 -b b o t t o m v i e w t o p v i e w e x p o s e d p a d pin 1 indica t or 4.10 4.00 sq 3.90 sea ting plane 0.80 0.75 0.70 0.05 max 0.02 nom 0.20 ref 0.25 min coplanarity 0.08 pin 1 indic a t or 2.65 2.50 sq 2.35 for proper connection of the exposed pad, refer to the pin confi gurat ion and function descriptions section of this data sheet. 1 2 0 6 1 0 1 1 1 5 1 6 5 figure 40 . 20 - lead lead frame chip scale package [lfcsp] 4 mm 4 mm body and 0.75 mm package height (cp - 20 - 10 ) dimensions shown in millimeters ordering guide model 1 , 2 , 3 temperature range package description package option ordering quantity ad798 5 bcpz ?40c to +85c 20- lead lead frame chip scale package [lfcsp], tray cp -20-10 490 ad798 5 bcpz -rl7 ?40c to +85c 20- lead lead frame chip scale package [lfcsp], 7 tape and reel cp -20-10 1,500 eval - ad7985 fmc z evaluation board eval - sdp - ch1z controller board 1 z = rohs compliant part. 2 the eval - ad7985fmcz can be used as a standalone evaluation board or in conjunction with the eval - sdp - ch1z for evaluation/demonstrat ion purposes. 3 the eval - sdp - ch1z allows a pc to control and communicate with all analog devices evaluation boards ending in the fmc designator.
ad7985 data sheet rev. c | page 28 of 28 notes ? 2009 C 2016 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. d07947 - 0- 3/16(c)

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