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touch screen digitizer ad7873 rev. d in fo rmation furn ished by an alog d e v i c e s is believed to be accurate and reliable. how e ver, n o resp on sibili ty is assume d b y a n alog de vices fo r its use, nor for an y i n fri n geme nt s of p a t e nt s or ot h e r ri ght s o f th ird parties th at may result fro m its use . s p ecificatio n s subj ec t to ch an g e witho u t n o tice. no licen s e is g r an te d by implicati o n or ot herwi s e u n der a n y p a t e nt or p a t e nt ri ghts of analog de v i ces. trademarks 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. 326. 87 03 ? 2004 analog de vices, i n c. al l r i ght s r e ser v ed . features 4-wire touch sc reen interface on-chip tempe r ature sensor: ?40 c to +8 5c on-chip 2.5 v r e ference direct battery measurement ( 0 v to 6 v) touch-pressur e measur emen t specified throughput rate of 125 ksps single supply, v cc of 2.2 v to 5 . 25 v ratiometric co nv ersion high speed s e r i al interface programmable 8-bit or 12-bit resolution one auxiliary a n alog input shutdown mode: 1 a max 16-lea d qsop, tssop, and lf csp packages applic ati o ns personal digita l assistants smart hand-he l d devices touch screen monitors point-of-sale te rminals pagers gener a l description the ad7873 is a 12-b i t s u cces s iv e a p p r o x ima t ion ad c wi t h a s y nc h r onou s s e r i a l i n t e r f a c e a n d l o w on re s i st a n c e s w itc h e s f o r dr i v in g t o uc h s c r e en s. th e ad7 873 o p era t es f r o m a sin g l e 2.2 v to 5 . 2 5 v p o we r supply and fe a t u r e s t h rou g h p ut r a te s g r e a te r tha n 125 ks ps. the ad7873 f e a t ur es dir e c t b a t t e r y m e as ur em en t, t e m p er a t ur e m e as ur em en t, and t o uc h-p r es sur e m e as ur em en t. th e ad7873 a l s o h a s an on - b o a rd re f e re nc e of 2 . 5 v t h a t c a n b e u s e d f o r t h e a u xi li a r y in p u t, b a t t er y m o ni t o r , a nd t e m p er a t ure m e as ur e m en t m o de s. w h en no t in us e , t h e i n ter n al r e fer e n c e ca n b e s h ut do wn t o con s er v e p o w e r . an ext e r n al r e fer e n c e ca n als o b e a p plie d an d can b e va r i e d f r o m 1 v to v cc , w h i l e t h e a n alog in p u t ra n g e is f r o m 0 v t o v ref . the de v i ce i n cl udes a s h u t do w n m o de th a t r e d u ce s th e curr e n t co n s um p t i o n t o le s s th a n 1 a . the ad7873 f e a t ur es on-bo a r d swi t ch es. this, co u p led wi t h lo w p o w e r a nd hig h sp e e d o p er a t ion, ma k e s t h e d e vice i d e a l fo r ba t t er y-p o w e r e d sys t ems s u c h as p e rs o n al dig i tal as sis t a n ts wi t h re s i st ive tou c h s c re e n s a n d ot he r p o r t abl e e q u i p m e n t . t h e p a r t is a v a i la b l e in a 16-lead 0.15" q u a r t e r size o u tline p a c k a g e (qso p), a 16-lead thin s h r i n k s m al l o u tline p a cka g e ( t sso p), a nd a 16 -le a d le ad f r a m e chi p s c ale p a cka g e (l fcs p ). func ti on a l bl ock di a g r a m 02164-d-001 ad7873 +v cc +v cc penirq sport din cs dout dclk busy pen interrupt charge redistribution dac sar + adc control logic 6-to-1 i/p mux t/h comp gnd temp sensor battery monitor y+ y? aux 2.5v ref x+ x? v ref v bat buf fi g u r e 1 . product highlights 1. r a t i om et r i c con v ersio n m o de a v a i lab l e , e l im ina t in g er r o rs d u e to on - b o a rd s w itch re s i st a n c e s . 2. on-bo a r d t e m p era t ur e s e n s o r : ?40c t o +85c. 3. ba t t e r y m o ni t o r i n p u t . 4. t o uc h- p r es s u r e m e as ur em en t ca pa b i li t y . 5. l o w p o w e r co ns um p t ion o f 1.37 mw max wi th th e re f e re nc e of f , or 2 . 4 1 m w t y p w i t h t h e re f e re nc e on , a t 125 ks ps and v cc a t 3.6 v . 6. p a ck age opt i ons i n clu d e 4 m m 4 m m l f c s p . 7. ana l og in p u t ran g e f r o m 0 v t o v ref . 8. ve r s a t i l e s e r i a l i / o p o r t s .
ad7873 rev. d | 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 te r m i no l o g y ...................................................................................... 8 typical performance characteristics ............................................. 9 circuit information ........................................................................ 13 adc transfer function ............................................................. 13 typical connection diagram ................................................... 13 analog input ............................................................................... 14 measurements ............................................................................. 16 pen interrupt request ................................................................ 18 control register ......................................................................... 19 power vs. throughput rate ....................................................... 20 serial interface ............................................................................ 21 grounding and layout .................................................................. 23 pcb design guidelines for chip scale package .................... 23 outline dimensions ....................................................................... 24 ordering guide .......................................................................... 25 revision history 6/04changed from rev. c to rev. d updated format..................................................................universal changes to absolute maximum ratings ....................................... 6 additions to pd0 and pd1 description...................................... 21 pbc guidelines for chip scale package added ......................... 23 additions to ordering guide........................................................ 25 4/03changed from rev. b to rev. c changes to formatting ......................................................universal updated outline dimensions ....................................................... 19 1/02changed from rev. a to rev. b addition of 16-lead lead frame chip scale package ..universal edits to features.................................................................................1 edits to general description ...........................................................1 addition of lfcsp pin configuration ...........................................4 edit to absolute maximum ratings................................................4 addition to ordering guide ............................................................4 addition of cp-16 outline dimensions ..................................... 19 2/01changed from rev. 0 to rev a edits to notes in the ordering guide
ad7873 rev. d | page 3 of 28 specifications v cc = 2.7 v to 3.6 v, v ref = 2.5 v internal or external, f dclk = 2 mhz; t a = ? 40c to +85c, unless otherwise noted. table 1. parameter ad7873a 1 ad7873b 1 unit test conditions/comments dc accuracy resolution 12 12 bits no missing codes 11 12 bits min integral nonlinearity 2 2 1 lsb max differential nonlinearity 2 C0.9/+1.5 lsb max offset error 2 6 6 lsb max +v cc = 2.7 v gain error 2 4 4 lsb max external reference noise 70 70 v rms typ power supply rejection 70 70 db typ switch drivers on resistance 2 y+, x+ 5 5 ? typ yC, xC 6 6 ? typ analog input input voltage ranges 0 to v ref 0 to v ref volts dc leakage current 0.1 0.1 a typ input capacitance 37 37 pf typ reference input/output internal reference voltage 2.45/ 2.55 2.45/2.55 v min/max internal reference tempco 15 15 ppm/c typ v ref input voltage range 1/v cc 1/v cc v min/max dc leakage current 1 1 a max v ref input impedance 1 1 g? typ cs = gnd or +v cc ; typically 260 ? when on- board reference enabled temperature measurement temperature range C40/+85 C40/+85 c min/max resolution differential method 3 1.6 1.6 c typ single conversion method 4 0.3 0.3 c typ accuracy differential method 3 2 2 c typ single conversion method 4 2 2 c typ battery monitor input voltage range 0/6 0/6 v min/max input impedance 10 10 k? typ sampli ng; 1 g? when battery monitor off accuracy 2.5 2 % max external reference 3 3 % max internal reference logic inputs input high voltage, v inh 2.4 2.4 v min input low voltage, v inl 0.4 0.4 v max input current, i in 1 1 a max typically 10 na, v in = 0 v or +v cc input capacitance, c in 5 10 10 pf max footnotes on next page.
ad7873 rev. d | page 4 of 28 parameter ad7873a 1 ad7873b 1 unit test conditions/comments logic outputs output high voltage, v oh v cc C 0.2 v cc C 0.2 v min i source = 250 a; v cc = 2.2 v to 5.25 v output low voltage, v ol 0.4 0.4 v max i sink = 250 a penirq output low voltage, v ol 0.4 0.4 v max 100 k? pull-up; i sink = 250 a floating-state leakage current 10 10 a max floating-state output capacitance 5 10 10 pf max output coding straight (natural) binary conversion rate conversion time 12 12 dclk cycles max track-and-hold acquisition time 3 3 dclk cycles min throughput rate 125 125 ksps max power requirements +v cc (specified performance) 2.7/3.6 2.7/3.6 v min/max functional from 2.2 v to 5.25 v i cc 6 digital i/ps = 0 v or v cc normal mode (f sample = 125 ksps) 380 380 a max internal reference off. v cc = 3.6 v, 240 a typ 670 670 a typ internal reference on. v cc = 3.6 v normal mode (f sample = 12.5 ksps) 170 170 a typ internal reference off. v cc = 2.7 v, f dclk = 200 khz normal mode (static) 150 150 a typ internal reference off. v cc = 3.6 v 580 580 a typ internal reference on. v cc = 3.6 v shutdown mode (static) 1 1 a max 200 na typ power dissipation 6 normal mode (f sample = 125 ksps) 1.368 1.368 mw max v cc = 3.6 v. internal reference disabled 2.412 2.412 mw typ v cc = 3.6 v. internal reference enabled shutdown 3.6 3.6 w max v cc = 3.6 v 1 temperature range as follows: a, b versions: C40c to +85c. 2 see the section. terminology 3 difference between temp0 and temp1 me asurement. no cali bration necessary. 4 temperature drift is C2.1 mv/c. 5 sample tested @ 25c to ensure compliance. 6 see the power vs. throughput rate section.
ad7873 rev. d | page 5 of 28 timi ng spe c ific a t io ns t a = t min to t ma x , u n l e s s o t h e r w i s e n o t e d ; v cc = 2.7 v t o 5.25 v , v ref = 2.5 v . table 2. timing sp ecification s 1 parameter limit at t min , t ma x unit description f dclk 2 10 khz min 2 mhz max t acq 1.5 s min acquisition time t 1 10 ns min cs falling edge to first dclk rising edge t 2 60 ns max cs falling ed ge to busy three-state d i sabled t 3 3 60 ns max cs falling edge to dout three-state disabled t 4 200 ns min dclk high pulse width t 5 200 ns min dclk low pulse width t 6 60 ns max dclk falling edge to busy rising edge t 7 10 ns min data setup time prior to dclk rising edge t 8 10 ns min data valid to dclk hold time t 9 3 200 ns max data access time after dclk falling edge t 10 0 ns min cs rising edge to dclk ignored t 11 100 ns max cs rising edge to busy high impedance t 12 4 100 ns max cs rising edge to dout high impedance 1 sa m p le t e st e d a t 2 5 c t o en sur e c o m p li a n ce. a l l i n put si g n a ls a r e speci f i e d w i t h t r = t f = 5 n s (10 % t o 90% of v cc ) a n d t i m e d fr om a vo lt a g e l e vel o f 1.6 v. 2 mark/ s pace ratio for the dc l k input is 40/60 to 60/40. 3 mea s ure d with the loa d circuit o f a n d d e f i ne d as the time re quire d fo r the o u tput to cro s s 0.4 v or 2.0 v. figu re 2 f i g u r e 2 4 t 12 i s deri ve d from t h e m e a s ur ed t i m e t a ken by t h e da t a o u t p ut s t o ch a n ge 0. 5 v wh en loa d ed wi t h t h e ci rcui t o f . th e m e a s ured numbe r i s the n extrapo l ate d back to re mo ve the e f f e cts of charging o r d i s c harging the 50 pf capacito r. this me ans that the ti me , t 12 , quoted in t h e timing character i s t ics is the true bus rel i nquis h time of the part and is in d e pe nde nt o f the bus lo ad ing. 02164-d-002 200
ad7873 rev. d | page 6 of 28 absolute maximum ra tings t a = 2 5 c , u n l e ss ot he r w i s e not e d. table 3. p a r a m e t e r r a t i n g +v cc to gnd C0.3 v to +7 v analog input voltage to gnd C0.3 v to v cc + 0.3 v digital input voltage to gnd C0.3 v to v cc + 0.3 v digital output v o ltage to gnd C0.3 v to v cc + 0.3 v v ref to gnd C0.3 v to v cc + 0.3 v input current to any pin except supplies 1 10 ma operating tem p erature range commercia l (a, b versions) C40c to +85c storage temperature range C65c to +150c junction tempe r ature 150c qsop, tssop, l f csp packages, power dissipation 450 mw ja thermal impedance 149.97c/w (qs o p) 1 5 0 . 4 c / w (tss o p ) 1 3 5 . 7 c / w (lfc s p ) jc thermal impedance 38.8c/w (qsop) 27.6c/w (tsso p ) ir reflow soldering peak temperature time-to-peak temperature ramp-down rat e 220c (5c) 10 sec to 30 sec 6c/sec max pb-free parts on ly: peak temperture 250c time-to-peak temperature ramp-up rate ramp-down rat e 20 sec to 40 sec 3c/sec max 6c/sec max _________ ___ _________ ___ ___ 1 t r ansient cur r en ts of up t o 100 m a do not cause scr la t c h-up . s t r e s s es a b o v e t h os e lis t e d u n de r a b s o l u t e m a xi m u m r a t i n g s ma y c a us e p e r m a n en t dama ge t o t h e de vice . this is a s t r e s s ra t i n g onl y ; f u n c t i o n al o p era t ion o f t h e de vice a t t h es e o r an y o t h e r con d i t io ns a b o v e t h os e list e d i n t h e o p era t io nal s e c t io n s o f t h is sp e c if ic a t io n is n o t i m pli e d . e x p o sur e t o a b s o l u t e max i m u m ra t i ng co ndi t i on s fo r ex ten d e d p e r i o d s ma y a f fe c t de vice r e l i ab i l i t y . esd c a ution esd (electrostatic discharge) sensitive device. ele c trosta tic charg e s as high as 4000 v readily accumulate on the human body and test eq uipment and can discharge with out detection. although this product features proprietary esd protection circu i try, permanent dama ge may occur on devices subjected to high energy electrostatic discharges. theref ore, prop er esd precautions a r e recommended to avoid perform a nce degradation or l o ss of functiona l ity.
ad7873 rev. d | page 7 of 28 pin conf igura t ions and f u ncti on descriptions 02164-d-003 ad7873 top view (not to scale) 1 2 3 4 12 11 10 9 16 15 14 13 56 7 8 pin 1 indicator aux v ref +v cc penirq y+ x+ +v cc v bat gnd y? x? dout bus y din cs dclk f i g u re 3. lfcs p pin conf ig ur at i o n 02164-d-004 1 2 3 4 5 6 7 10 8 9 11 12 13 14 15 16 ad7873 top view (not to scale) +v cc x+ y+ x? y? gnd v bat aux dclk cs din busy dout penirq +v cc v ref f i gure 4.qso p /t ss op pin configu r ation ta ble 4. pi n f u nct i on d e s c ri pt i o ns mnemonic function +v cc power supply input. the +v cc range for the ad7 873 is from 2.2 v to 5.25 v. both +v cc pins s h oul d be connected directly together. x+ x+ position input. adc input c h annel 1. y+ y+ position input. adc input c h annel 2. xC xC position input. yC yC position inpu t. adc input channel 3. gnd analog ground. ground reference point for all circuitry on th e a d 7873. all analog input signa ls and any external reference signal s should b e referred to this gnd voltage. v bat battery monitor input. adc inpu t channel 4. aux aux i liary input. adc input channel 5. v ref reference output for the ad7 8 73. alternativel y an external refe rence can be applied to this inp ut. the voltage range for the external refere n c e is 1.0 v to +v cc . for specified performance, it is 2.5 v on the a d 7873. the inte rnal 2.5 v reference is available on this pin for use ex ternal to the device. the reference output must be buffe red before it is applied elsewhere in a system. a 0.1 f capacitor i s rec o mmended between th is pin and gnd to re du ce system noise effects. penirq pen interrupt. c m os logic open drain output (re q uires 10 k? to 100 k? pull-up resistor exter n ally). dout data out. logic output. the conversion re su lt from the ad7873 is provided on this outp ut as a serial data stream. the bits are clocked out on t h e falling edge of the dclk input. this output i s high impedan ce when cs is high. busy busy output. l o gic output. thi s output is high impedance when cs is high. din data in. logic input. data to be written to the a d 7873 control r e gister is prov id ed on this input an d is clocked into the register on the rising edge of dclk (see the control register se ction). cs chip select inpu t. active low logic inp ut. this input provides the dual function of initiating conversions on the ad7873 and enabling the ser i al in put/ output register. dclk external clock i n put. logic input. dclk pr ovides the serial c l oc k for a ccessing data from the part. this clock in put is also us ed as the clock sou r ce for the ad7873 conversi on process.
ad7873 rev. d | page 8 of 28 terminology integral nonlinearity this is the maximum deviation from a straight line passing through the endpoints of the adc transfer function. the endpoints of the transfer function are zero scale, a point 1 lsb below the first code transition, and full scale, a point 1 lsb above the last code transition. differential nonlinearity this is the difference between the measured and the ideal 1 lsb change between any two adjacent codes in the adc. offset error this is the deviation of the first code transition (00 . . . 000) to (00 . . . 001) from the ideal, that is, agnd + 1 lsb. gain error this is the deviation of the last code transition (111 . . . 110) to (111 . . . 111) from the ideal (that is, v ref C 1 lsb) after the offset error is adjusted out. track-and-hold acquisition time the track-and-hold amplifier enters the acquisition phase on the fifth falling edge of dclk after the start bit has been detected. three dclk cycles are allowed for the track-and-hold acquisition time. the input signal is fully acquired to the 12-bit level within this time even with the maximum specified dclk frequency. see the analog input section for more details. on-resistance this is a measure of the ohmic resistance between the drain and source of the switch drivers.
ad7873 rev. d | page 9 of 28 typical perf orm ance cha r acte ristics 198 199 200 201 202 203 204 205 206 207 s u p p l y curre nt ( a) 02164-d-005 ?40 ? 20 0 2 0 4 0 6 0 8 0 100 temperature (c) f i gure 5. su p p ly cu rr e n t v s . t e m p er atu r e 150 160 170 180 190 200 s u p p l y curre nt ( a) 210 220 230 02164-d-006 2.2 2.6 3.0 3.4 3.8 4.2 4.6 5.0 +v cc (v) f sample = 12.5khz v ref = +v cc f i gure 6. su p p ly cu rr e n t v s . +v cc ? 0.20 ? 0.15 ? 0.10 ? 0.05 0 0.05 delta from 25 c (lsb) 0.10 0.15 0.20 02164-d-007 ?40 ? 20 0 2 0 4 0 6 0 8 0 100 temperature (c) f i gure 7 . ch a n ge i n g a in vs . t e m p er a t ur e 134 135 136 137 138 139 140 141 s u p p l y curre nt (na) 02164-d-008 ?40 ? 20 0 2 0 4 0 6 0 8 0 100 temperature (c) f i gu r e 8 . p o w e r - d o w n s u ppl y c u rr en t vs . t e m p er a t ur e 3.2 3.7 2.2 2.7 4.2 4.7 5.2 +v cc (v) 02164-d-009 100 1000 sa m p le r a t e ( k sps) v ref = +v cc f i gure 9. m a xi m u m s a mpl e rate v s . + v cc ?0.6 ?0.4 ?0.2 0 0.2 0.4 0.6 delta from 25 c (lsb) 02164-d-010 ? 4 0 ? 20 0 2 0 4 0 6 0 8 0 100 temperature (c) f i gure 1 0 . change in o ffset vs . t e m p er a t ur e
ad7873 rev. d | page 10 of 28 0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 re fe re nce curre nt ( a) 70 55 25 4 0 10 85 10 0 115 13 0 sample rate (khz) 02164-d-011 f i gure 11. r e ference cur r ent v s . s a mpl e ra te y+ x+ x? y? 4 5 6 7 8 9 10 r on ( ? ) 02164-d-012 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 +v cc (v) f i gure 12. s w itch o n r e sistance v s . +v cc (x+, y + : +v cc to pin ; x - , y - : p i n to gnd) 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 e rror (ls b ) 15 35 55 75 95 115 135 155 175 195 sampling rate (ksps) 02164-d-013 inl: r = 2k ? inl: r = 500 ? dnl: r = 2k ? dnl: r = 500 ? f i g u re 13. m a x i mu m s a mpl i ng r a te v s . r in 2 4 6 5 3 8 7 re fe re nce curre nt ( a) 10 9 12 11 14 13 02 0 ? 4 0 ? 20 40 60 80 temperature ( c) 02164-d-014 f i gure 14. r e ference cur r ent v s . t e mper atu r e y+ x+ x? y? 3 4 5 6 7 8 9 r on ( ? ) 02164-d-015 ? 4 0 ? 20 0 2 0 4 0 6 0 8 0 100 temperature (c) f i gure 15. s w itch o n r e sistance v s . t e mpe r atu r e (x+, y + : +vc c to pin; x - , y - : pin to gn d ) 2.4988 2.4990 2.4992 2.4994 2.4996 2.4998 2.5000 2.5002 2.5004 2.5006 inte rnal v re f (v ) temperature ( c) 02164-d-016 ?40 ? 30 ?20 ? 10 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 f i gure 16. inte rn al v ref v s . t e mper ature
ad7873 rev. d | page 11 of 28 2.484 2.486 2.488 2.490 2.492 2.494 2.496 2.498 2.500 2.502 2.504 v re f (v ) 2.9 3.1 2.5 2.7 3.3 3.5 3.7 +v cc (v) 02164-d-017 f i gure 17. inte rn al v ref vs . + v cc ? 4 0 ? 30 ?20 ? 10 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 450 500 550 600 650 700 tem p d i od e volta ge ( m v) 750 800 850 temperature ( c) 02164-d-018 95.95mv temp1 temp0 142.15mv f i g u re 18. t e mp d i ode v o lt ag e v s . t e mpe r at u r e ( 2 .7 v s u p p ly ) 720 721 722 723 724 725 726 727 728 729 730 tem p 1 d i od e volta ge ( m v) 02164-d-019 v supply (v) 2.7 3.3 3.0 3.6 f i g u re 19. t e mp 1 d i od e v o lt ag e v s . v su ppl y (25c ) inte rnal v re f (v ) 02164-d-020 turn-on time ( s) 0 1 2 3 4 5 0 200 400 600 800 1000 1200 1400 1600 1800 no cap (7 s) settling time 1 f cap (1800 s) settling time f i gure 20. inte rn al v ref vs . t u r n - o n tim e 600 601 602 603 604 605 606 607 608 609 610 tem p 0 d i od e volta ge ( m v) 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 v supply (v) 02164-d-021 f i g u re 21. t e mp 0 d i od e v o lt ag e v s . v su ppl y (25c ) 120 100 80 60 40 20 0 s nr (db) 30 . 0 22. 5 7. 5 15. 0 0 3 7. 5 45. 0 5 2 . 5 60. 0 frequency (khz) 02164-d-022 f sample = 125khz f in = 15khz snr = 68.34db f i gure 22. auxi li ar y chann e l d y n a mi c p e r f o r m a nce ( f sa mple = 1 2 5 kh z, f in p u t = 15 kh z)
ad7873 rev. d | page 12 of 28 ? 120 ? 100 ?80 ?60 ?40 ?20 0 p s rr (db) 0 1 02 0 3 04 0 5 0 6 07 0 8 09 0 1 0 0 v cc ripple frequency (khz) 02164-d-023 v cc = 3v 100mv p-p sinewave on +v cc v ref = 2.5v ext reference f sample = 125khz, f in = 20khz f i g u r e 2 3 . a c p s r r v s . s u pp l y ri ppl e f r e q u e n c y f i g u re 2 3 shows t h e p o we r su p p ly re j e c t ion r a t i o ve rsu s v dd su p p ly f r e q u e nc y for t h e a d 7 8 7 3 . the p o we r sup p ly re j e c t ion ra ti o i s d e f i n e d a s th e ra ti o o f th e po w e r i n t h e a d c o u t p u t a t f u l l -s cale f r eq uen c y , f , t o th e p o w e r o f a 100 mv sine wa ve ap p l i e d t o t h e a d c v cc su p p ly of f r e q u e nc y f s : pss r ( d b ) = 10 log ( pf/ p f s ) w h er e: pf is p o w e r a t f r eq uen c y , f , in ad c o u t p u t . pf s is p o w e r a t f r e q uen c y , f s , c o u p l e d o n t o t h e a d c v cc su p p ly . h e r e a 100 mv p-p sine wa ve is co u p led o n t o th e v cc sup p ly . deco u p lin g c a p a ci t o rs o f 10 f a nd 0.1 f w e r e us ed o n t h e su p p ly .
ad7873 rev. d | page 13 of 28 circuit i n forma t ion the ad7873 is a fast, lo w-p o w e r , 12-b i t, sin g le-s u p p l y a/d co n v er t e r . th e ad7873 can be o p era t e d f r o m a 2.2 v t o 5.25 v su p p ly . whe n op e r a t e d f r om e i t h e r a 5 v su p p ly or a 3 v su p p ly , th e ad7873 is c a p a b l e o f thr o ug h p u t r a t e s o f 125 ks ps w h en p r o v ide d w i t h a 2 mh z clo c k. the ad7873 p r o v ides the us er wi t h an o n -c hi p trac k-and-h o ld , m u l t i p lexer , a/ d co n v er t e r , r e f e r e n c e , t e m p era t ur e s e n s o r , an d s e r i a l in t e r f ace h o us e d in a t i n y 16-le ad qsop , t ssop , o r lfcs p p a cka g e , w h ich o f fers t h e us er co n s idera b le s p ace- s a vi ng ad van t a g e s o v er al t e r n a t i v e s o l u t i o n s. th e s e r i al clo c k in pu t (d clk) acces s es da ta f r o m t h e p a r t a n d als o p r o v ides the c l o c k s o ur ce fo r t h e successi v e a p p r o x ima t ion a d c. the a n a l og in pu t ra n g e is 0 v t o v ref (w h e r e t h e ext e r n al l y a p plie d v ref can b e bet w een 1 v a n d +v cc ). th e ad7873 has a 2.5 v r e f e r e n c e o n b o a r d wi t h t h is r e fer e n c e v o l t a g e a v a i la b l e fo r us e ext e r n al l y if bu f f e r e d . the a n alog in pu t t o t h e a d c i s p r o v ide d v i a an o n -chi p m u l t i p lexer . this a n alog in p u t c a n be an y on e of th e x, y , an d z p a ne l c o ordi n a t e s , b a tte r y vol t age, or chi p te m p e r a t u r e. t h e m u l t i p lexer is co nf igur e d w i t h l o w-r e sist a n c e s w i t ch es t h a t al lo w a n un s e le c t e d ad c in p u t c h a n n e l t o p r o v ide p o w e r an d a n accom p an yin g p i n t o p r o v ide g r o u n d f o r a n ext e r n al de vice . f o r s o m e m e asur em en ts, t h e on-r esis t a n c e o f t h e s w i t ch es co u l d p r es en t a s o ur ce o f er r o r . h o w e v e r , wi t h a dif f er e n t i al in p u t t o t h e con v er t e r and a dif f er en t i al r e fer e n c e a r chi t e c t u r e , t h is er r o r ca n b e nega te d . adc tr ans f er func tion the o u t p u t co din g o f th e ad78 73 is s t ra ig h t b i na r y . th e desig n e d co de t r a n si tio n s o c c u r a t s u cces s i v e in t e g e r ls b va l u es (t ha t is, 1 ls b , 2 ls bs, a nd s o on). the ls b size is v ref /4096. the ideal tran sf er c h a r ac t e r i s t ic f o r th e ad7873 is s h o w n in f i gur e 24. 02164-d-024 adc code analog input 1lsb 0v +v ref ?1lsb 1lsb = v ref /4096 111...111 111...110 111...000 011...111 000...010 000...001 000...000 f i g u re 24. t r ans f er char ac t e ris t i c t y p i c a l c o nnec t i o n di a g r a m f i gur e 25 s h o w s a typ i cal co nn e c tio n dia g ram f o r th e ad7873 in a t o uc h s c r een co n t r o l a p p l ic a t io n. th e ad7 873 f e a t ur es a n in ter n a l r e fer e nce, b u t t h is can b e o v er dr i v en w i t h an ex ter n a l lo w im p e dan c e s o ur ce b e tw e e n 1 v a nd +v cc . th e va l u e o f t h e r e fer e n c e v o l t a g e s e ts t h e in p u t ra n g e o f t h e con v er t e r . th e c o n v e r s i on re su lt i s output m s b f i r s t , f o l l owe d by t h e re m a i n i n g 11 b i t s an d t h r e e t r a i lin g zer o es , dep e n d i n g o n t h e n u m b er o f clo c ks us e d p e r co n v ersio n . (s e e t h e s e r i al i n t e r f ace s e c t ion.) f o r a p plica t ion s w h er e p o w e r co n s um pt io n is a co n c er n, t h e p o we r m a n a ge me n t opt i o n s h ou l d b e u s e d to i m prove p o we r p e r f o r ma n c e . s e e t a b l e 7 fo r a v ai la b l e p o w e r ma na g e m e n t opt i ons . 02164-d-025 16 15 14 13 12 11 9 8 1 2 3 4 7 6 5 10 touch screen 1 f to 10 f (optional) 2.2v to 5v 0.1 f 0.1 f auxiliary input ad7873 +v cc x+ y+ x? y? gnd v bat aux dclk cs din busy dout penirq +v cc v ref serial/conversion clock chip select serial data in converter status serial data out pen interrupt 50k ? to battery voltage regulator f i gure 25. t y pic a l a p plic at ion cir c u i t
ad7873 rev. d | page 14 of 28 anal og input f i gur e 26 s h o w s a n eq u i valen t cir c ui t o f th e ana l og in p u t s t r u c t ur e o f th e ad7873, which co n t a i n s a b l o c k dia g ra m o f t h e in p u t m u l t i p lexer , t h e dif f er en t i al in p u t o f t h e ad c, and t h e dif f er en t i al r e fe r e n c e . t a b l e 5 sh o w s th e m u l t i p lexer addr es s co r r es p o n d in g t o e a ch a n alog i n p u t , bo th f o r th e s e r/ df r bit i n t h e c o n t ro l re g i ste r s e t hig h an d lo w . th e con t r o l b i t s a r e p r o v ide d s e r i al l y t o t h e de vice v i a t h e din p i n. f o r m o r e info r m a t io n on t h e co n t r o l r e g i s t er , s e e t h e c o n t r o l reg i st e r s e c t io n. w h en t h e con v er t e r en t e rs h o l d m o de , t h e v o l t a g e dif f er e n ce bet w een t h e +in a n d C i n in p u t s (see f i gur e 26) i s ca p t ur ed o n t h e i n t e r n al c a p a ci t o r a r ra y . the in p u t c u r r en t o n t h e a n alog in p u ts dep e nds o n t h e con v ersio n ra te o f t h e de v i ce . d u r i n g t h e sa m p l e pe ri o d , th e so u r c e m u s t c h a r g e th e in t e r n al sa m p l i n g ca p a c i t o r (typ ic al l y 37 pf). on ce th e ca p a c i t o r is f u l l y c h a r g e d , t h er e is n o f u r t h e r i n p u t c u r r en t. th e r a t e o f cha r g e t r a n sfer f r o m t h e a n alog s o ur ce t o t h e c o n v er t e r is a f u n c t i on o f co n v ersio n r a t e . 02164-d-026 x+ y+ ref int/ ext x? y ? gnd 6-to-1 mux 3-to-1 mux 3-to-1 mux in+ in+ in? ref? ref+ adc core data out v cc y? y+ x+ x? on-chip switches x+ y+ y? v bat aux temp f i g u re 26. equiv a le nt a n al og input c i rcuit table 5. analo g input, ref e re nce, a n d touch screen c o ntro l a2 a1 a0 ser/ dfr analog input x switches y switches +ref 1 Cref 1 0 0 0 1 temp0 off off v ref gnd 0 0 1 1 x+ off on v ref gnd 0 1 0 1 v b a t o f f o f f v ref gnd 0 1 1 1 x+ (z1) x+ off y+ on v ref gnd xC on yC off 1 0 0 1 yC (z2) x+ off y+ on v ref gnd xC on yC off 1 0 1 1 y+ on off v ref gnd 1 1 0 1 aux off off v ref gnd 1 1 1 1 temp1 off off v ref gnd 0 0 0 0 invalid ad dres s. test mode: s w itches out the temp0 diod e t o the penirq pin. 0 0 1 0 x+ off on y+ yC 0 1 0 0 invalid ad dres s 0 1 1 0 x+ (z1) x+ off y+ on y+ xC xC on yC off 1 0 0 0 yC (z2) x+ off y+ on y+ xC xC on yC off 1 0 1 0 y+ on off x+ xC 1 1 0 0 outputs identi ty code, 1000 0000 0000. 1 1 1 0 invalid addr ess . test mode: s w itches out the temp1 diod e t o the penirq pin. 1 inte rnal no de , no t d i re ctl y acce ss ibl e by the use r.
ad7873 rev. d | page 15 of 28 acquisition time the t r ack - an d - h o ld am plif ier e n t e rs t r ack i n g m o d e o n t h e fal l in g e d g e o f t h e f i f t h d c l k af t e r t h e st ar t b i t is det e c t e d (see f i gur e 35). t h e tim e r e q u i r ed f o r th e tra c k - a n d - h o ld a m plif ier t o acquir e a n i n p u t si g n a l dep e n d s on h o w q u ick l y t h e 37 pf in p u t ca p a ci tan c e is c h a r g e d . w i th zer o s o ur ce im p e dan c e o n t h e a n alog in p u t, t h r e e d c l k c y cles a r e al wa ys s u f f i cien t t o acq u ir e t h e sig n al t o t h e 12 -b i t l e v e l . w i t h a s o u r ce im p e dan c e r in o n t h e ana l og in p u t, t h e ac t u a l acq u isi t ion t i m e r e q u ir e d is c a l c ul a t ed u s i n g th e f o rm ul a : () pf 37 100 4 . 8 ? + = in acq r t w h er e r in i s t h e s o urce i m p e dance o f t h e i n pu t si g n a l , and 1 00 ?, 37 pf is th e in p u t r c . dep e ndin g o n t h e f r eq u e n c y o f d c lk u s ed , th r ee d c lk c y c l e s ma y o r ma y n o t be s u f f ic i e n t t o a c q u i r e t h e a n alog in p u t sig n al w i t h v a r i o u s s o ur ce im p e dan c e val u es. touch screen settling i n s o me a p pl i c a t i o ns , e x te r n a l c a p a c i tors c o u l d b e re qu i r e d acr o s s t h e t o uch s c r e en t o f i l t er n o is e as s o c i a t e d wi t h i t , fo r exa m ple , n o is e g e n e r a t e d b y t h e l c d p a ne l o r b a ckl i g h t cir c ui t r y . th e va l u e o f t h es e c a p a ci t o rs c a us es a s e t t ling t i m e r e q u ir em e n t w h en t h e p a n e l is to uch e d . th e s e t t lin g t i m e typ i cal l y a p p e a r s as a ga i n er r o r . ther e a r e s e v e ral m e t h o d s fo r mini miz i n g o r elimin a t in g t h i s issue. t h e p r ob le m ma y b e t h a t th e in p u t si gn al , r e f e r e n c e , o r bo th , h a v e n o t se t t led t o th ei r f i n a l v a l u e be f o r e t h e sa m p li n g i n sta n t o f th e ad c . a d di ti o n all y , th e r e fer e n c e v o l t a g e co u l d s t i l l b e cha n g i n g d u r i n g t h e con v ersio n c y c l e . on e o p tion is t o s t o p o r s l o w do wn t h e d c lk f o r th e re qu i r e d tou c h s c re e n s e tt l i ng t i m e . t h i s a l l o w s t h e i n put a n d r e fer e n c e t o s t ab i l i z e fo r t h e ac q u isi t io n t i m e , w h ich r e s o l v es t h e issue fo r b o t h si n g le-e n d e d and dif f er en t i a l mo des. the o t h e r o p tion is t o o p era t e t h e ad7873 in dif f er en tial m o de o n l y f o r th e t o uc h s c r e en, and p r og ra m th e ad7873 t o k e ep t h e t o uc h s c r e en dr iv ers o n an d n o t g o in t o p o w e r - do wn (p d0 = pd1 = 1). s e ver a l co n v ersio n s c o u l d b e r e q u ir e d , dep e ndin g on th e s e t t lin g tim e r e q u ir ed and the ad7873 da t a ra t e . o n ce t h e r e q u ir ed n u m b er o f co n v ersio n s ha v e been made , th e ad7873 ca n t h en be p l aced in a po w e r - d o w n s t a t e o n t h e l a s t m e a s ur e- m e n t . the las t m e t h o d is t o us e t h e 15 d c l k c y cle m o de , w h i c h ma in t a in s t h e to uch s c r e en dr iv ers o n un t i l i t i s co mman d e d b y th e p r oces so r t o s t o p . i n te rna l re fere nce the ad7873 has a n in t e r n al r e f e r e n c e v o l t a g e of 2.5 v . th e i n te r n a l re f e re n c e i s a v a i l a bl e o n t h e v ref p i n fo r ext e r n al us e in t h e sys t em; h o w e v e r , i t m u s t b e b u f f er e d b e fo r e i t is a p plie d el s e w h e r e. t h e on - c h i p re f e re n c e c a n b e tu r n e d on or of f w i t h th e p o w e r - do wn addr ess, p d 1 = 1 (s ee t a b l e 7 a nd f i gur e 27). t y p i cal l y t h e r e fer e n c e v o l t a g e i s o n l y us e d in si n g le-e n d e d m o de fo r b a t t er y m o ni t o r i n g , t e m p er a t ur e m e asur em en t, an d fo r usin g t h e a u xi lia r y in p u t. o p t i mal t o uch s c r e en p e r f o r ma nce is achie v e d w h e n usin g t h e dif f er en t i a l m o de . the p o w e r - u p time o f th e 2.5 v r e f e r e n c e is typ i cal l y 10 s wi t h o u t a lo ad; h o w e v e r , a 0.1 f ca p a ci t o r o n t h e v ref p i n is r e co m m e n de d fo r o p ti m u m pe rf o r m a n c e , wh i c h a f f e ct s th e po w e r - u p ti m e . (s ee f i gur e 20.) 02164-d-027 x+ y+ v ref 260 ? reference inpu t the v o l t a g e dif f er en ce b e tw e e n +ref a nd ?re f (s e e f i gur e 26) s e ts t h e a n alog in p u t ran g e . th e ad7873 o p er a t es wi t h a r e f e r - en c e in p u t in t h e ra n g e o f 1 v to +v cc . f i gur e 27 s h o w s t h e on- c h i p r e f e r e n c e c i r c ui tr y o n th e ad7873. th e in t e r n al r e f e r e n c e o n the ad7873 ca n be o v er dr i v en wi t h a n ext e r n al r e f e r e n c e; for b e st p e r f or manc e, how e ve r , t h e i n te r n a l re fe re nc e shou l d b e dis a b l e d w h e n an ext e r n al r e fer e n c e is a p plie d , b e ca us e s w 1 in f i gur e 27 o p en s o n the ad7873 when t h e in t e r n al r e f e r e n c e is dis a b l e d . th e on-chi p r e fer e n c e al wa ys is a v a i l a b l e a t t h e v ref p i n as lo ng as t h e r e fer e n c e is ena b le d . the in p u t im p e dan c e s e en a t t h e v ref p i n is a p p r o x ima t e l y 260 ? w h en the in t e r n al re fe re nc e i s e n a b l e d. whe n it i s d i s a bl e d , t h e i n put i m p e d a nc e s e en a t t h e v ref p i n i s in th e gi ga o h m r e gi o n . w h en ma kin g to uch s c r e en m e as ur emen t s , co n v ersio n s ca n b e made i n dif f er e n t i a l (r a t iom e t r i c ) m o de o r sing le-ende d m o de. if t h e s e r / df r b i t is s e t t o 1 in t h e co n t r o l r e g i s t er , th en a s i ng l e - e nd e d c o n v e r s i on i s p e r f or me d. f i g u re 2 8 show s t h e co nf igur a t io n fo r a sin g le-e n d e d y co o r di na te m e a s ur e m en t. the x+ i n p u t is co nne c t e d t o t h e a n alog-t o-dig i t a l co n v er t e r , t h e y+ and y ? dr i v ers a r e t u r n e d o n , and t h e vol t a g e on x+ is di g i t i ze d . t h e con v ers i on i s p e r f o r me d w i t h t h e ad c refere n c e d fr o m g n d t o v ref . t h i s v ref is ei t h er t h e on-chi p r e fer e n c e o r t h e v o l t a g e a p pl ie d a t t h e v ref pin ex ter n a l ly , a nd is deter m i n e d b y th e set t i n g o f th e po w e r m a na g e m e n t b i t s pd 0 a n d pd 1 (se e t a b l e 6). the adva n t a g e o f t h is m o de is t h a t t h e sw i t ch es t h a t
ad7873 rev. d | page 16 of 28 s u p p l y th e ext e r n al t o uc h s c r een can be t u r n e d o f f o n ce the acq u isi t ion is com p let e , r e su l t i n g in a p o w e r s a v i n g s. h o w e v e r , t h e on-r esis t a n c e o f t h e y dr i v ers a f fe c t s t h e i n pu t v o l t a g e t h a t ca n b e acq u ir e d . the f u l l t o uch s c r e en r e sis t an c e co u l d b e in t h e o r der o f 200 ? t o 900 ?, dep e ndin g on the man u fac t ur er . th u s , if t h e o n -r esis t a n c e o f t h e s w i t ch es is a p p r o x ima t e l y 6 ?, t r ue f u l l - s c a l e a n d z e ro - s c a l e vo lt age s c a n n ot b e a c q u i r e d , re g a rd l e s s o f w h er e t h e p e n/sty l us is o n t h e t o uch s c r e e n . n o t e t h a t t h e m i n i m u m tou c h s c re e n re s i st a n c e re c o m m e n d e d f o r u s e w i t h th e ad7873 is a p p r o x ima t e l y 7 0 ?. i n this mo de o f o p era t ion, t h er efo r e , s o me v o l t a g e is li k e l y t o b e los t acr o s s t h e i n t e r n al s w it c h e s , a n d it i s u n l i k e l y t h a t t h e i n t e r n a l s w it c h re s i s t a n c e wi l l t r ack t h e r e sis t a n ce o f t h e to uch s c r e en o v e r t e m p er a t ur e an d supply , prov i d i n g a n a d d i t i on a l s o u r c e of e r ror . 02164-d-028 +v cc v ref gnd y+ y? x+ in+ in+ in? ref+ adc core ref? f i gure 28. sing le -ended refe r e nce m o de (ser/ dfr = 1) th e a l te r n a t ive to t h i s s i tu a t i o n i s to s e t t h e se r / df r bi t l o w . a g a i n, ma k i n g a y co o r di na te m e a s ur e m en t is co n s ider e d , b u t n o w t h e +ref and Cref n o des o f t h e a d c a r e co nne c t e d dir e c t ly to t h e y + a nd y C p i n s . this m e a n s t h e a n a l o g -to - dig i t a l co n v ersio n is r a t i o m et r i c. th e res u l t o f t h e con v ersio n is al wa y s a p e r c en t a ge o f t h e ext e r n al r e sis t an ce , i n dep e nden t o f h o w i t co u l d c h a n g e wi th r e s p ec t t o t h e o n r e sis t an ce o f th e in t e r n al sw i t ch es. f i gur e 29 s h o w s t h e c o nf igura t io n fo r a ra t i om et r i c y co o r dina te m e a s ur em e n t. 02164-d-029 +v cc gnd y+ y? x+ in+ in+ in? ref+ adc core ref? f i gure 2 9 . di ffer e nt i a l refer e nc e m o de (ser/ dfr = 0) t h e d i s a dv a n t a ge of t h i s mo d e of op e r a t i o n i s t h a t d u r i ng b o t h t h e ac q u isi t ion phas e and con v ersio n p r o c es s, t h e ext e r n al t o uch s c re e n m u s t re m a i n p o we re d. t h i s re su lt s i n a d d i t i o n a l supp l y c u r r en t fo r t h e d u ra t i on o f t h e co n v ersio n . measurement s tem p er at ur e meas ur em ent t w o t e m p era t ur e m e as ur e m en t o p t i o n s a r e a v a i la b l e on t h e ad7873, the sing le co n v ersio n m e t h o d and the dif f er en tial c o n v e r s i on me t h o d . b o t h me t h o d s are b a s e d o n an o n - c h i p d i od e m e a s ur em en t . i n t h e sin g le con v ersio n m e t h o d , a dio d e vol t age is dig i t i ze d and re c o rd e d at a f i x e d c a l i br a t i o n te m p e r a t u r e. a n y sub s e q u e n t p o l l in g o f t h e di o d e p r o v ides an es t i ma t e o f t h e a m b i e n t tem - p e r a t u r e t h ro ug h e x t r a p ol a t i o n f r o m t h e cal i b r a t i o n te m p er a t ure dio d e r e s u l t . this as s u m e s a dio d e t e m p er a t ur e dr if t o f a p p r o x im a t e l y C 2 .1 mv/ c. this m e t h o d p r o v id es a r e s o l u t i o n of a pprox i m atel y 0 . 3 c an d a pre d i c te d a c c u r a c y of 3 c . the dif f er en t i al co n v ersio n m e t h o d is a 2- p o i n t m e as ur emen t. the f i rst m e asu r em e n t is p e r f o r m e d wi t h a f i xe d b i as c u r r en t in to a dio d e, and t h e s e cond me asur emen t is p e r f o r m e d w i t h a f i xe d m u l t i p le of t h e b i as c u r r en t i n t o t h e s a m e dio d e . the v o l t a g e dif f er ence in t h e dio d e r e adin gs is p r o p o r t i o n al t o a b s o l u t e t e m p er a t ur e and is g i v e n b y t h e fol l o w in g fo r m u l a: ( ) ( ) = ? / w h er e v be r e p r es en ts t h e dio d e v o l t a g e , n is t h e b i as c u r r en t mu l t i p l e , k is b o l t zma n n s c o n s t a n t and q i s t h e e l ectr o n c h a r g e . t h i s me t h o d prov i d e s more a c c u r a te ab s o lute t e m p e r a tu re m e as ur e m en t o f 2c. h o w e v e r , t h e r e s o l u t i on is r e d u ce d t o a p p r o x ima t e l y 1.6c. a s s u min g a c u r r en t m u l t i p le o f 105 (wh i c h i s t y p i cal f o r th e a d 7873) taki n g b o l t zm a n n s co n s ta n t , k = 1.38054 10 C23 e l ectr o n s v o l t s / d e gr ee s k e lv in , th e e l ectr o n charge q = 1.602189 10 C19 , th en t , t h e am b i e n t t e m p era t ur e i n deg r e e s cen t ig r a de, w o u l d b e c a lc u l a t e d as fol l o w s: ( ) ( ) () () 273 10 49 . 2 / / 3 ? ? = ? = = ? v be is calc u l a t e d f r o m t h e dif f er en ce in r e adings f r o m t h e f i rst co n v ersio n an d s e con d con v ersio n . f i gur e 30 sh o w s a b l o c k di a g ram o f t h e t e m p er a t ur e me a s u r e m e n t mo d e . 02164-d-030 i temp0 temp1 105*i mux adc f i g u re 30. bl ock d i ag r a m of t e m p er at ur e m e as u r e m e n t circuit
ad7873 rev. d | page 17 of 28 battery m e as urement the ad7873 can m o ni t o r a b a t t e r y v o l t a g e f r o m 0 v t o 6 v . f i g u r e 3 1 shows a bl o c k di ag r a m of a b a tte r y volt age mon i tore d th r o u g h th e v ba t p i n. th e v o l t ag e t o t h e +v cc of th e ad7873 is ma in t a i n e d a t t h e desir e d su p p ly v o l t a g e v i a t h e dc/dc r e gu l a t o r whi l e t h e i n p u t t o t h e r e gu l a t o r is m o ni t o r e d . this v o l t a g e on v ba t is di vi de d b y 4 s o t h a t a 6 v b a t t er y v o l t a g e is p r es en te d t o th e ad c as 1.5 v . t o co nser v e p o w e r , th e divider is o n o n l y d u r i ng t h e s a m p l i ng of a vo lt ag e on v ba t . t a b l e 5 sh o w s t h e c o n t rol b i t s e t t i n g s re qu i r e d to p e r f or m a b a tte r y me asu r e m e n t . 02164-d-031 + battery 0v to 6v dc/dc converter +v cc v bat 0v to 1.5v adc core 7.5k ? 2.5k ? f i g u re 31. bl ock d i ag r a m of b a t t er y m e as ure m ent c i rc uit pressu re measu r ement the p r essur e a pplie d t o t h e t o u c h s c r e en v i a a p e n o r f i n g er can als o be m e as ur e d wi th t h e ad7 873 wi t h s o m e s i m p le calc u l a - ti o n s . th e 8-b i t r e so l u ti o n m o de w o uld b e s u f f i ci e n t f o r th i s m e as u r em e n t , b u t th e f o llo w i n g calcula t i o n s a r e s h o w n w i t h t h e 12-b i t r e s o l u t i on m o de . the con t ac t r e sis t an c e b e tw e e n t h e x a n d y pl a t es is m e a s ur e d , w h ich p r o v ides a go o d i n di c a t i o n o f t h e si ze o f t h e dep r ess e d a r e a and t h e a p pli e d pr essur e . the a r e a of t h e sp o t tou c he d i s prop or t i on a l to t h e s i z e o f t h e o b j e c t t o uc hin g i t . th e size o f t h is r e sist a n ce (r tou c h ) ca n be calc u l a t ed usin g tw o dif f er en t m e t h o d s. the f i rst m e t h o d r e q u ir es t h e us er t o kn o w t h e t o t a l r e sist an ce o f t h e x-pl a t e t a b l et. thr e e t o uch s c r e en con v ersio n s a r e r e q u i r ed , a m e as u r e m en t o f th e x - posi ti o n , z 1 -posi t i o n , a n d z 2 - p o si t i o n (s e e f i gur e 32). th e fol l o w in g e q u a t i on calc u l a t es t h e tou c h re s i st anc e : ( )( ) ( ) [ ] ? = the s e cond m e t h o d r e q u ir es t h a t t h e r e sist an c e o f b o t h t h e x-pl a t e an d y - pla t e t a b l e t s a r e k n o w n. a g a i n t h re e t o uch s c r e e n co n v ersio n s a r e r e q u ir e d , a m e asur em e n t o f t h e x-p o si t i on, y- p o s i t i o n , a n d z 1 - p osi t i o n (see f i g u r e 32). the fol l o w in g e q ua t i o n als o cal c u l a t es t h e t o uch r e sist an ce: ( )( ) ( ) [ ] { } () {} ? ? = + ? + ? + ? 02164-d-032 x+ x? x+ x? y+ y? y? touch touch z 2 -position z 1 -position touch x-position y+ x+ x? y? y+ measure x-position measure z 1 -position measure z 2 -position f i g u re 32. p r es s u r e m e as ure m ent b l o c k d i ag r a m
ad7873 rev. d | page 18 of 28 pen interrupt request the p e n in t e r r u p t e q ui valen t circ ui t r y is o u t l in e d in f i gur e 33. b y connec tin g a p u l l - u p r e s i st or (10 k? t o 100 k?) betw e e n + v cc a n d t h is cm os log i c o p en dra i n o u t p u t , t h e pe ni rq output re m a i n s h i g h n o r m a l ly . i f pe ni rq i s en a b le d (s ee t a b l e 7), wh en th e t o uch s c r een co nn ec ted t o th e ad7873 is to uc h e d b y a p e n o r f i n g er , t h e pe ni rq o u t p u t g o es lo w , ini t ia t i n g an in t e r r u p t t o a m i cr o p r o ces s o r , whic h can t h en ins t r u c t a co n t r o l w o r d t o b e wr i t t e n t o the ad7873 t o ini t ia te a con v ersio n . this o u t p u t can also be enab led betw een co n v ersio n s d u r i n g p o w e r - do wn (see t a b l e 7) al lo wing p o w e r - u p t o b e ini t i a t e d o n ly when t h e s c r e e n i s tou c he d. t h e re su lt of t h e f i rs t tou c h s c re e n c o ordi na te c o n - v e rsio n a f t e r p o w e r - u p is va li d , assumi n g an y e x t e rn a l r e f e r e n c e i s set t led t o th e 1 2 - b i t o r 8-b i t lev e l a s r e q u i r ed . f i gur e 34 assu m e s t h a t t h e pe ni rq f u n c tio n was ena b led in t h e l a st wr i t e o r t h a t t h e p a r t was j u st p o w e r e d u p s o pe ni rq is ena b le d b y def a u l t. on c e t h e s c r e en is t o uch e d , t h e pe ni rq ou t p ut go e s l o w a t i me t pen l a t e r . this dela y is a p p r o x ima t e l y 5 s, as s u m i n g a 10 nf t o uc h scr e en c a p a ci tan c e , a n d va r i es wi th th e t o u c h scr een r e s i s t a n ce a c t u all y u s ed . o n ce th e s t ar t b i t i s de te c t e d , t h e p e n i n ter r u p t f u n c t i on is di s a b l e d and t h e pe ni rq c a n n ot re s p on d to s c re e n tou c h e s . t h e pe ni rq output r e m a i n s lo w un t i l t h e four t h fal l in g e d ge o f d c lk a f t e r t h e s t ar t b i t is c l o c k e d in, a t w h ic h p o in t i t r e t u r n s high as so o n as p o s s i b le , i r r e s p ecti v e o f th e t o u c h scr een ca pa ci ta n c e . t h i s d o e s n o t m e a n t h a t t h e p e n i n ter r u p t f u n c t i o n is n o w enab le d a g a i n b e c a us e t h e p o w e r - do wn b i ts ha v e n o t yet b e e n lo ade d t o t h e con t r o l re g i ste r . r e g a rd l e ss of w h e t he r pe ni rq is t o be enab le d a g a i n, th e pe ni rq o u t p u t n o r m al l y al wa ys idl e s high . a s s u m i n g th e pe ni rq is ena b le d a g a i n as sh o w n i n f i gur e 34, t h en on ce t h e co n v ersio n is com p let e , t h e pe ni rq output ag ai n re sp ond s to a s c r e en t o uch. th e fac t t h a t pe ni rq r e t u r n s hig h al m o st i m m e dia t e l y a f t e r th e f o u r th falli n g ed g e o f d c l k m e a n s t h e u s e r a v oi d s a n y spu r i o u s i n te r r upt s on t h e m i c r opro c e ss or or ds p , which co u l d o c c u r if t h e i n t e r r u p t r e q u est line o n t h e m i cr o/ds p w e r e unm a sk e d d u r i n g o r t o wa r d t h e end o f co n v ersio n an d t h e pe ni rq p i n w a s st i l l lo w . on ce t h e nex t s t ar t b i t is det e c t e d b y th e ad7843, th e pe ni rq f u nc tio n is ag ai n d i s a bl e d . i f t h e con t r o l r e g i st er wr i t e o p e r a t io n o v erl a ps wi t h t h e da t a r e ad , a s t ar t b i t is al wa ys det e c t ed p r io r t o the end o f co n v ersio n , m e anin g t h a t e v en i f t h e pe ni rq f u n c tio n is ena b le d in t h e c o n t r o l r e g i st er , i t is dis a b l e d b y t h e st ar t b i t a g a i n b e fo r e t h e end o f t h e con v ersio n is r e ach e d , s o t h e pe ni rq f u n c tio n ef f e c t i v e l y ca nn o t be used in t h is m o de . h o w e v e r , as co n v ersio n s a r e o c c u r r i n g co n t in uously , t h e pe ni rq f u nc t i o n i s no t ne c e ss ar y an d i s t h e r e f ore re d u n d a n t . 02164-d-033 y+ y? on x+ touch screen external pull-up penirq enable +v cc +v cc penirq 100k ? f i g u re 33. pe nir q f u nc t i o n al bl ock d i ag r a m 02164-d-034 sa 2 a 1 a 0 1 0 8 1 1 13 16 mode ser/ dfr screen touched here interrupt processor no response to touch t pen penirq cs dclk din (start) pd1 = 1, pd0 = 0, penirq enabled again f i g u re 34. pe nir q tim i n g d i ag ra m
ad7873 rev. d | page 19 of 28 control register the control word provided to the adc via the din pin is shown in table 6. this provides the conversion start, channel addressing, adc conversion resolution, configuration, and power-down of the ad7873. table 6 provides detailed information on the order and description of these control bits within the control word. initiate start the first bit, the s bit, must always be set to 1 to initiate the start of the control word. the ad7873 ignores any inputs on the din line until the start bit is detected. channel addressing the next three bits in the control register, a2, a1, and a0, select the active input channel(s) of the input multiplexer (see table 5 and figure 26), touch screen drivers, and the reference inputs. mode the mode bit sets the resolution of the analog-to-digital converter. with a 0 in this bit, the following conversion has 12 bits of resolution. with a 1 in this bit, the following conversion has 8 bits of resolution. ser/ dfr the ser/ dfr bit controls the reference mode, which can be either single-ended or differential if a 1 or a 0 is written to this bit respectively. the differential mode is also referred to as the ratiometric conversion mode. this mode is optimum for x-position, y-position, and pressure-touch measurements. the reference is derived from the voltage at the switch drivers, which is almost the same as the voltage to the touch screen. in this case, a separate reference voltage is not needed because the reference voltage to the adc is the voltage across the touch screen. in single-ended mode, the reference voltage to the converter is always the difference between the v ref and gnd pins. see table 5 and figure 26 through figure 29 for further information. if x-position, y-position, and pressure touch are measured in single-ended mode, an external reference voltage or +v cc is required for maximum dynamic range. the internal reference can be used for these single-ended measurements; however, a loss in dynamic range is incurred. if an external reference is used, the ad7873 should also be powered from the external reference. because the supply current required by the device is so low, a precision reference can be used as the supply source to the ad7873. it might also be necessary to power the touch screen from the reference, which could require 5 ma to 10 ma. a ref19x voltage reference can source up to 30 ma, and, as such, could supply both the adc and the touch screen. care must be taken however, to ensure that the input voltage applied to the adc does not exceed the reference voltage and therefore the supply voltage. see the absolute maximum ratings section. note that the differential mode can only be used for x-position, y-position, and pressure touch measurements. all other measurements require single-ended mode. pd0 and pd1 the power management options are selected by programming the power management bits, pd0 and pd1, in the control register. table 7 summarizes the options available and the internal reference voltage configurations. the internal reference can be turned on or off independent of the analog-to-digital converter, allowing power saving between conversions using the power management options. on power-up, pd0 defaults to 0, while pd1 defaults to 1. table 6. control register bit function description msb lsb s a2 a1 a0 mode ser/dfr pd1 pd0 bit no. mnemonic comment 7 s start bit. the control word starts with the first high bi t on din. a new control word can start every 15th dclk cycle when in the 12-bit conversion mode or every 11t h dclk cycle when in 8-bit conversion mode. 6C4 a2Ca0 channel select bits. these three address bits along with the ser/dfr bit control the setting of the multiplexer input, switches, and reference inp uts, as detailed in table 5. 3 mode 12-bit/8-bit conversion select bit. this bit controls the resolution of the followi ng conversion. with a 0 in this bit, the conversion has 12-bit resolution or, wi th a 1 in this bit, 8-bit resolution. 2 ser/ dfr single-ended/differential reference select bit. along with bits a2Ca0, this bit controls the setting of the multiplexer input, switches, and reference inputs as described in table 5. 1, 0 pd1, pd0 power management bits . these two bits decode the power-down mo de of the ad7873 as shown in table 7.
ad7873 rev. d | page 20 of 28 ta ble 7. power ma na geme nt opt i ons pd1 pd0 penirq description 0 0 enabled this configuration results in im mediate power- d own of the on chip reference as soon as pd1 is set to 0. the ad c power s d o wn o n ly between co nversio n s. whe n pd0 is set to 0, the conversion is performed first and the adc power s down upon com p letio n of that conversi on (or upon the rising edge of cs if it occurs first). a t the start of the nex t conversion, the adc instantly power s up to full power . t h is means if the device is bei n g used in the differential mode, or an external refere nce is use d , there is no ne ed for addi tional delays to ensure full operation and the very first conversio n is v a lid. the yC switch is on w h ile in power-dow n. when the device is performing differential table conversion s, the reference an d referenc e buffer do not attempt to pow er up with bits pd1 and pd0 programm ed in this way. 0 1 enabled this configuration results in swi t ching the reference off immediately and the adc on permane n tly. when the device is perfor ming differential tablet conv ersi ons, th e referen c e and referenc e buffe r do not attempt to power up with bits pd1 and pd0 programmed in this way. 1 0 enabled this configuration results in swi t ching the reference on and p o w ering the adc down between conver sion s. the adc powers d o wn only betw ee n conver sion s. when pd0 is set to 0, the conversion is performed first, and the adc powers do wn upon com p l e tion of the con version (or upo n the rising edge of cs if it occu rs first). at the st art of the next conv ersion, the adc instan tly powers up to full power. there is no need for additional delays to ensure full oper a tion as the refe rence remain s permanently powered up. 1 1 disabl ed t h is configuration results in a l w a ys keep ing the device power ed up. the reference and the adc are on. po wer v s . throug hput r a te b y usin g t h e p o w e r - do wn o p t i on s o n t h e ad78 73 when n o t co n v er t i ng, t h e a v era g e p o w e r co n s um pt io n o f t h e de vice decr eas e s a t lo w e r t h r o ug h p u t ra t e s. f i gur e 35 sh o w s h o w , as t h e th r o ugh p u t ra t e i s r e d u ced wh ile m a in ta in i n g t h e d c lk f r eq uen c y a t 2 mh z, t h e de vic e r e m a in s in i t s p o w e r - do wn s t a t e lo n g er a n d t h e a v era g e c u r r en t c o n s um pt io n o v er t i m e dr o p s acco r d in g l y . s u p p l y curre nt ( a) 1 100 10 1000 0 120 throughput (ksps) 02164-d-035 40 60 0 2 0 8 0 100 f dclk = 16 f sample f dclk = 2mhz v cc = 2.7v t a = ? 40c to +85c f i gure 35. sup p l y current v s . through p ut (a ) f o r ex am p l e , if t h e ad7 873 is o p er a t e d in a 2 4 d c lk con t in uo u s s a m p ling m o de, wi t h a t h r o ug h p u t ra te o f 10 k s ps an d a d c l k o f 2 mh z, an d t h e de vice is plac ed in the p o w e r - do wn m o de betw een con v ersio n s, (p d0, p d 1 = 0, 0), tha t is, th e ad c s h u t s d o w n b e t w e e n c o n v e r s i ons but t h e re f e re n c e re m a i n s p o we re d d o w n pe rm a n en tl y , th en th e cu rr e n t co n s u m p t io n i s calcula t e d a s f o l l ow s . t h e c u r r e n t c o nsu m pt i o n d u r i ng no r m a l op e r a t i o n wi t h a 2 m h z d c lk is 210 a ( v cc = 2.7 v). a s s u min g an ext e r n al r e fer e n c e is us e d , t h e p o w e r - u p t i m e o f t h e ad c is in s t an taneo u s, so wh en t h e p a r t is co n v er ting, i t co n s um es 210 a. i n this m o de o f o p era t io n, th e p a r t p o w e rs u p o n the fo ur t h fal l in g e d g e o f d c l k a f t e r t h e s t a r t b i t i s r e cog n ize d . i t g o es bac k in t o p o w e r - do wn a t t h e end o f co n v er sio n o n the 20th fal l in g e d ge o f d c lk, w h ic h m e a n s tha t t h e p a r t co n s um es 210 a f o r 16 d c lk c y c l es o n l y , 8 s d u r i n g each con v ersio n c y c l e . i f th e thr o ug h p u t r a t e is 1 0 ks ps, the c y c l e tim e is 100 s a n d t h e a v era g e p o w e r dis s i p a t e d d u r i n g each c y c l e is (8/100) (210 a) = 16.8 a.
ad7873 rev. d | page 21 of 28 serial interf a c e f i gur e 36 sh o w s t h e typ i cal op er a t io n o f t h e s e r i al in t e r f ace o f th e ad7873. the s e r i al c l o c k p r o v ides the con v ersio n c l o c k and also co n t r o ls th e tra n sf er o f inf o r m a t io n t o and f r o m the ad7873. one c o m p let e con v ersio n can be ac hiev e d wi th 24 dc l k cy c l e s . the cs sig n al ini t ia t e s t h e da t a t r an sfer a n d con v ersio n p r o c es s. the f a l l in g e d ge o f cs tak e s t h e b u s y o u t p u t a n d th e se rial b u s o u t o f t h r e e-st a t e . the f i rst eig h t d c lk c y cles a r e us e d t o wr i t e t o t h e con t r o l r e g i st er v i a t h e d i n p i n. th e con t r o l r e g i st er is u p da te d in st a g es as e a ch b i t is clo c k e d i n . on c e t h e con v er t e r h a s e n ou g h i n f o r m a t i o n a b out t h e f o l l ow i n g c o n v e r s i on to s e t t h e i n p u t m u l t i p lexer a n d s w i t ch es a p p r o p r i a t e l y , t h e con v er t e r en ters t h e a c q u i s i t ion mo de and, if re quire d , t h e i n ter n a l s w i t ch e s a r e t u r n e d o n . d u r i n g acq u isi t io n m o de , t h e refer e n c e i n p u t da ta i s u p d a t e d . a f t e r th e th r e e d c l k c y c l e s o f a c q u i s i t i o n , th e c o n t ro l word i s c o m p l e te ( t he p o we r m a n a ge m e n t bit s are now u p da te d) and t h e co n v er t e r e n t e rs co n v ersio n mo de . a t t h is p o in t, t r ack- a n d-h o ld go es in t o h o ld m o de , t h e in p u t sig n a l is sa m p le d , a n d t h e b u s y o u t p u t g o es high (b us y r e t u r n s lo w o n th e n e xt fal l ing edg e o f d c lk). t h e in t e r n al swi t ch es c a n also t u r n o f f a t t h is p o in t if in si n g le-e n d e d m o de , b a t t er y-m o ni t o r mo d e , or te m p e r a t u r e me a s u r e m e n t mo d e . the n e xt 12 d c lk c y cles a r e us e d t o p e r f o r m t h e con v ersion a n d t o clo c k o u t t h e con v ersio n r e su l t . i f t h e con v ersio n is ra tiom etr i c (s er/ df r lo w), t h e in t e r n al swi t ch es a r e o n d u r i n g t h e con v ersio n . a 13t h d c lk c y cle is n e e d e d to al lo w t h e ds p/ micr o t o clo c k i n t h e ls b . thr e e m o r e d c lk c y cles clo c k o u t t h e t h r e e t r ai lin g zer o es an d co m p let e t h e 2 4 d c lk t r an sfer . t h e 2 4 d c l k c y cl e s c a n b e prov i d e d f r om a d s p or v i a t h re e b u r s ts o f eigh t clo c k c y c l es f r o m a micr o c o n tr o l ler . 02164-d-036 cs dclk din busy dout x/y switches 1 (ser/dfr high) x/y switches 1,2 (ser/dfr low) three-state (start) idle off off (msb) (lsb) on on off off acquire conversion idle zero filled three-state three-state three-state t acq 1 888 11 10 9 8 7 6 5 4 3 2 1 0 1 1 s a 2 pd1 pd0 a1 a0 mode ser/ dfr notes 1 y drivers are on when x+ is selected input channel (a2?a0 = 001), x drivers are on when y+ is selected input channel (a2 ?a0 = 101). 1 when pd1, pd0 = 00, 01 or 10, y ? will turn on at end of conversion. 2 drivers will remain on if power-down mode is 11 (no power-down) until selected input channel, reference mode, 1 or power-down mode is changed, or cs is high. f i g u re 36. co nvers i on ti ming, 2 4 dclks per conve r s i o n c y cl e , 8-bi t b u s inte r f ace . n o dcl k d e l a y requir ed with dedic a t e d serial po r t . 02164-d-037 cs dclk din busy dout db11 pd0 db10 t 1 t 4 t 5 t 6 t 6 t 9 t 10 t 11 t 7 t 2 t 3 t 8 t 12 f i g u re 37. d e t a il ti ming d i ag r a m
ad7873 rev. d | page 22 of 28 16 clocks per cycle the co n t r o l b i t s fo r t h e n e xt con v ersio n ca n b e o v erla p p e d wi t h t h e c u r r en t con v ersio n t o al lo w fo r a co n v ersio n e v er y 16 d c l k c y c l es, as s h o w n in f i gur e 38. this tim i n g dia g r a m also al lo ws t h e p o ss ibi l it y of c o mm u n i c a t i o n w i t h ot he r s e r i a l p e r i p h e r a l s bet w een ea ch b y t e (e i g h t d c lk ) tra n sf e r be tw ee n t h e p r ocesso r a n d t h e con v er ter . h o w e v e r , t h e co n v ersio n m u st co m p le t e w i t h i n a s h or t e n ou g h t i me f r a m e to a v oi d c a p a c i t i ve d r o o p ef fe c t s t h a t co u l d dist o r t t h e con v ersio n r e su lt. i t sh o u ld als o b e n o ted tha t t h e ad7873 is f u l l y p o w e r e d while o t h e r s e r i al co mm uni c a t io ns a r e t a kin g plac e b e tw e e n b y t e t r a n sfers. 15 clocks per cycle f i gur e 39 s h o w s th e fas t est wa y t o c l o c k the ad7873. this sc h e m e do es n o t w o r k wi t h m o s t m i cr o c o n tr o l ler s o r ds p s b e ca us e t h e y a r e n o t c a p a b l e o f g e n e r a t i n g a 15 clo c k c y cle p e r ser i al tra n sf er . h o w e v e r , so m e ds p s al lo w t h e n u m b er o f c l o c ks p e r c y c l e t o be p r ogra m m e d . this m e t h o d co ul d also be use d wi t h fpga s (f ield p r ogra m m a ble ga t e a r ra ys) o r a s i c s (a p p li- ca tio n sp ecif ic in t e gr a t e d cir c ui ts). a s in t h e 16 c l o c ks p e r c y c l e cas e , t h e con t r o l b i t s fo r t h e n e xt co n v ersion a r e o v erla p p e d wi t h t h e c u r r en t co n v ersio n t o al lo w a co n v ersion e v er y 15 d c lk c y cles usin g 12 d c l k s to p e r f o r m t h e co n v ersio n and 3 d c lks t o acq u i r e t h e a n alog i n p u t. this ef fe c t iv e l y in cr e a s e s th e thr o ug h p u t ra t e o f the ad 7 873 beyon d tha t us ed f o r th e sp e c if ic a t ion s t h a t a r e t e st e d usin g 16 d c l k s p e r c y cle , a n d dc l k = 2 m h z . 8-bit conv ersion the ad7873 can b e s e t u p t o o p era t e in a n 8-b i t m o de ra t h er tha n a 12 -b i t m o de b y set t in g t h e m o d e b i t in th e co n t r o l r e gis t er t o 1. t h is m o de al lo ws a fas t er thr o ugh p u t ra te t o b e a c h i ev ed , a s s u mi n g 8-b i t r e so l u t i o n i s s u f f i ci e n t . w h en u s i n g 8- b i t m o de , a co n v er sio n is co m p let e f o ur c l o c k c y c l es ea r l ier tha n in 12-b i t m o de . t h is co uld be used wi t h ser i al in t e r f aces tha t p r o v ide 12 clo c k t r an sfers, o r two con v ers i on s cou l d b e com p le t e d w i t h th r e e ei gh t- c l oc k tra n s f e r s . t h e th r o u g h p u t ra t e in cr e a se s b y 25% as a r e sul t o f th e sh o r t e r co n v er sio n c y c l e , b u t t h e co n v ersio n i t s e l f ca n o c c u r a t a fast er clo c k r a t e b e c a us e t h e in t e r n al s e t t ling time o f th e ad7873 is n o t as cr i t ical , be ca us e set t ling t o eig h t b i ts is al l tha t is r e q u ir ed . th e c l o c k ra te can be as m u ch as 50% faster . th e fa ste r clo c k ra te and fe w e r clo c k c y cles co m b i n e t o p r o v ide do uble t h e con v ersion ra te . 02164-d-038 cs dclk din busy dout 1 s s 11 10 9 8 7 6 5 4 3 2 1 0 11 10 9 11 1 88 8 control bits control bits f i g u re 38. co nvers i on ti ming, 1 6 dclks per c y c l e , 8-bit b u s in t e r f a c e . no d c l k d e l a y r e qu ir e d w i t h d e di c a t e d s e ria l p o r t . 02164- d - 039 cs dclk din busy dout s a 2 pd1 pd0 a1 a0 mode ser/ dfr mode ser/ dfr 1 11 10 9 8 7 6 5 4 3 2 1 0 11 10 9 8 7 6 5 4 15 1 15 1 sa 2 sa 2 a1 pd1 pd0 a0 f i gure 39. co n v ers i on ti ming, 1 5 dclks per c y c l e , m a xi m u m t h roughp ut rate
ad7873 rev. d | page 23 of 28 grounding and layout for information on grounding and layout considerations for the ad7873, refer to application note an-577, layout and grounding recommendations for touch screen digitizers. pcb design guidelines for chip scale package the lands on the chip scale package (cp-32), are rectangular. the printed circuit board pad for these should be 0.1 mm longer than the package land length and 0.05 mm wider than the package land width. the land should be centered on the pad. this will ensure that the solder joint size is maximized. the bottom of the chip scale package has a central thermal pad. the thermal pad on the printed circuit board should be at least as large as this exposed pad. on the printed circuit board, there should be a clearance of at least 0.25 mm between the thermal pad and the inner edges of the pad pattern. this ensures that shorting is avoided. thermal vias may be used on the printed circuit board thermal pad to improve thermal performance of the package. if vias are used, they should be incorporat ed in the thermal pad at 1.2mm pitch grid. the via diameter should be between 0.3 mm and 0.33 mm and the via barrel should be plated with 1 oz. copper to plug the via. the user should connect the printed circuit board thermal pad to gnd.
ad7873 rev. d | page 24 of 28 outline dimensions 16 9 8 1 pin 1 sea t i n g pl a n e 0 . 010 0 . 004 0. 01 2 0. 00 8 0. 02 5 bs c 0. 0 1 0 0. 0 0 6 0 . 050 0 . 016 8 0 coplanarity 0.004 0. 0 6 5 0. 0 4 9 0. 06 9 0. 05 3 0. 15 4 bs c 0. 2 3 6 bs c compliant to jedec standards mo-137ab 0. 19 3 bs c f i gure 40. 1 6 -l ead shrink sm al l o u t lin e p a ckage [qs o p ] (r q - 16) d i mensions sh o w n in inc h es 16 9 8 1 pin 1 seating plane 8 0 4.50 4.40 4.30 6.40 bsc 5.10 5.00 4.90 0.65 bsc 0.15 0.05 1.20 max 0.20 0.09 0.75 0.60 0.45 0.30 0.19 coplanarity 0.10 compliant to jedec standards mo-153ab f i gure 41. 1 6 -l ead thin shr i nk s m a l l o u tline p a ckage [ t ssop ] (ru - 16) di me nsio ns sho w n i n mi ll im e t e r s 16 5 13 8 9 12 1 4 bo t t o m view 2.25 2.10 sq 1.95 0.75 0.60 0.50 0.65 bsc 1. 95 bs c 0.35 0.28 0.25 12 max 0. 20 re f seating plane pin 1 indicator top view 4.0 bs c s q 3.75 bs c s q 0.60 max 0.60 max 0.05 max 0.02 nom 0 . 8 0 ma x 0.65 typ pin 1 indicator 1.00 0.85 0.80 coplanarity 0.08 0. 25 m i n compliant to jedec standards mo-220-vggc f i gure 42. 1 6 -l ead l e ad f r a m e ch ip s c a l e p a ck ag e [lfcs p ] (c p - 1 6 ) di me nsio ns sho w n i n mi ll im e t e r s
ad7873 rev. d | page 25 of 28 ordering guide model temperature range linearity error (lsb) 1 package option 2 ad7873arq C40c to +85c 2 rq-16 ad7873arq-reel C40c to +85c 2 rq-16 ad7873arq-reel7 C40c to +85c 2 rq-16 ad7873brq C40c to +85c 1 rq-16 ad7873brq-reel C40c to +85c 1 rq-16 ad7873brq-reel7 C40c to +85c 1 rq-16 ad7873aru C40c to +85c 2 ru-16 ad7873aru-reel C40c to +85c 2 ru-16 ad7873aru-reel7 C40c to +85c 2 ru-16 ad7873aruz-reel 3 C40c to +85c 2 ru-16 ad7873aruz-reel7 3 C40c to +85c 2 ru-16 ad7873acp C40c to +85c 2 cp-16 ad7873acp-reel C40c to +85c 2 cp-16 ad7873acp-reel7 C40c to +85c 2 cp-16 ad7873acpz 3 C40c to +85c 2 cp-16 ad7873acpz-reel 3 C40c to +85c 2 cp-16 ad7873acpz-reel7 3 C40c to +85c 2 cp-16 ad7873bcp C40c to +85c 1 cp-16 ad7873bcp-reel C40c to +85c 1 cp-16 ad7873bcp-reel7 C40c to +85c 1 cp-16 eval-ad7873cb 4 evaluation board eval-control brd2 5 controller board 1 linearity error here refers to integral linearity error. 2 rq = qsop = 0.15 quarter size outline package; ru = tssop. 3 z = pb-free part. 4 this can be used as a standalone evaluation board or in conjunction with the evaluation board controller for evaluation/demons tration purposes. 5 this evaluation board controller is a complete unit allowing a pc to control and communicate with all analog devices evaluatio n boards ending in cb designators.
ad7873 rev. d | page 26 of 28 notes
ad7873 rev. d | page 27 of 28 notes
ad7873 rev. d | page 28 of 28 notes ? 2004 analo g de vices, inc. all rights reserve d . tra d em arks and registered tra d em ar ks are the proper t y of their respective owne rs. c02164C0 C 6/04(d)

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