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ultralow input bias current operational amplifier ad549 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 ultralow bias c u rrent 60 fa max (a d 549l) 250 f a max ( a d549j) input bias curr ent guarantee d over common mode voltage range low offset voltage 0.25 mv max ( ad54 9 k) 1.00 mv max ( ad54 9 j) low offset drift 5 v/c max (a d549k) 2 0 v / c ma x (a d 5 49 j) low power 700 a m a x su pply current low input voltage noise 4 v p-p 0.1 hz to 10 h z mil-std-883b parts avai lable applic ation s electrometer a m plifiers photod iod e preamp ph electrode b u ffer vacuum ion gauge meas urem ent produc t de scription the ad549 1 is a m o n o li t h ic e l e c t r o m et er o p era t io na l a m plif ier wi t h v e r y lo w in p u t b i a s c u r r en t. i n p u t o f fs et v o lt a g e and i n p u t o f fs et v o l t a g e dr if t a r e la s e r t r imm e d fo r p r e c isio n p e r f o r ma n c e. the p a r t s u l t r alo w in p u t c u r r en t is achie v e d w i t h t o pga t e jfet t e chn o log y , a p r o c es s de v e l o p men t excl usi v e t o analog d e vi ces, i n c. this t e ch n o log y a llo ws fa b r ica t ion o f ext r eme l y lo w in p u t c u r r en t jfet s co m p a t i b le wi th a s t anda rd j u n c t i o n is ol a t e d bi p o l a r pro c e s s . t h e 1 0 15 ? co mm o n -m o d e im p e d a n c e , which re su l t s f r om t h e b o ot st r a pp e d i n p u t st ag e, e n su re s t h a t t h e in pu t c u r r en t is ess e n t ia l l y i n dep e nde n t o f co m m o n - m o d e vol t a g e. the ad549 is sui t e d f o r a p p l ic a t io n s tha t r e q u ir e v e r y l o w in p u t c u r r en t a nd lo w in p u t o f fs et v o l t a g e . i t exce ls as a p r e a m p fo r a wide va r i ety o f c u r r en t o u t p u t t r a n s d ucers, s u ch as ph o t o d io de s, p h o t om u l t i p l ier t u bes, o r o x yg en s e n s o r s. the ad549 can als o b e us e d as a p r e c isio n in teg r a t or o r lo w dr o o p s a m p le and h o ld . the ad549 is p i n com p a t i b le wi th s t anda r d fet an d e l e c t r o m e t er o p a m ps, al lo win g desig n ers t o u p g r ade t h e p e r f o r ma n c e o f p r es en t sys t em s a t li t t le addi t i onal cos t . connection diagram 00511-001 ad549 offset null output nc v? offset null noninverting input v+ guard pin, connected to case inverting input v os trim ?15v 10k ? nc = no connection 6 1 5 4 7 3 2 8 1 5 4 fi g u r e 1 . the ad549 is a v a i lab l e in a t o -99 h e r m etic p a c k a g e . th e c a s e is co nne c t e d to pin 8 s o t h a t t h e m e t a l c a s e c a n b e i ndep e n- den t ly co nn e c te d t o a p o in t a t t h e s a me p o ten t i a l as t h e in p u t ter m ina l s, mini miz i n g st r a y le a k a g e to t h e cas e . the ad549 is a v a i lab l e in f o ur p e r f o r ma n c e g r ades. th e j , k, a nd l v e rsio n s ar e ra t e d o v er t h e co mm er cial t e m p er a t ur e ra n g e o f 0c t o +70c . th e s g r ad e is sp e c if ie d o v er t h e m i li t a r y t e m p era t ur e ra ng e o f ?55c t o +125c, a nd is a v a i lab l e p r o c es s e d t o mi l - s t d-883b , rev c. e x t e n d e d r e liab ili t y p l us s c r e enin g is als o a v a i lab l e . p l us s c r e enin g in c l u d es 168-h o ur b u r n -i n, as wel l as o t h e r e n vir o nm e n t a l an d ph y s ica l t e sts der i v e d f r o m m i l - s t d-883b , rev c. produc t h i ghligh ts 1. the ad549 s in p u t c u r r en ts a r e s p ecif ie d , 100% t e s t e d , an d g u ar an t e e d af te r t h e d e v i c e i s w a r m e d up . i n put c u r r e n t i s gua r a n te e d o v er t h e e n t i r e co mm on- m o d e i n pu t v o l t a g e ra n g e . 2. the ad549 s in p u t o f fs et v o l t a g e a nd dr if t a r e l a s e r tr imm e d t o 0.2 5 mv and 5 v/c (ad549 k), a nd 1 mv a nd 20 v/c ( a d549j). 3. a maxim u m q u ies c en t s u p p l y c u r r en t o f 700 a minimizes h e a t in g ef fe c t s o n in p u t c u r r en t a nd o f fs et v o l t a g e . 4. a c s p eci f ica t i o n s in c l ud e 1 mh z uni t y ga in b a n d w i d t h a nd 3 v/s s l e w ra t e . s e t t lin g t i m e f o r a 10 v in p u t s t ep is 5 s t o 0.01%. 1 pr otected by patent no. 4,639,683.
ad549 rev. d | page 2 of 20 table of contents specifications ..................................................................................... 3 absolute maximum ratings ............................................................ 5 esd caution .................................................................................. 5 typical performance characteristics ............................................. 6 functional description .................................................................. 10 minimizing input current ........................................................ 10 circuit board notes ................................................................... 10 offset nulling .............................................................................. 11 ac response with high value source and feedback resistance .................................................................................... 12 common-mode input voltage overload ................................ 12 differential input voltage overload ........................................ 13 input protection ......................................................................... 13 sample and difference circuit to measure electrometer leakage currents ........................................................................ 13 photodiode interface ................................................................. 14 temperature compensated ph probe amplifier ................... 17 outline dimensions ....................................................................... 18 ordering guide .......................................................................... 18 revision history 5/04data sheet changed from rev. c to rev. d updated format..............................................................universal changes to features...................................................................... 1 updated outline dimensions ................................................... 18 added ordering guide .............................................................. 18 10/02data sheet changed from rev. b to rev. c. deleted product highlights #5 ..........................................1 edits to specifications..............................................................3 deleted metallization photograph ..............................3 updated outline dimensions .............................................13 7/02data sheet changed from rev. a to rev. b. edits to specifications..............................................................2
ad549 rev. d | page 3 of 20 specifications @ 25c and v s = 15 v dc, unless otherwise noted. all min and max specifications are guaranteed. specifications in boldface are tested on all production units at final electrical test. results from those tests are used to calculate outgoing quality levels. table 1. ad549j ad549k ad549l ad549s parameter min typ max min typ max min typ max min typ max unit input bias current 1 either input, v cm = 0 v 150 250 75 100 40 60 75 100 fa either input, v cm = 10 v 150 250 75 100 40 60 75 100 fa either input at t max , v cm = 0 v 11 4.2 2.8 420 pa offset current 50 30 20 30 fa offset current at t max 2.2 1.3 0.85 125 pa input offset voltage 2 initial offset 0.5 1.0 0.15 0.25 0.3 0.5 0.3 0.5 mv offset at t max 1.9 0.4 0.9 2.0 mv vs. temperature 10 20 2 5 5 10 10 15 v/c vs. supply 32 100 10 32 10 32 10 32 v/v vs. supply, t min to t max 32 100 10 32 10 32 32 50 v/v long-term offset stability 15 15 15 15 v/month input voltage noise f = 0.1 hz to 10 hz 4 4 6 4 4 v p-p f = 10 hz 90 90 90 90 nv/ hz f = 100 hz 60 60 60 60 nv/ hz f = 1 khz 35 35 35 35 nv/ hz f = 10 khz 35 35 35 35 nv/ hz input current noise f = 0.1 hz to 10 hz 0.7 0.5 0.36 0.5 fa rms f = 1 khz 0.22 0.16 0.11 0.16 fa/ hz input impedance differential v diff = 1 10 13 ||1 10 13 ||1 10 13 ||1 10 13 ||1 ?||pf common mode v cm = 10 10 15 ||0.8 10 15 ||0.8 10 15 ||0.8 10 15 ||0.8 ?||pf open-loop gain v o @ 10 v, r l = 10 k? 300 1000 300 1000 300 1000 300 1000 v/mv v o @ 10 v, r l = 10 k?, t min to t max 300 800 300 800 300 800 300 800 v/mv v o = 10 v, r l = 2 k? 100 250 100 250 100 250 100 250 v/mv v o = 10 v, r l = 2 k?, t min to t max 80 200 80 200 80 200 25 150 v/mv input voltage range differential 3 20 20 20 20 v common-mode voltage ?10 +10 ?10 +10 ?10 +10 ?10 +10 v common-mode rejection ratio v = +10 v, ?10 v 80 90 90 100 90 100 90 100 db t min to t max 76 80 80 90 80 90 80 90 db output characteristics voltage @ r l = 10 k?, t min to t max ?12 +12 ?12 +12 ?12 +12 ?12 +12 v voltage @ rl = 2 k?, t min to t max ?10 +10 ?10 +10 ?10 +10 ?10 +10 v short-circuit current 15 20 35 15 20 35 15 20 35 15 20 35 ma t min to t max 9 9 9 6 ma load capacitance stability g = +1 4000 4000 4000 4000 pf
ad549 rev. d | page 4 of 20 ad549j ad549k ad549l ad549s parameter min typ max min typ max min typ max min typ max unit frequency response unity gain, small signal 0.7 1.0 0.7 1.0 0.7 1.0 0.7 1.0 mhz full power response 50 50 50 50 khz slew rate 2 3 2 3 2 3 2 3 v/s settling time, 0.1% 4.5 4.5 4.5 4.5 s settling time, 0.01% 5 5 5 5 s overload recovery, 50% overdrive, g = ?1 2 2 2 2 s power supply rated performance 15 15 15 15 v operating 5 18 5 18 5 18 5 18 v quiescent current 0.60 0.70 0.60 0.70 0.60 0.70 0.60 0.70 ma temperature range operating, rated performance 0 +70 0 +70 0 +70 ?55 +125 c storage ?65 +150 ?65 +150 ?65 +150 ?65 +150 c package option to-99 (h-08a) chips AD549JH ad 549kh ad549lh ad549sh/883b 1 bias current specifications are guaranteed after five minutes of operation at t a = 25c. bias current increases by a factor of 2.3 for every 10c rise in temperature. 2 input offset voltage specifications are guara nteed after five minutes of operation at t a = 25c. 3 defined as max continuous voltage between the inputs, such that neither input exceeds 10 v from ground.
ad549 rev. d | page 5 of 20 absolute maximum ratings table 2. p a r a m e t e r r a t i n g supply voltage 18 v internal power dissipation 500 mw input voltage 1 18 v output short ci rcuit du r a t i o n i n d e f i n i t e differential input voltage +vs and ?vs storage temperature range (h) ?65c to +125c operating tem p erature range ad549j (k, l) 0c to +70c ad549s ?55c to +125c lead temperature range (soldering, 60 sec) +300c 1 for supp ly vo lt a g e s les s t h a n 18 v, t h e a b so lut e m a x i m u m i n put volt a g e i s eq ua l t o t h e suppl y volt a g e 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 o s e i ndic a te d i n t h e op er a t io na l s e c t io n o f t h is sp e c if ic a t io n is no t im plie d . e x p o sur e t o a b s o l u te 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 caution esd (electrostatic discharge) sensitive device. ele c tros tatic charg e s as high as 4000 v readily accumulate on the human body and test eq uipment and can discharge wi thout 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.
ad549 rev. d | page 6 of 20 typical perf orm ance cha r acte ristics 20 15 10 5 0 0 5 10 15 20 +v in ?v in 00511-002 supply voltage (v) input voltage (v) f i g u re 2. input v o lt ag e r a ng e v s . sup p ly v o lt ag e 20 15 10 5 0 0 5 10 15 20 +v out 25 c r l = 10k ? ?v out 00511-003 supply voltage (v) otuput voltage sw ing ( v ) f i g u re 3. o u t p ut v o lt ag e sw ing v s . sup p ly v o lt ag e 30 25 20 15 10 5 0 10 100 1k 10k 100k 00511-004 load resistance ( ? ) otuput voltage swing (v p-p) v s = 15v f i g u re 4. o u t p ut v o lt ag e sw ing v s . l o a d r e s i s t anc e 800 700 600 500 400 0 5 10 15 20 00511-005 supply voltage ( v) amp l ifie r quie s c e n t curre nt ( a) f i gure 5. q u i e s c ent current v s . sup p l y v o ltag e 120 100 110 90 80 70 ? 2 0 ? 10 0 1 0 2 0 00511-006 input common-mode voltage (v) common-mode re j e ction ratio (db) f i gure 6. c m rr vs . input c o mmon-m ode v o ltage 3000 1000 300 100 0 5 10 15 20 00511-007 supply voltage (v) open- l oop gain ( v /mv) f i gure 7. o p en-l o o p g a i n v s . sup p ly v o ltag e
ad549 rev. d | page 7 of 20 3000 1000 300 100 ? 5 5 ? 25 5 3 5 6 5 9 5 125 00511-008 temperature ( c) open- l oop gain ( v /mv) f i gur e 8 . op e n -l oop ga in vs . t e m p er atur e 30 25 20 15 10 5 0 0123 4567 00511-009 warmup time (minutes) ? iv os i ( v) f i gure 9 . ch a n ge i n o ffset v o l t a g e vs . w a rm -up t i m e 50 45 40 35 30 25 20 ?1 0 ? 5 0 5 1 0 00511-010 common-mode voltage (v) inp u t curre nt (fa) f i g u re 10. input b i a s current v s . co mm on-m od e v o lt ag e 50 45 40 35 30 25 20 0 5 10 15 20 00511-011 power supply voltage (v) inp u t curre nt (fa) f i gure 11. input b i a s current v s . sup p ly v o lt age 160 140 120 100 80 60 40 20 10 100 1k 10k 00511-012 frequency (hz) n o ise spec tr a l d e n s ity ( n v/ h z ) f i gure 12. input v o l t age n o ise spec t r a l d e nsit y 100k 10k 1k 100 10 1 0.1 100k 1m 10m 100m 1g 10g 100g 00511-013 source resistance ( ? ) in pu t n o ise volta ge ( v p-p) whenever johnson noise is greater than amplifier noise, amplifier noise can be considered negligible for the application resistor johnson noise 10hz bandwidth amplifier generated noise 1khz bandwidth f i gure 13. no ise v s . s o u r c e r e sistance
ad549 rev. d | page 8 of 20 100 80 60 40 20 0 ?2 0 ?4 0 100 ?4 0 ?2 0 0 20 40 60 80 10 100 1k 10k 100k 1m 10m 00511-014 frequency (hz) oepn- l oop gain ( d b) ph a se m a r g in ( d egrees) f i gure 14. o p en-l oop f r equ e nc y response 40 35 30 25 20 15 10 5 0 10 100 1k 10k 100k 1m 00511-015 frequency (hz) output voltage sw ing ( v ) f i gure 1 5 . la r g e s i gna l f r equenc y r e sp o n se 100 80 60 40 20 0 ?2 0 10 100 1k 10k 100k 10m 1m 00511-016 frequency (hz) cmrr (db) fi g u r e 1 6 . c m r r v s . fr e q u e n c y 120 100 80 60 40 20 0 ?2 0 10 100 1k 10k 100k 10m 1m 00511-017 frequency (hz) p s rr (db) +supply ? supply f i gur e 1 7 . p s rr vs . f r e q ue nc y re sp onse 10 ?1 0 ?5 0 5 01 3 24 00511-018 settling time ( s) output voltage sw ing ( v ) 5 10mv 5mv 1mv 10mv 5mv 1mv f i gure 18. o u tput v o ltag e s w ing a n d e rror v s . s e ttling t i m e
ad549 rev. d | page 9 of 20 00511-019 2 3 5 7 4 ad549 r l 10k ? c l 100pf 0.1 f 0.1 f v in ?v s +v s v out square wave input f i g u re 19. u n it y g a in f o l l ower 00511-020 f i g u re 20. u n it y g a in f o l l ower la r g e s i g n al p u ls e r e s p ons e 00511-021 f i g u re 21. u n it y g a in f o l l ower s m a ll si g n al p u ls e r e s p ons e 00511-022 2 3 5 7 4 ad549 r l 10k ? c l 100pf 0.1 f 0.1 f v in ?v s +v s v out square wave input 10k ? 10k ? f i g u re 22. u n it y g a in inve r t er 00511-023 f i g u re 23. u n it y g a in inve r t er lar g e s i g n al p u ls e r e s p ons e 00511-024 f i g u re 24. u n it y g a in inve r t er sm a ll s i g n al p u ls e r e s p ons e
ad549 rev. d | page 10 of 20 functional description minimizing inpu t cur r ent the ad549 has been o p timize d f o r l o w in p u t c u r r en t a nd o f fs et v o l t a g e . c a r e f u l a t t e n t io n t o h o w t h e a m pl if ier is us e d w i l l r e d u ce i n p u t c u r r en ts in ac t u a l a p plic a t ion s . the am plif ier o p era t i n g t e m p er a t ur e sh o u l d b e k e pt as lo w as p o ssi b le t o mini mi ze i n p u t c u r r en t. l i k e o t h e r j f et in p u t a m p l if iers, th e ad549 s in p u t c u r r en t is s e n s i t iv e t o c h i p t e m p era t ur e , r i s i n g b y a fac t o r of 2.3 f o r ev er y 1 0 c. f i gur e 25 is a plo t o f ad549 s in p u t c u r r en t v e rsus i t s am b i e n t t e m p er a t ur e . 1na 100pa 10pa 1pa 100fa 10fa 1fa ? 5 5 ? 25 5 3 5 6 5 125 95 00511-025 temperature ( c) f i gure 25. input b i a s current v s . a m bi e n t t e mper atu r e on-ch i p p o w e r dissi p a t io n ra is e s t h e ch i p o p er a t in g t e m p er a- t u r e , ca usin g a n in cr e a s e i n in p u t b i as c u r r en t. d u e t o t h e ad549 s lo w q u ies c en t s u p p l y c u r r en t, th e c h i p t e m p era t ur e is le s s th a n 3 c h i gh e r th a n i t s a m b i en t t e m p e r a t ur e w h en th e (un l o a de d) a m plif ier is o p era t ing wi t h 15 v supplies. th e dif f er en ce i n t h e in p u t c u r r en t is n e g l ig ib le . h o w e v e r , h e a v y o u t p u t lo ads can c a us e a sig n if ica n t i n cr e a s e in chi p t e m p er a t ure a nd a co r r es p o n d in g i n cr e a s e in t h e i n p u t c u r r en t. m a in t a i n in g a min i m u m lo ad r e sist ance o f 10 ? is r e co mme n d e d . i n p u t c u r r en t v e rsus addi t i o n a l p o w e r dissi p a t io n d u e to o u t p ut dr i v e c u r r en t is plo t te d in f i gur e 26. 6 5 4 3 2 1 0 2 5 5 0 7 5 100 125 150 175 200 00511-026 additional internal power dissipation (mw) normalize d inp u t bias curre nt based on typical i b = 40fa f i gure 26. input b i a s current v s . additiona l p o w e r d i s s i pa tion circuit board notes ther e a r e a n u m b er o f ph ysica l ph e n om ena t h a t g e n e ra t e s p ur io us c u rr en ts, w h ich deg r ade t h e acc u rac y o f lo w c u r r en t m e as ur em en ts. f i gur e 27 is a s c h e ma tic o f a n i - t o -v con v er t e r wi t h t h es e p a ras i t i c c u r r en ts m o de le d . 00511-027 2 3 6 8 ad549 + v out ? f s c f r f v s r p c p i i' = +v + c p v r p dc p dt dv dt f i g u re 27. s o urc e s of p a r a s i t i c l e ak ag e cur r ent s f i ni te r e sis t an ce f r o m in p u t lin e s t o v o l t a g es o n th e bo a r d , mo d e l e d b y re s i stor r p , r e s u l t s in p a rasi tic le a kag e . i n s u l a tion re s i st anc e of m o re t h an 1 0 15 ? m u st b e main t a in e d b e tw e e n t h e a m plif ier s sig n a l a nd su p p ly li nes in o r der to c a p i t a li z e o n t h e ad549 s lo w in p u t c u r r en ts. s t a nda rd pc bo a r d ma t e r i al do es n o t ha v e hig h e n o u g h in s u la t i on r e sis t an ce . ther efo r e , t h e ad549 s in p u t l e ads sh o u ld be co nnec t e d t o s t a n do f f s made o f in s u l a ting ma t e r i al wi th adeq ua t e v o l u m e r e sis t i v i t y (e . g ., t e f l o n ). th e in su la t o r s s u r f ace m u s t be k e p t c l e a n in o r der t o p r es er v e s u r f ace r e sis t i v i t y . f o r t e f l o n , an ef fe c t i v e cle a nin g p r o c e d ur e co n s i s ts o f swa b b i n g t h e s u r f ace w i t h hig h g r ade is o p r o p y l a l coh o l, r i n s in g wi t h deio ni ze d w a ter , a nd b a k i n g t h e bo a r d a t 80c fo r 10 min u t e s . i n ad di t i on to h i g h vol u m e and sur f ace r e sist i v i t y , o t h e r p r o p er t i es a r e desira b l e in t h e in s u l a t i n g ma ter i al ch os en. resist an c e t o w a t e r a b s o r p t i on is im p o r t an t si nce sur f ace wa t e r f i l m s dr ast i c a l l y re d u c e su r f ac e re s i st iv ity . t h e i n su l a tor cho s e n s h o u ld als o exhib i t minimal p i e z o e le c t r i c ef f e c t s (c ha rg e emissio n d u e t o m e ch a n ic a l st ress) a nd t r ib o e l e c t r i c ef fe c t s ( c harge ge ne r a te d b y f r ic t i on) . c h arge imb a l a nc e s ge ne r a te d by t h es e me cha n is m s can a p p e a r as p a rasi t i c le akag e c u r r en ts. th e s e ef fe c t s a r e m o de le d b y var i a b le c a p a ci t o r c p in f i gur e 27. t a b l e 3 lis t s va r i o u s in s u la t o rs and t h e i r p r o p er t i es 2 . g u ard i ng t h e i n put l i ne s b y c o m p l e tely su r r ou nd i n g t h e m w i t h a metal cond uc to r b i as e d n e a r th e in p u t lin e s p o t e n t ial has tw o ma jo r b e n e f i ts. f i rst, p a rasi t i c l e a k a g e f r o m t h e sig n a l line is r e d u ce d si n c e t h e v o l t a g e b e twe e n t h e i n p u t li n e and t h e gu a r d is v e r y lo w . s e co nd , st ra y c a p a ci t a n c e a t t h e in p u t n o de is 2 e l ect r on ic m e asur e m en t s , pp. 15C17, keithley inst ruments, inc., cleveland, ohio, 1977.
ad549 rev. d | page 11 of 20 mini mi ze d . i n pu t c a p a ci t a nce c a n subst a n t ia l l y deg r ade sig n a l ba nd wid t h and th e sta b ili t y o f th e i-t o -v con v er t e r . th e cas e o f th e ad549 is conn ec t e d t o p i n 8 s o tha t i t can be bo o t s t ra p p e d n e a r t h e i n p u t p o ten t ia l. t h is m i nim i zes p i n le a k a g e an d i n p u t co mm o n -mo d e ca p a ci tan c e d u e t o th e cas e . g u a r d s c h e m e s f o r in v e r t ing an d no ni n v er t i n g am plif ier t o p o log i es a r e i l l u st ra t e d in f i gur e 28 and f i gur e 29. 00511-028 2 3 6 8 ad549 + v out ? i n c f r f guard f i g u re 28. inve r t ing a m pl if ie r w i t h g u a r d 00511-029 3 2 6 8 ad549 + v out ? v s + ? guard r f r i f i gure 29. nonin v e r ting a m plifier w i t h guar d o t h e r guide l i n e s in cl ud e k e eping t h e cir c ui t l a yo u t as com p ac t as p o ssib le an d k e e p in g t h e in pu t li n e s sh o r t. k e ep in g t h e ass e m b ly r i g i d and min i mi zi n g s o ur ces o f vib r a t io n wi l l r e d u c e tri b o e l e ctri c a n d p i ez oe l e ctri c ef f e ct s . all p r ec i s i o n , h i gh im p e d a n c e cir c ui t r y r e q u ir es shie ldi n g a g a i n s t in t e r f er en c e n o is e . l o w n o i s e co axia l o r t r ia xia l cab l es sh o u ld b e us e d fo r r e m o t e co nn e c t i o n s t o the in p u t sig n al lin e s. offset nulling the ad549 s in p u t o f fs et v o l t a g e can b e n u l l ed b y usin g balan c e p i n s 1 an d 5, as s h own in f i gur e 30. n u l l in g th e in p u t o f fs et v o l t a g e i n t h is f a shion in t r o d uc es a n ad de d in pu t o f fs et v o l t a g e dr if t co m p onen t o f 2.4 v/c p e r mi l l i v ol t o f n u l l e d o f fs et (a maxim u m addi tio n al dr if t o f 0.6 v/c f o r the ad549 k, 1.2 v/c f o r the ad549l, and 2.4 v/c f o r the ad549j). 00511-030 2 3 6 5 1 7 4 ad549 + v out ? ?v s +v s 10k ? f i gure 30. stand a rd o ffs et null ci r c u i t the a p p r o a ch i n f i gur e 31 can b e us e d w h e n t h e am plif ier is us e d as an i n ver t er . this m e t h o d in t r o d uces a s m a l l vol t a g e r e fer e n c e d t o t h e p o w e r s u p p lie s in s e r i es wi t h t h e am plif ier s p o si t i v e i n p u t t e r m ina l . the am plif ier s in p u t o f fs et v o l t a g e dr if t wi t h t e m p era t u r e is n o t a f fe c t e d . h o w e v e r , va r i a t io n o f t h e p o w e r s u p p l y v o l t a g es c a us es of fs et s h if ts. 00511-031 2 3 6 ad549 + v out ? v i + ? r f r i 200 ? 100k ? 499k ? 499k ? 0.1 f ?v s +v s f i gure 31. a l te rn ate o ffs et nul l c i rcuit for i n ve r t e r tab l e 3. in sulatin g materials an d characteristic s material volume resi sti v ity (vCcm) minimal triboelectric effects minimal piezoelectric effect resistance to water absorption tefl o n ? 1 0 17 ? 10 18 w w g k e l - f ? 1 0 17 ? 10 18 w m g s a p p h i r e 1 0 16 ? 10 18 m g g p o l y e t h y l e n e 1 0 14 ? 10 18 m g m p o l y s t y r e n e 1 0 12 ? 10 18 w m m c e r a m i c 1 0 12 ? 10 14 w m w glass ep oxy 10 10 ? 10 17 w m w p v c 1 0 10 ? 10 15 g m g p h e n o l i c 1 0 5 ? 10 12 w g w gC goo d with re gard to p r o p e r ty m C m o de rate with re gard to pro p e r ty wC we a k with re ga rd to p r o p e r ty
ad549 rev. d | page 12 of 20 ac response with hig h value source and feedback resistance s o ur ce an d fe e d b a ck r e sist an ces g r e a t e r t h a n 10 0 k? ma g n if y t h e ef fe c t o f t h e in p u t c a p a ci t a nces (s t r a y an d i n h e r e n t t o t h e ad549) o n t h e ac beha vio r o f th e cir c ui t. th e ef f e c t s o f co m m o n - m o d e a nd dif f er en t i a l in p u t c a p a ci t a nces sh o u l d b e t a k e n i n to accoun t si n c e t h e circ ui t s b a ndwid t h an d st ab i l i t y c a n b e a d ve rs el y af fe c t e d . 00511-032 f i gur e 3 2 f o l l o w er p u l s e resp onse fr om 1 m ? s o ur c e r e si sta n ce , c a se no t bo o t str a p p ed 00511-033 f i gur e 3 3 . f o l l o w er p u l s e resp onse fr om 1 m ? s o ur c e r e si sta n ce , c a se bo o t str a p p ed i n a fol l o w er , t h e s o ur ce r e sist an ce an d i n p u t c o mm on- m o d e c a pa ci ta n c e f o rm a po l e tha t l i m i t s th e ba n d w i d t h t o ? r s c s . b o o t s t ra p p i n g t h e m e tal case b y co n n ectin g p i n 8 t o th e o u t p u t mini mi zes c a p a ci t a n c e d u e to t h e p a cka g e. f i gur e 32 a nd f i g u re 3 3 show t h e f o l l owe r pu ls e re sp ons e f r om a 1 m ? s o u r c e r e s i s t a n ce w i t h a n d w i th o u t th e pa c k a g e c o n n ect e d t o th e o u t p u t . t y p i cal co mm o n -mo d e in p u t c a p a ci tance f o r th e ad54 9 is 0.8 pf . i n an i n v e r t in g co nf igura t io n, t h e dif f er en t i a l i n p u t ca p a c i t a n c e fo r m s a p o le in th e cir c ui t s lo op tra n smis sio n . this can cr e a t e p e a k i ng in t h e a c re sp ons e and p o ss ibl e i n st ab i l i t y . a fe e d b a ck ca p a c i t a n c e can b e us e d t o s t ab i l ize t h e c i r c ui t. the i n v e r t er pu l s e re s p ons e w i t h r f an d r s eq ual t o 1 m? a p p e a r s in f i gur e 34. f i gur e 35 s h o w s t h e res p o n s e o f th e s a me cir c ui t wi t h a 1 pf f eed back ca p a c i tan c e . t y p i cal dif f er en tia l in p u t ca p a c i tan c e f o r th e ad549 is 1 pf . 00511-034 f i g u re 34. inve r t e r p u ls e r e s p o n s e w i t h 1 m ? s o u r ce a n d f eedb a ck r e sista n c e 00511-035 f i gure 35. in ve r t e r p u lse r e spo n se w i t h 1 m ? s o u r ce a n d f eedb a ck r e sistance , 1pf f e e dback capa cita nc e commo n-m o de i n pu t voltage o v erload the ra ted comm on-m o d e in p u t v o l t a g e ran g e o f th e ad549 is f r o m 3 v les s tha n t h e p o si t i v e su p p l y v o l t a g e t o 5 v g r ea t e r th a n t h e n e ga ti v e s u p p l y v o l t a g e . ex ceed i n g th i s ra n g e d e gra d e s t h e am plif ier s cmrr . dr ivin g t h e co m m o n - m o d e v o l t a g e a b o v e t h e p o si t i v e s u p p l y c a us es t h e am plif ier s o u t p ut t o s a t u r a t e a t t h e up p e r limi t o f t h e o u t p ut v o l t a g e. re co v e r y t i m e is typ i cal l y 2 s a f t e r th e in p u t has be en r e t u r n e d t o wi thin t h e n o r m al o p er a t i n g ra n g e . dr i v i n g t h e in p u t comm on- m o d e v o l t a g e wi t h in 1 v o f t h e n e g a t i v e s u p p l y c a us es phas e r e v e rs al of t h e output s i g n a l . i n t h i s c a s e , nor m a l op e r at i o n i s t y pi c a l l y r e s u m e d w i thi n 0. 5 s o f th e in p u t v o l t a g e r e t u rn i n g wi th in ra n g e .
ad549 rev. d | page 13 of 20 differential input voltage overload a p l o t o f th e ad549 s in p u t c u r r en ts v e rs us dif f er en tial in p u t v o l t a g e (def i n e d as v in + ? v in ?) a p p e a r s in f i gu r e 36. th e in p u t c u r r en t a t ei t h er t e r m inal s t a y s b e lo w a fe w h u n d r e d fem t o a m p s un til on e in p u t t e r m inal is f o r c ed hig h er than 1 v t o 1.5 v a b ov e t h e o t h e r t e r m i n al . u n der t h es e co ndi t i on s, t h e in p u t c u r r en t limi ts a t 30 a . 100 10 1 100n 10n 1n 100p 10p 1p 100f 10f ?5 ? 4 ?3 ? 2 ?1 0 1 2 3 4 5 00511-036 differential input voltage (v) (v in +? v in ?) inp u t curre nt (a) i in ?i in + f i gure 3 6 . input c u rr ent vs . di ffer e ntial input v o l t a g e inpu t pro t ectio n the ad549 s a f e l y ha n d les an y in p u t v o l t a g e wi t h in t h e su p p l y vol t age r a ng e. sub j e c t i ng t h e i n put te r m i n a l s to vol t age s b e yond t h e p o we r sup p ly c a n de st roy t h e de v i c e or c a u s e shif ts in i n p u t c u r r en t o r o f fs e t v o l t a g e if t h e am plif ier is n o t pr o t e c t e d . a p r o t e c t i on s c h e m e fo r t h e am plif ier as an i n v e r t er is sh o w n in f i gur e 37. r p is ch os en t o limi t t h e c u r r en t t h r o ug h t h e in v e r t ing in p u t t o 1 ma fo r exp e c t e d t r an sien t (less t h a n 1 s) o v er v o l t a g e co ndi tion s, o r t o 100 a f o r a co n t in uo us o v erlo ad . sinc e r p is in sid e t h e fe e d b a ck l o o p , a nd is m u ch lo w e r in va l u e t h a n t h e am plif i e r s in p u t r e sis t an ce , i t do es n o t a f fe c t t h e in v e r t er s dc gai n . h o w e v e r , t h e j o hn s o n n o is e of t h e r e sis t o r a d d s ro ot su m o f s q u a re s to t h e am p l i f i e r s i n put noi s e. 00511-037 2 3 6 ad549 c f source r protect r f f i gure 3 7 . in v e r t er wi th input curr ent li mi t i n t h e co r r esp o n d in g v e rsio n of t h is s c h e m e for a fol l o w er , s h own in f i gur e 38, r p a n d th e ca pa c i ta n c e a t th e po s i t i v e i n p u t t e r m inal p r o d u c e a p o le i n t h e sig n al f r e q uen c y r e s p o n s e a t a f = ?rc. a g a i n, t h e j o h n s o n n o is e, r p , adds to t h e am plif ier s in p u t v o l t a g e n o is e . 00511-038 3 2 6 ad549 source r protect f i gure 3 8 . f o l l o w er wi th input curr ent li mi t f i gur e 39 is a s c h e ma tic o f the ad549 as an in v e r t er wi t h a n in p u t v o l t a g e cl a m p . b o o t s t ra pp in g t h e cl a m p dio d es a t t h e in ver t ing in p u t mini mi zes t h e vol t a g e acr o ss t h e clam ps an d k e eps t h e le a k a g e d u e t o t h e dio d es lo w . l o w le aka g e dio d es, s u c h as t h e fd3 33s, s h o u ld be u s e d and sh o u ld be shie lde d f r om l i g h t to ke e p photo c u r re n t s f r om b e i n g ge ne r a te d. e v e n wi t h t h es e p r e c a u t i o n s, t h e dio d es m e as ura b l y i n cr e a s e in p u t cu rr e n t a n d ca pa c i ta n c e . 00511-039 2 3 6 ad549 source r f protect diodes f i gure 39. input v o ltage clamp w i th d i odes sample and difference circuit to measur e electrometer leakage currents ther e a r e a n u m b er o f m e t h o d s us e d t o t e st ele c t r o m et er le a k a g e c u r r en t s , in cl ud in g c u r r en t in t e g r a t io n a nd dir e c t c u r r en t-t o -v ol t a g e co n v ersion. rega r d les s o f t h e m e t h o d us e d , b o a r d and in terco nne c t cle a n l i n ess, p r o p er cho i ce o f in su l a t i ng ma ter i a l s (such as t e f l o n o r k e l - f ) , co r r e c t gua r din g and shie ld in g te chni q u es, a nd ca r e i n ph y s ica l la yo ut a r e ess e n t i a l t o m aki n g a c cura t e leak a g e m e a s ur em en t s . f i gur e 40 is a s c h e ma tic o f the s a m p le and dif f er en ce cir c ui t. i t us es tw o ad54 9 ele c t r o m e t er am plif iers (a and b) as c u r r en t- to - v olt a g e c o n v e r te rs w i t h h i g h v a lu e ( 1 0 10 ?) s e n s e r e sis t o r s (rsa and rs b). r1 a nd r2 p r o v ide fo r a n o v er a l l cir c ui t s e n s i t ivi t y o f 10 fa/mv (10 pa f u l l s c ale). c c a nd c f prov i d e n o is e s u p p r es si o n an d lo o p com p e n s a t i on. c c s h o u ld b e a lo w le aka g e p o l y s t yren e ca p a c i t o r . a n u l t r alo w le akag e k e l - f t e s t s o ck e t is us e d fo r co n t ac t i ng t h e de vic e under test. r i g i d t e f l o n coa x i a l ca b l e i s used t o m a k e co n n ect i o n s t o all h i gh im p e da n c e n o des. th e us e o f r i g i d co axial ca b l e a f fo r d s imm u ni ty t o er r o r ind u ce d b y me cha n ica l v i b r a t ion an d p r o v ides a n o u t e r co nd uc t o r fo r shie ldi n g. t h e en t i r e cir c ui t is e n clos e d in a gr o u n d ed m e tal bo x.
ad549 rev. d | page 14 of 20 th e te s t a p p a r a tu s i s c a l i b r a t e d w i t h out a d e v i c e u n d e r te st p r e s e n t . a f t e r p o w e r i s t u rn ed o n , a f i v e -m in u t e s t a b iliz a t i o n p e r i o d is r e q u ire d . f i rst, v err1 and v err2 a r e m e as ur e d . th es e v o l t a g es a r e t h e er r o rs ca us e d b y t h e o f fs et v o l t a g es an d le a k a g e c u r r en ts o f t h e c u r r en t-t o -v ol t a g e co n v er t e rs. v err1 = 10 ( v os a C i b a rsa ) v err2 = 10 ( v os b C i b b rsb ) 00511-040 v os + ? 2 3 6 8 a ad549l r2 9.01k ? r1 1k ? rsa 10 10 ? + verr1/v a verr2/v b v out ? cal/test c c 20pf c f 0.1 f + ? 3 2 6 8 b ad549l r2 9.01k ? r1 1k ? rsb 10 10 ? c c 20pf c f 0.1 f c f 0.1 f r1 1k ? r2 9.01k ? device under test i (+) i ( ? ) guard f i gure 4 0 . sam p l e a n d d i ffer e nc e cir c ui t fo r mea s uring e l ec trom ete r l e ak a g e cur r ents on ce me as ur e d , t h es e er r o rs a r e s u b t rac t e d f r om t h e r e adi n gs t a k e n wi t h a de vice u n der t e s t p r es en t. a m plif i e r b clos es t h e f eed back lo o p to th e de vice u nde r t e st ing, in a ddi t i on t o p r o- vidin g t h e c u r r en t- t o -v ol t a ge con v ersio n . the o f fs et er r o r o f t h e de vice u n d e r t e st in g a p p e a r s as a co m m on- m o d e sig n a l and do e s not af fe c t t h e te st me a s u r e m e n t. a s a re su l t , on ly t h e le aka g e c u r r en t o f t h e de vi ce u nder t e s t in g is me as ur e d . v a C v err1 = 10[ rsa i b (+)] v x C v err2 = 10[ rsb i b (C)] a l t h ou g h a s e r i e s of d e v i c e s c a n b e te s t e d af te r on ly one ca lib r a t ion m e a s ur em e n t, ca l i b r a t io n sh o u ld b e u p d a te d pe ri od i c all y t o co m p en sa t e f o r a n y th e r m a l d r i f t o f th e curr e n t - t o -v ol t a ge con v er t e rs o r cha n g e s in t h e am b i e n t en vir o nm e n t. l a bo r a t o r y r e s u l t s ha ve sh o w n tha t r e p e a t ab le m e as ur e m en ts wi t h in 10 f a can b e r e a l i z e d w h e n t h is a p p a ra t u s is p r o p erly i m p l em en t e d . th e s e r e s u l t s a r e a c h i ev ed in pa r t b y th e d e s i gn o f t h e cir c ui t, w h ich e l im in a t es r e l a y s a nd o t h e r p a rasi t i c le a k a ge p a th s in the hig h im p e dan c e sig n al lin e s, and in p a r t b y th e i n he re n t c a nc el l a t i on of e r ror s t h rou g h t h e c a l i br a t i o n a n d m e as ur em e n t p r oced ur e . p h ot od i o d e i n t e rf ace the ad549 s lo w in p u t c u r r en t a nd lo w in p u t of fs et v o l t a g e mak e i t an exce l l en t ch o i ce f o r v e r y s e n s i t i v e pho t o d io de p r e a m p s (f igur e 41). th e ph oto d io de de ve lo ps a sig n al c u r r en t, i s , eq ual t o i s = r p w h er e p is lig h t p o w e r in cide n t o n t h e dio d e s sur f ace ,in w a t t s, a nd r is t h e ph ot o d io de r e sp o n s i v i ty i n a m ps/ w a t t. r f co n v er ts t h e sig n al c u r r en t t o an o u t p u t v o l t a g e v ou t = r f i s 00511-041 2 3 6 5 1 4 ad549 ?v s c f 10pf r f 10 9 ? 10k ? 1 f v out + ? f i g u re 41. phot od i o de pr e a mp d c er r o r s o ur c e s a nd an eq ui valen t cir c ui t f o r a smal l a r ea (0.2 mm s q ua r e ) ph o t o d io de a r e indic a te d i n f i gur e 42. 00511-042 a + v out ? v os + ? i s ? i s r s 10 9 ? c s 20pf r f 10 9 ? c f 10pf f i g u re 42. phot od i o de pr e a mp dc e rror s o u r ces i n p u t c u r r en t, i b , c o n t r i b u t e s a n o u t p u t v o l t a g e e r r o r , v e1 , prop or t i on a l to t h e f e e d b a c k re s i st a n c e v e1 = i b r f
ad549 rev. d | page 15 of 20 the o p a m p s in p u t v o l t a g e o f fs et ca us es a n er r o r c u r r en t th r o ugh th e ph o t od i o d e s s h un t r e si s t a n ce , r s i = v os / r s the er r o r c u rr en t r e s u l t s i n an er r o r v o l t a g e ( v e2 ) a t the a m plif ier s o u t p u t e q ua l t o v e2 = (1 + r f / r s ) v os gi ven ty p i ca l v a l u es o f ph o t o d i o de sh un t r e sist a n c e (o n t h e ord e r of 1 0 9 ?), r f / r s ca n ea s i l y be gr ea t e r th a n o n e , e s peci all y i f a la rg e f e e d back r e sis t a n ce is us ed . als o , r f /r s in cr e a s e s w i t h t e m p era t ur e , since p h ot o d io de sh un t r e sis t ance typ i cal l y dr o p s by a f a c t or of 2 for e v e r y 1 0 c r i s e i n te m p e r atu r e. a n op am p wi t h lo w o f fs et vol t a g e and lo w dr if t m u st b e us e d i n o r der to ma in t a i n acc u ra c y . th e ad549 k o f fers gua r a n te e d max i m u m 0.25 mv o f fs et v o l t a g e and 5 mv/c dr if t f o r v e r y s e n s i t i v e ap p l i c at i o n s . photodiode preamp noise n o i s e lim i ts th e si gn al r e so l u ti o n o b t a i n a b le w i t h th e p r ea m p . the o u t p u t v o l t a g e n o is e divide d b y t h e fe e d b a ck r e sis t an ce is t h e mini m u m c u r r en t sig n al t h a t c a n b e det e c t e d . this mini m u m dete c t a b le c u r r en t divide d b y t h e r e sp o n sivi ty o f t h e photo d io d e r e pr es en ts t h e l o west l i g h t p o wer t h a t c a n b e dete c t e d b y t h e p r e a m p . n o is e s o ur ces a s s o ci a t e d w i t h t h e ph o t o d io de , a m plif ier , a nd f eed back r e sis t an ce a r e sh o w n in f i gur e 43; f i g u r e 44 is th e sp e c t r a l d e ns it y ve r s u s f r e q u e nc y pl ot of t h e c o n t r i but i on of e a ch o f t h e n o i s e s o ur ces t o t h e o u t p ut v o l t a g e n o is e (cir c u i t p a ra m e ters in f i gur e 42 a r e as sum e d). e a c h n o is e s o ur ce s r m s co n t ri b u ti o n t o th e t o tal o u t p u t v o l t a g e n o i s e i s o b ta in ed b y i n t e gra t in g th e s q ua r e o f i t s s p ec tral d e n s i t y fun c ti o n o v e r f r e q uen c y . th e r m s val u e o f t h e o u t p u t v o l t a g e n o is e is t h e s q u a re ro ot of t h e su m of a l l c o n t r i but i ons . m i n i m i z i n g t h e tot a l a r e a under t h es e c u r v es o p t i mi zes t h e p r e a m p l i f i er s r e s o l u t i o n fo r a g i ven b a ndwi d t h . 00511-043 a i s r s c s c f r f if en in f i g u re 43. phot od i o de pr e a mp n o is e s o u r c e s the ph o t o d io de p r e a m p i n f i gu r e 41 ca n de te c t a sig n a l c u r r en t o f 26 fa r m s a t a ban d wid t h o f 16 h z , w h ic h, ass u min g a p h o t o d io de r e sp o n sivi ty o f 0.5 a/w , tra n s l a t es t o a 52 fw r m s mini m u m dete c t a b le p o w e r . t h e ph o t o d io de us e d has a hig h so u r c e r e s i s t a n ce a n d l o w j u n c tio n ca pa c i ta n c e . c f se t s th e sig n al ban d wid t h wi th r f , a nd a l s o limi ts t h e p e ak in t h e n o is e ga in t h a t m u l t i p lies t h e o p am p s in p u t v o l t a g e no is e co n t r i b u - tio n . a sing le p o le f i l t er a t t h e am p l if ier s o u t p u t limi ts the o p am p s out p ut vo lt age noi s e b a nd w i d t h to 2 6 hz , c o m p ar abl e to th e si gn al b a n d w i d t h . th i s gr ea tl y i m p r o v e s th e p r ea m p li f i e r s sign al-t o-n o ise ra tio (in th is ca s e , b y a f a ct o r o f 3 ) . 10 1 100n 10n 1 1 0 100 1k 10k 100k 1m 00511-044 frequency (hz) v o ltage nois e contributions n o ise spec tr a l d e n s ity ( n v h z ) if and cs, no filters if and cs, with filters en contribution, with filter en contribution, no filter ad549 open-loop gain f i g u re 44. phot od i o de pr e a mp n o is e s o u r c e s ' spec t r a l d e ns it y v s . f r equenc y log ratio amplifier l o ga r i t h mic ra t i o cir c ui ts a r e us ef u l fo r p r o c es sin g sig n als w i t h wide d y na mic r a n g e . th e ad54 9l s 60 fa maxim u m in p u t c u r r e n t m a k e s it p o ss ibl e to b u i l d a l o g r a t i o am pl i f i e r w i t h 1 % log co nf o r ma n c e f o r in p u t c u r r en ts ra n g in g f r o m 10 pa t o 1 ma, a d y na mic ra n g e o f 160 db . the log ra t i o am plif ier in f i gur e 45 p r o v ides an o u t p u t v o l t a g e prop or t i on a l to t h e l o g b a s e 1 0 of t h e r a t i o of i n put c u r r e n t s i 1 a nd i2. resist o r s r1 an d r2 a r e p r o v ide d fo r v o l t a g e in p u ts. sin c e np n de vi ces a r e us e d i n t h e fe e d b a ck lo op o f t h e f r o n t e n d a m p l i f i e r s th a t p r o v i d e th e log tra n sf e r fun c ti o n , th e o u t p u t is va lid o n ly fo r p o si t i v e i n p u t v o l t a g es and in pu t c u r r en ts. the in p u t c u r r en ts s e t t h e col l e c t o r c u r r en ts i c 1 and i c 2 o f a m a tc he d p a i r of l o g t r ans i stor s , q 1 an d q 2 , to d e vel op vo lt age s va and vb : va , vb = C( kt / q )ln ic / ies w h er e ies i s t h e tra n si s t o r s sa t u ra ti o n curr e n t . the dif f er ence o f va and vb i s tak e n b y th e s u b t ra ct o r secti o n to ob t a i n vc = ( kt / q )ln( ic2 / ic 1 )
ad549 rev. d | page 16 of 20 the v e r y lo w in p u t c u r r en t o f th e ad549 ma k e s this cir c ui t us ef u l o v er a v e r y wide ra n g e of sig n al c u r r en ts. th e t o t a l i n p u t c u r r en t (w hich det e r m in es t h e l o w le v e l acc u rac y o f t h e cir c ui t ) is t h e s u m o f t h e a m plif ier i n p u t c u r r en t, t h e le aka g e acr o s s t h e co m p e n s a t i n g c a p a ci t o r ( n eg l i g i b l e if a p o l y s t yren e o r t e f l o n c a p a c i tor i s u s e d ) , and t h e c o l l e c tor - to - c ol l e c t or and c o l l e c tor to - b a s e l e a k age s of one s i d e of t h e d u a l l o g t r an s i stor s . t h e ma g n i t udes o f t h es e last tw o le a k a g es de p e nd on t h e am plif ier s i n put of f s e t volt age, a n d are t y pi c a l l y l e ss t h an 1 0 f a w i t h 1 m v o f fs ets. th e lo w le v e l acc u rac y is limi te d p r ima r i l y b y th e a m plif ier s i n p u t c u r r en t, o n ly 60 fa maxim u m w h en t h e ad549l is us e d . vc is s c ale d u p b y th e ra tio o f (r9 + r10)/r8, whic h is eq ua l to ap p r ox i m at e l y 1 6 at r o o m t e m p e r at u r e , r e s u l t i n g i n t h e o u t p u t vol t age v ou t = 1 l o g( ic 2 / ic 1 )v r8 is a r e sis t o r wi t h a p o si ti v e 3500 p p m /c t e m p er a t ur e co ef f i cien t t o p r o v ide t h e n e ces s a r y t e m p era t ur e co m p e n s a t i on. the p a ra l l e l com b in a t ion o f r15 a nd r7 is p r o v ide d t o k e ep t h e s u b t rac t o r sectio n s gain f o r p o si ti v e an d n e ga tiv e in p u ts ma tc he d ove r te m p e r a t u r e. f r eq uen c y co m p en s a tio n is p r o v ide d b y r11, r12, c1, a n d c2. the bandwid t h o f th e cir c ui t is 300 kh z a t in p u t sig n als g r ea t e r t h a n 50 a; b a nd w i d t h de cr e a s e s sm o o t h ly wi t h de cr e a sin g sig n al le v e l s. the ef fe c t s o f t h e e m i t t e r r e sis t a n c e o f q1 a nd q2 ca n deg r ade t h e cir c ui t s acc u rac y a t in p u t c u r r en ts a b o v e 1 00 a. the n e tw o r ks co m p os ed o f r13, d1, r16, r14, d2, a nd r17 co m p en sa t e f o r th e s e e r r o r s , so th a t th i s ci r c ui t h a s le s s th a n 1% log co nfo r ma n c e er r o r a t 1 ma in p u t c u r r en ts. the co r r e c t val u e f o r r13 a nd r14 dep e n d s o n the typ e o f log tra n sis t o r s us ed . 49.9 k? r e sis t o r s w e r e c h o s en f o r us e wi th lm394 tra n sis t o r s. s m a l ler r e sis t a n ce v a l u es a r e n e e d e d fo r smal ler log tra n sis t o r s. t o t r im t h e cir c ui t, s e t t h e i n p u t c u r r en ts t o 10 a an d t r i m a3 s o f fs et usin g t h e a m plif ier s t r im p o t e n t iom e t e r s o t h e o u t p u t eq uals 0. th en s e t i1 t o 1 a and ad j u st the o u t p u t t o eq ual 1 v b y t r immin g r1 0. a ddi t i o n a l o f fs et t r im s o n a m plif iers a1 an d a2 can b e us ed t o in cr eas e t h e v o l t a g e in p u t ac c u rac y a n d dy n a m i c r a nge. 00511-045 3 2 6 5 1 4 a3 ad549 r10 2k ? 10k ? output offset scale factor adj v out r9 14.3k ? r8 1k ? * * r4 20k ? r6 20k ? r7 15k ? r15 1k ? r5 20k ? r3 20k ? 0.1 f 0.1 f +v s for each amplifie r pin 7 pin 4 ?v s 2 3 6 5 1 4 a2 ad549 v 2 offset 10k ? d4 i 2 in v 2 in c2 100pf r2 10k ? q2 b d2 d1 i 1 in 2 6 5 1 4 a1 ad549 v 1 offset 10k ? d3 3 v 1 in r1 10k ? q1 a c1 100pf r11 49.9k ? r14 49.9k ? r16 10k ? r17 10k ? r13 49.9k ? q1, q2 = lm394 dual log transistors d1, d4 1n4148 diodes r8, r15 1k ? + 350 ppm/ c tc resistor * tellab qb1 or precision resistor pt146 all other resistors are 1% metal film v out = 1v log 10 v 2 v 1 v out = 1v log 10 i 2 i 1 f i g u re 45. l o g r a t i o a m p lif ier
ad549 rev. d | page 17 of 20 temperature compensated ph probe amplifier a ph p r ob e can b e m o d e le d as a mv -le v el vol t a g e s o ur ce wi t h a se ri e s so u r c e r e s i s t a n c e d e pen d en t u p o n t h e e l ec tr od e s co m p o s i t ion and co nf igur a t ion. the g l a s s b u lb r e sist a n c e o f a t y p i cal ph e l ect r od e pa i r fall s b e t w een 10 6 ? and 10 9 ?. i t is t h er efo r e im p o r t a n t t o s e le c t an a m plif ier w i t h l o w en o u g h in p u t c u r r en ts such t h a t t h e v o l t a g e dr o p p r o d u c e d b y t h e a m plif ier s i n p u t b i as c u r r en t and t h e e l e c t r o d e r e sis t a n ce do es n o t b e come an a p p r e c ia b l e p e r c en t a g e o f a ph uni t . the cir c ui t in f i gur e 46 il l u s t ra tes th e us e o f the ad549 as a ph p r ob e am plif ier . a s w i t h o t h e r ele c t r o m et er a p plica t ion s , t h e us e o f gua r din g , shi e lding, t e f l o n st a n do f f s, a nd s o o n is a m u st i n o r der t o ca p i talize on the ad54 9 s lo w in p u t c u r r en t. i f a n ad549l (60 fa max in p u t c u r r en t) is us ed , t h e er r o r c o n t r i but e d by t h e i n pu t c u r r e n t i s h e l d b e l o w 6 0 v f o r p h e l e c t r o d e s o ur c e im p e dan c es up t o 10 9 ? i n p u t o f fs et v o l t a g e (whic h ca n b e t r imm e d) wil l b e be lo w 0.5 mv . the ph p r ob e ou t p ut is ide a l l y 0 v a t a p h o f 7 in d e p e n d e n t o f t e m p era t ur e . the s l o p e o f t h e p r ob e s t r a n sfer f u n c t i on, t h o u g h p r edic ta b l e , is tem p er a t ur e dep e n d en t (?54.2 mv/ph a t 0 and ?74.04 mv/ph a t 100c). b y usin g a n ad590 tem p er a t ur e s e n s o r an d a n ad535 a n alog divider , a n acc u ra t e t e m p er a t ur e co m p e n s a t i o n netw o r k can b e a dde d to t h e b a si c ph p r ob e a m p l if ier . t a b l e 4 s h o w s v o l t a g e s a t va r i o u s p o in ts and il l u s t ra tes th e com p en s a t i o n . th e ad549 is s e t f o r a n o nin v er tin g ga in o f 13.51. th e ou t p u t o f th e ad590 cir c ui tr y (p o i n t c) is eq ual t o 10 v a t 100c, a nd decr eas e s b y 26.8 mv/c. th e o u t p u t o f th e ad535 a n alog divider (p o i n t d) is a t e m p er a t ur e co m p ens a t e d o u t p ut v o l t a g e c e n t er e d a t 0 v fo r a ph o f 7, a nd has a t r a n sfer f u nc t i o n of C 1 . 0 0 v / p h u n it . t h e output r a nge s p ans f r om ?7.00 v (ph = 14) t o +7.00 v ( p h = 0). 00511-046 5 14 10 z 2 12 out 7 y 2 6 y 1 11 z 1 1 x 1 2 x 2 ad535 0.1 f +15v 0.1 f ? 15v output (d) (b) (c) 26.6k ? + ? ad590 in stainless steel probe or ac2626 +15v 3 2 6 4 8 7 ad549 0.1 f 0.1 f ph probe output (a) 1k ? 12k ? 1k ? scale factor adjust f i gure 46. t e mp er atur e comp ensat e d ph a m p lif ied table 4. illustration of tem p e r ature compe n sation point probe temperature a (probe output) b (a 3 1 3 .51) c (590 output) d (10 b/c) 0 54.20 mv 0.732 v 7.32 v 1.00 v 25c 59.16 mv 0.799 v 7.99 v 1.00 v 37c 61.54 mv 0.831 v 8.31 v 1.00 v 60c 66.10 mv 0.893 v 8.93 v 1.00 v 100c 74.04 mv 1.000 v 10.00 v 1.00 v
ad549 rev. d | page 18 of 20 outline dimensions 0.2500 (6.35) min 0.5000 (12.70) min 0.1850 (4.70) 0.1650 (4.19) reference plane 0.0500 (1.27) max 0.0190 (0.48) 0.0160 (0.41) 0.0210 (0.53) 0.0160 (0.41) 0.0400 (1.02) 0.0100 (0.25) 0.0400 (1.02) max base & seating plane 0.0340 (0.86) 0.0280 (0.71) 0.0450 (1.14) 0.0270 (0.69) 0.1600 (4.06) 0.1400 (3.56) 0.1000 (2.54) bsc 6 2 8 7 5 4 3 1 0.2000 (5.08) bsc 0.1000 (2.54) bsc 45 bsc 0.3700 ( 9 .40) 0.3350 ( 8 .51) 0.3350 ( 8 .51) 0.3050 ( 7 .75) controlling dimensions are in inches; millimeter dimensions (in parentheses) are rounded-off inch equivalents for reference only and are not appropriate for use in design compliant to jedec standards mo-002ak f i g u re 47. 8-l e ad m e t a l can [ t o - 99] (h-08) di me nsio ns sho w n i n i n che s a n d ( m il lim e t e r s) ordering guide model temperature r a nge package descri ption package option AD549JH 0c to +70c 8-lead metal can (to-99) h-08 ad549kh 0c to +70c 8-lead metal can (to-99) h-08 ad549lh 0c to +70c 8-lead metal can (to-99) h-08 ad549sh/883b C55c to +125c 8-lead metal can (to-99) h-08
ad549 rev. d | page 19 of 20 notes
ad549 rev. d | page 20 of 20 notes ? 2004 analo g de vices, inc. all rights reserve d . tra d em arks and registered tra d ema r ks are the prop erty of their respective owners . c00511C0 C 5/04(d)

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