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low noise, 90 mhz variable gain amplifier ad603 rev. g 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. 461. 31 13 ? 2005 analog de vices, i n c. al l r i ght s r e ser v ed . features linear-in-db g a in control pin programmable gain rang es ?11 db to +31 db with 90 mh z bandwidth 9 db to 51 db w i th 9 mhz band width any interme di ate range, fo r e x ample ?1 db t o +41 db with 30 mhz b a ndwidth bandwidth independent of variable gain 1.3 nv/hz inp u t noise spectr al densit y 0.5 db ty pical gain accu racy applic ati o ns rf/if agc amp l ifier video gain co ntro l a/d range extension signal measur ement general description the ad603 is a lo w n o is e , v o l t ag e-co n t r o l l ed am p l if ier f o r us e in rf and if a g c sys t em s. i t p r o v ides acc u ra t e , p i n s e lec t a b le ga in s o f ?11 db t o +31 db wi th a ban d wid t h o f 90 mh z o r 9 db t o 51 d b w i t h a ba n d w id th o f 9 mh z . an y in t e rm e d ia t e ga in ra n g e ma y b e a r ra n g e d usin g one ext e r n al r e sis t o r . th e i n p u t r e fer r e d n o is e sp e c t r a l den s i t y i s o n ly 1.3 nv/ h z and p o w e r co n s um p t io n is 125 mw a t t h e r e co mmen d e d 5 v s u p p lies. the de c i b e l ga i n is li n e a r i n db , acc u r a tely ca l i b r a t e d , and st able o v er t e m p era t ure a nd s u p p l y . th e ga i n is con t r o l l e d a t a hig h im p e dan c e (50 m?), l o w b i as (200 na) dif f er en tial in p u t; t h e s c a l in g is 25 m v /d b , r e q u ir in g a ga in con t r o l vol t a g e o f o n ly 1 v t o s p a n t h e ce n t ral 40 db o f t h e ga in ra n g e . a n o v er ra n g e a n d un der r a n g e o f 1 db is p r o v ide d wha t ever th e s e l e c t ed ran g e . the ga in con t r o l r e s p o n s e t i m e is les s tha n 1 s f o r a 40 db c h an g e . the dif f er en t i al ga in con t r o l in ter f ace al lo ws t h e us e o f ei t h er di f f e r e n t i a l or s i ng l e - e nde d p o s i t i ve or ne g a t i ve c o n t rol vol t age s . s e v e ra l o f t h es e a m plif iers ma y b e cas c ade d and t h e i r ga in co n t r o l ga in s o f fs et t o o p t i mi ze t h e sys t em s / n ra t i o . the ad603 can dr i v e a lo ad im p e dan c e as lo w as 100 ? wi th lo w dis t o r tio n . f o r a 500 ? lo a d in s h u n t wi t h 5 pf , th e t o tal h a r m on i c d i stor t i on f o r a 1 v s i n u s o i d a l output a t 1 0 m h z i s ty p i ca l l y ?60 d b c. the p e a k sp e c if ie d o u t p ut is 2.5 v minim u m in t o a 500 ? lo ad . the ad603 us es a p a t e n t ed p r op r i eta r y cir c ui t t o p o log y th e x-am p?. the x - amp com p r i s e s a va r i ab le a t t e n u a t o r o f 0 db t o ?42.14 db f o l l o w ed b y a f i xed-ga in am p l if ier . b e ca us e o f th e a t t e n u a t o r , t h e am plif ier ne v e r h a s t o co p e w i t h l a rge in p u t s an d ca n us e nega t i ve fe e d b a ck to de f i n e i t s (f ixe d ) ga in and d y na mi c p e r f o r ma n c e . th e a t t e n u a t o r has a n in p u t r e sis t a n ce o f 100 ?, las e r t r im m e d to 3%, a nd co m p r i s e s a s e ve n- st a g e r - 2r ladde r n e tw o r k, r e su l t i n g in an a t t e n u a t io n b e t w e e n t a p p o in t s o f 6.021 db . a p r op r i eta r y in t e r p ola t ion t e chniq u e p r o v ides a co n t i n uo us ga in co n t r o l f u n c t i on w h ich is li n e ar in db . t h e a d 60 3 i s sp e c if ie d fo r o p er a t ion f r om ?4 0 c t o + 8 5 c . func tio n a l block di agram scaling reference v g gain- control interface ad603 precision passive input attenuator fixed-gain amplifier 1 nominal values. r-2r ladder network vpos vneg gpos gneg vinp comm 0db ? 6.02db ? 12.04db ?18.06db ?24.08db ? 30.1db ?36.12db ? 42.14db rr r r r r r 2r 2r 2r 2r 2r 2r r 20 ? 1 694 ? 1 6.44k ? 1 v out fdbk 00539-001 8 6 1 2 5 7 4 3 fi g u r e 1 .
ad603 rev. g | page 2 of 20 table of contents specifications ..................................................................................... 3 absolute maximum ratings ............................................................ 4 esd caution .................................................................................. 4 pin configuration and function descriptions ............................. 5 typical performance characteristics ............................................. 6 theory of operation ...................................................................... 11 noise performance ..................................................................... 11 the gain control interface ....................................................... 12 programming the fixed-gain amplifier using pin strapping ................................................................... 12 using the ad603 in cascade ........................................................ 14 sequential mode (optimal s/n ratio) .................................... 14 parallel mode (simplest gain control interface) .................. 16 low gain ripple mode (minimum gain error) ................... 16 applications ..................................................................................... 18 a low noise agc amplifier .................................................... 18 caution ........................................................................................ 19 outline dimensions ....................................................................... 20 ordering guide .......................................................................... 20 revision history 3/05rev. f to rev. g updated format.................................................................. universal change to features ............................................................................1 changes to general description .....................................................1 change to figure 1 ............................................................................1 changes to specifications .................................................................3 new figure 4 and renumbering subsequent figures...................6 change to figure 10 ..........................................................................7 change to figure 23 ..........................................................................9 change to figure 29 ........................................................................12 updated outline dimensions ........................................................20 4/04rev. e to rev. f changes to specifications .................................................................2 changes to ordering guide .............................................................3 8/03rev. d to rev e updated format.................................................................. universal changes to specifications .................................................................2 changes to tpcs 2, 3, 4.....................................................................4 changes to sequential mode (optimal s/n ratio) section .........9 change to figure 8 ..........................................................................10 updated outline dimensions ........................................................14
ad603 rev. g | page 3 of 20 specifications @ t a = 25c, v s = 5 v, C500 mv v g +500 mv, gneg = 0 v, C10 db to +30 db gain range, r l = 500 ?, and c l = 5 pf, unless otherwise noted. table 1. parameter conditions min t y p max unit input characteristics input resistance pin 3 to pin 4 97 100 103 ? input capacitance 2 pf input noise spectral density 1 input short-circuited 1.3 nv/hz noise figure f = 10 mhz, gain = max, r s = 10 ? 8.8 db 1 db compression point f = 10 mhz, gain = max, r s = 10 ? ?11 dbm peak input voltage 1.4 2 v output characteristics ?3 db bandwidth v out = 100 mv rms 90 mhz slew rate r l 500 ? 275 v/s peak output 2 r l 500 ? 2.5 3.0 v output impedance f 10 mhz 2 ? output short-circuit current 50 ma group delay change vs. gain f = 3 mhz; full gain range 2 ns group delay change vs. frequency v g = 0 v; f = 1 mhz to 10 mhz 2 ns differential gain 0.2 % differential phase 0.2 degree total harmonic distortion f = 10 mhz, v out = 1 v rms ?60 dbc third order intercept f = 40 mhz, gain = max, r s = 50 ? 15 dbm accuracy gain accuracy, f = 100 khz; gain (db) = (40 v g + 10) db ?500 mv v g +500 mv, ?1 0.5 +1 db t min to t max ?1.5 +1.5 db gain, f = 10.7 mhz v g = -0.5 v ?10.3 ?9.0 ?8.0 db v g = 0.0 v +9.5 +10.5 +11.5 db v g = 0.5 v +29.3 +30.3 +31.3 db output offset voltage 3 v g = 0 v ?20 +20 mv t min to t max ?30 +30 mv output offset variation vs. v g ?500 mv v g +500 mv ?20 +20 mv t min to t max ?30 +30 mv gain control interface gain scaling factor 100 khz 39.4 40 40.6 db/v t min to t max 38 42 db/v 10.7 mhz 38.7 39.3 39.9 db/v gneg, gpos voltage range 4 ?1.2 +2.0 v input bias current 200 na input offset current 10 na differential input resistance pin 1 to pin 2 50 m? response rate full 40 db gain change 80 db/s power supply specified operating range 4.75 6.3 v quiescent current 12.5 17 ma t min to t max 20 ma 1 typical open or short-circuited input; noise is lower when syst em is set to maximum gain and input is short-circuited. this fi gure includes the effects of both voltage and current noise sources. 2 using resistive loads of 500 ? or greate r, or with the addition of a 1 k? pull-down resistor when driving lower loads. 3 the dc gain of the main amplifier in the ad603 is 35.7; thus , an input offset of 100 v be comes a 3.57 mv output offset. 4 gneg and gpos, gain control, and voltage range are guaranteed to be within the range of ?v s + 4.2 v to +v s ? 3.4 v over the full temperature range of ?40c to +85c.
ad603 r e v. g | pa ge 4 o f 2 0 absolute maximum ratings table 2. p a r a m e t e r r a t i n g supply voltage v s 7.5 v internal voltage vinp (p in 3) 2 v continuous v s for 10 ms gpos, gneg (pi n s 1, 2) v s internal power dissip a tion 1 400 mw operating tem p erature range ad603a ?40c to +85c ad603s ?55c to +125c storage temperature range ?65c to +150c lead temperature range (soldering 60 sec) 300c 1 thermal c h aracteris t ics : 8- lea d soic pa c k a g e: ja = 155c/ w, jc = 33 c/w, 8-le ad c e rdip pack age : ja = 140c/w, jc = 15c/w. 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) se nsit ive device. electrostatic charges as high as 4000 v readily accumulate on the human body and test eq ui pment and can discharge wi thout detection. although this product features proprietary esd protection circ uitry, permane n t dama ge may occur on devices subj ected to high energy electrostatic discharges. theref ore, pr oper esd prec a utions are recomm ended to avoid perfor mance degrada - tion or los s of functional ity.
ad603 r e v. g | pa ge 5 o f 2 0 pin conf iguration and fu nction descriptions gpos 1 gneg 2 vinp 3 comm 4 vpos 8 vout 7 vneg 6 fdbk 5 ad603 top view (not to scale) 00539-002 f i g u re 2. 8-l e ad pl as t i c soic (r ) p a ckag e 00539-003 gpos 1 gneg 2 vinp 3 comm 4 vpos 8 vout 7 vneg 6 fdbk 5 ad603 top view (not to scale) f i gure 3. 8-l e ad ce r a mi c ce r d ip (q) p a ckage ta ble 3. pi n f u nct i on d e s c ri pt i o ns pin o. neonic description 1 gpos gain control input high (p ositive voltage incre a ses gai n ) . 2 gneg gain control input low (n egati ve voltage incre a ses gai n ). 3 v i n p a m p l i f i e r i n p u t . 4 c o m m a m p l i f i e r groun d . 5 fdbk connectio n to feedback netwo r k. 6 vneg negative supply input. 7 v o u t a m p l i f i e r o u t p u t . 8 vpos positive supply input.
ad603 r e v. g | pa ge 6 o f 2 0 typical perf orm ance cha r acte ristics @ t a = 25c, v s = 5 v , C500 mv v g +500 mv , g n eg = 0 v , C10 db t o +3 0 db ga in ran g e , r l = 500 ?, a nd c l = 5 pf , unles s o t h e r w ise not e d. 00539-004 v g (v) 0.6 ?0.6 ?0.4 ?0.2 0 0.2 0.4 gain ( d b) 40 30 20 10 0 ?10 1 0.7m hz 100 kh z fi g u r e 4 . g a i n v s . v g a t 100 khz and 10 .7 mhz 00539-005 gain voltage (volts) 0.5 ?0.5 ? 0.4 ?0.3 ?0.2 ? 0.1 0 0.1 0.2 0.3 0.4 gain e rror (db) 2.5 2.0 1.5 1.0 0.5 0 ?0.5 ?1.0 ?1.5 45mhz 70mhz 10.7mhz 455khz 70mhz f i g u re 5. g a in e rror v s . g a in cont ro l v o lt ag e at 4 5 5 k h z, 10. 7 m h z, 4 5 m h z, 70 m h z 00539-006 ph a se ( d egrees) ? 225 225 45 90 135 180 0 ?4 5 ?9 0 ? 135 ? 180 frequency (hz) 100k 1m 10m 100m gain ( d b) 4 2 3 0 1 ?2 ?1 ?4 ?5 ?3 ?6 gain phase f i g u re 6. f r equ e nc y and p h as e r e s p o n s e v s . g a in (g ain = ?1 0 db , p in = ? 3 0 db m) 00539-007 ? 225 225 180 135 90 45 0 ?4 5 ?9 0 ? 180 ? 135 ph a se ( d egrees) frequency (hz) 100k 1m 10m 100m gain ( d b) 4 2 1 3 0 ?1 ?2 ?3 ?4 ?5 ?6 gain phase f i g u re 7. f r equ e nc y and p h as e r e s p o n s e v s . g a in (g ain = +1 0 db , p in = ? 3 0 db m) 00539-008 ph a se ( d egrees) ? 225 225 45 90 135 180 0 ?4 5 ?9 0 ? 135 ? 180 frequency (hz) 100k 1m 10m 100m gain ( d b) 4 2 3 0 1 ?2 ?1 ?4 ?3 ?6 ?5 gain phase f i g u re 8. f r equ e nc y and p h as e r e s p o n s e v s . g a in (g ain = +3 0 db , p in = ? 3 0 db m) 00539-009 gain control voltage (v) 0.6 ?0.6 ? 0.4 ? 0.2 0 0.2 0.4 group delay ( n s) 7.60 7.40 7.20 7.00 6.80 6.60 6.40 f i g u re 9. g r oup d e l a y v s . g a in co nt r o l v o lt ag e
ad603 r e v. g | pa ge 7 o f 2 0 datel dvc 8500 hp3326a dual- channel synthesizer 100 ? +5v 511 ? 10 probe hp3585a spectrum analyzer 0.1 f 00539-010 ad603 8 5 4 7 3 6 1 2 ?5v 0.1 f f i g u re 10. thi r d o r der in ter m odul at i o n d i s t or t i on t e s t s e t u p 00539-011 10db/div f i g u re 11. thi r d o r der in ter m odul at i o n d i s t or t i on at 4 55 k h z (1 0 p r o b e use d to h p 35 85 a sp ectr u m a n al yz e r , gai n = 0 db , p in = 0 db m) 00539-012 10db/div f i g u re 12. thi r d o r der in ter m odul at i o n d i s t or t i on at 1 0 .7 m h z (1 0 p r o b e use d to h p 35 85 a sp ectr u m a n al yz e r , gai n = 0 db , p in = 0 db m) 00539-013 load resistance ( ? ) 0 5 0 100 200 500 1000 2000 negative output voltage ( v ) ?1.0 ?1.2 ?1.4 ?1.6 ?1.8 ?2.0 ?2.2 ?2.4 ?2.6 ?2.8 ?3.0 ?3.2 ?3.4 f i g u re 13. t y pic a l o u t p ut v o lt ag e sw ing v s . l oad r e s i s t a n c e (nega t i v e o u tput swi n g l i mi ts f i rst) 00539-014 frequency (hz) 100m 100k 1m 10m inp u t imp e dance ( ? ) 102 100 98 94 96 f i gure 14. input i m p e dance vs . f r equenc y ( g ain = ?10 db ) 00539-015 frequency (hz) 100m 100k 1m 10m in pu t im ped a n c e ( ? ) 100 102 98 96 94 f i gure 15. input i m p e dance vs . f r equenc y ( g ain = +10 db )
ad603 r e v. g | pa ge 8 o f 2 0 00539-016 frequency (hz) 100m 100k 1m 10m inp u t imp e dance ( ? ) 100 102 98 96 94 f i gure 16. input i m p e dance vs . f r equenc y ( g ain = +30 db ) 00539-017 1v 200ns 1v 10 0% 100 90 f i g u re 17. g a in co nt r o l ch anne l r e s p ons e ti me 00539-018 451ns ?49ns 50ns 4.5v 500mv ? 500mv input gnd 1v/div output gnd 500mv/div f i gure 18. input sta g e o v er lo ad r e c o v e r y t i m e (input is 500 ns p e riod , 50% d u t y c y cl e squ a r e w a ve , ou tpu t i s c a ptur e d usi n g t e ktr o ni x 114 02 d i gi tiz i ng os c ill o s c o p e ) 00539-019 451ns ? 49ns 50ns 3v 1v ?2v input gnd 100mv/div output gnd 1v/div f i g u re 19. o u t p ut s t ag e o v e r l oad r eco ver y tim e (input is 50 0 ns p e r i od , 50% d u t y c y c l e s q uare w a v e , o u t p ut is capt u r ed usi n g t e ktr o ni x 114 02 d i gi tiz i ng os c ill o s c o p e ) 00539-020 gnd gnd 456ns ?44ns 50ns 3.5v 500mv ?1.5v input 500mv/div output 500mv/div f i gur e 2 0 . t r a n sient resp o n se , g = 0 d b (input is 500 ns p e riod , 50% d u t y c y cl e squ a r e w a ve , ou tpu t i s c a ptur e d usi n g t e ktr o ni x 114 02 d i gi tiz i ng os c ill o s c o p e ) 00539-021 456ns ?44ns 50ns 3.5v 500mv ?1.5v input gnd 100mv/div output gnd 500mv/div f i gur e 2 1 . t r a n sient resp o n se , g = +20 db (input is 500 ns p e riod , 50% d u t y c y cl e squ a r e w a ve , ou tpu t i s c a ptur e d usi n g t e ktr o ni x 114 02 d i gi tiz i ng os c ill o s c o p e )
ad603 r e v. g | pa ge 9 o f 2 0 00539-022 frequency (hz) 100m 100k 1m 10m p s rr (db) 0 ?10 ?20 ?30 ?40 ?50 ?60 fi g u r e 2 2 . p s r r v s . fr e q u e n c y ( w o r st ca se i s neg a ti v e sup p l y p s rr, sho w n her e ) datel dvc 8500 hp3326a dual- channel synthesizer 100 ? +5v 50 ? hp3585a spectrum analyzer 0.1 f 00539-023 ad603 8 5 4 7 3 6 1 2 ?5v 0.1 f f i gure 23. t e s t s e tu p u s ed for : n o is e f i gur e , th ir d o r de r i n terc ept and 1 db co mpres s i on p o int m e as u r e m e n t s 00539-024 gain (db) 30 20 21 22 23 24 25 26 27 28 29 nois e figure (db) 23 21 19 17 15 13 11 9 7 5 t a = 25 c r s = 50v test setup figure 23 70mhz 30mhz 50mhz 10mhz f i gure 24. noise f i g u r e in ?10 db /+30 db mode 00539-025 gain (db) 40 30 31 32 33 34 35 36 37 38 39 nois e figure (db) 21 17 19 15 11 13 7 9 5 t a = 25 c r s = 50 ? test setup figure 23 20mhz 10mhz f i g u re 25. no is e f i g u r e in 0 db /4 0 d b m o de 00539-026 input frequency (mhz) 70 10 30 50 in pu t level ( d b m ) 0 ?5 ?10 ?15 ?20 ?25 t a = 25 c test setup figure 23 f i g u re 26. 1 d b co mpres s i on p o int , ? 10 d b / + 3 0 db m o d e , g a i n = + 30 d b 00539-027 input level (dbm) 0 ?20 ? 10 ou tpu t level ( d b m ) 20 18 16 12 14 10 0 t a = 25 c test setup figure 23 30mhz 40mhz 70mhz f i gure 27. thir d o r der in t e rc ept ?10 d b /+30 db m o de , g a in = +10 db
ad603 rev. g | page 10 of 20 00539-028 input level (dbm) ?20 ?40 ? 30 ou tpu t level ( d b m ) 20 16 18 14 12 10 8 t a = 25 c r s = 50 ? r in = 50 ? r l = 100 ? test setup figure 23 30mhz 40mhz 70mhz f i gure 28. thir d o r der in t e rc ept ?10 d b /+30 db m o de , g a in = +30 db
ad603 rev. g | page 11 of 20 theory of operation the ad603 comprises a fixed-gain amplifier, preceded by a broadband passive attenuator of 0 db to 42.14 db, having a gain control scaling factor of 40 db per volt. the fixed gain is laser- trimmed in two ranges, to either 31.07 db (35.8) or 50 db (358), or may be set to any range in between using one external resistor between pin 5 and pin 7. somewhat higher gain can be obtained by connecting the resistor from pin 5 to common, but the increase in output offset voltage limits the maximum gain to about 60 db. for any given range, the bandwidth is independent of the voltage-controlled gain. this system provides an underrange and overrange of 1.07 db in all cases; for example, the overall gain is ?11.07 db to +31.07 db in the maximum bandwidth mode (pin 5 and pin 7 strapped). this x-amp structure has many advantages over former methods of gain control based on nonlinear elements. most importantly, the fixed-gain amplifier can use negative feedback to increase its accuracy. since large inputs are first attenuated, the amplifier input is always small. for example, to deliver a 1 v output in the ?1 db/+41 db mode (that is, using a fixed amplifier gain of 41.07 db) its input is only 8.84 mv; thus the distortion can be very low. equally important, the small-signal gain and phase response, and thus the pulse response, are essentially independent of gain. figure 29 is a simplified schematic. the input attenuator is a seven-section r-2r ladder network, using untrimmed resistors of nominally r = 62.5 ?, which results in a characteristic resistance of 125 ? 20%. a shunt resistor is included at the input and laser trimmed to establish a more exact input resistance of 100 ? 3%, which ensures accurate operation (gain and hp corner frequency) when used in conjunction with external resistors or capacitors. the nominal maximum signal at input vinp is 1 v rms (1.4 v peak) when using the recommended 5 v supplies, although operation to 2 v peak is permissible with some increase in hf distortion and feedthrough. pin 4 (comm) must be connected directly to the input ground; significant impedance in this connection will reduce the gain accuracy. the signal applied at the input of the ladder network is attenuated by 6.02 db by each section; thus, the attenuation to each of the taps is progressively 0 db, 6.02 db, 12.04 db, 18.06 db, 24.08 db, 30.1 db, 36.12 db, and 42.14 db. a unique circuit technique is employed to interpolate between these tap points, indicated by the slider in figure 29, thus providing continuous attenuation from 0 db to 42.14 db. it will help in understanding the ad603 to think in terms of a mechanical means for moving this slider from left to right; in fact, its position is controlled by the voltage between pin 1 and pin 2. the details of the gain control interface are discussed later. the gain is at all times very exactly determined, and a linear-in- db relationship is automatically guaranteed by the exponential nature of the attenuation in the ladder network (the x-amp principle). in practice, the gain deviates slightly from the ideal law, by about 0.2 db peak (see, for example, figure 5). noise performance an important advantage of the x-amp is its superior noise performance. the nominal resistance seen at inner tap points is 41.7 ? (one third of 125 ?), which exhibits a johnson noise spectral density (nsd) of 0.83 nv/hz (that is, 4ktr) at 27c, which is a large fraction of the total input noise. the first stage of the amplifier contributes a further 1 nv/hz, for a total input noise of 1.3 nv/hz. it will be apparent that it is essential to use a low resistance in the ladder network to achieve the very low specified noise level. the signals source impedance forms a voltage divider with the ad603s 100 ? input resistance. in some applications, the resulting attenuation may be unacceptable, requiring the use of an external buffer or preamplifier to match a high impedance source to the low impedance ad603. the noise at maximum gain (that is, at the 0 db tap) depends on whether the input is short-circuited or open-circuited: when shorted, the minimum nsd of slightly over 1 nv/hz is achieved; when open, the resistance of 100 ? looking into the first tap generates 1.29 nv/hz, so the noise increases to a total of 1.63 nv/hz. (this last calculation would be important if the ad603 were preceded by, for example, a 900 ? resistor to allow operation from inputs up to 10 v rms.) as the selected tap moves away from the input, the dependence of the noise on source impedance quickly diminishes. apart from the small variations just discussed, the signal-to- noise (s/n) ratio at the output is essentially independent of the attenuator setting. for example, on the ?11 db/+31 db range, the fixed gain of 35.8 raises the output nsd to 46.5 nv/hz. thus, for the maximum undistorted output of 1 v rms and a 1 mhz bandwidth, the output s/n ratio would be 86.6 db, that is, 20 log (1 v/46.5 v).
ad603 rev. g | page 12 of 20 scaling reference v g gain- control interface ad603 precision passive input attenuator fixed-gain amplifier 1 nominal values. r-2r ladder network vpos vneg gpos gneg vinp c omm 0db ? 6.02db ? 12.04db ?18.06db ? 24.08db ? 30.1db ? 36.12db ?42.14db r r r rrr r 2r 2r 2r 2r 2r 2r r 20 ? 1 694 ? 1 6.44k ? 1 v out fdbk 00539-029 8 6 1 2 5 7 4 3 f i g u re 29. si mpl i f i e d bl ock d i ag r a m the gain contr o l interface the a t t e n u a t ion is co n t r o l l e d t h r o ug h a dif f er en t i al , hig h im p e d a n c e (50 m?) in p u t, wi t h a s c a l in g fac t or w h ich is l a s e r - t r imm e d to 40 db p e r vol t , t h a t is, 25 mv/ d b . a n in ter n a l b a nd g a p re f e re nc e e n su re s st a b i l it y of t h e s c a l i n g w i t h re sp e c t to su p p ly and te m p e r a t u r e v a r i a t i o ns . w h en t h e dif f er en t i al i n p u t v o l t a g e v g = 0 v , the a t t e n u a t o r slider is cen t er e d , p r o v iding a n a t t e n u a t io n o f 2 1 .07 db . f o r t h e maxim u m ban d wid t h ra n g e , this r e s u l t s in an o v eral l ga in o f 10 db (= ?21.07 db + 31.07 db). w h en t h e con t r o l in p u t is ?500 mv , the gain is lo w e r e d b y 20 db (= 0.500 v 40 db/v) t o ?10 db; w h en s e t t o +500 mv , t h e ga in is in cr eas e d b y 20 db t o 30 db . w h en t h is in t e r f ace is o v er dr i v en i n ei t h er dir e c t io n, t h e ga in a p p r o a c h es ei ther ?11.07 db (= ? 42.14 db + 31.07 db) o r 31.07 db (= 0 + 31.07 db), r e s p e c ti v e l y . th e o n l y co n s tra i n t o n th e ga in co n t r o l v o l t a g e i s tha t i t be k e p t w i thi n th e co mm o n - m o de ra n g e (?1 . 2 v t o +2.0 v as s u ming +5 v su p p lies) o f t h e ga in con t r o l in ter f ace . the basic ga in o f th e ad603 can th us b e calc u l a t e d usin g t h e fol l o w in g sim p le exp r es sio n : ga i n ( d b) = 40 v g + 1 0 ( 1 ) w h er e v g is in vol t s. w h e n pi n 5 a nd pi n 7 a r e st r a p p e d (s e e n e xt s e c t ion), t h e ga in b e com e s ga i n ( d b) = 40 v g + 20 fo r 0 to +40 db a nd ga i n ( d b) = 40 v g + 30 fo r +10 to +50 db (2) the hig h i m p e da n c e ga in co n t rol in p u t e n s u r e s minimal lo adin g w h e n d r i v in g m a n y a m plif iers in m u l t i p le cha n n e l o r cas c ade d a p plic a t io n s . t h e dif f e r en t i a l ca p a b i l i t y p r o v ides f l exi b ili t y in cho o sin g th e a p p r o p r i a t e sig n al le v e l s a nd po l a ri ti e s f o r v a ri o u s c o n t r o l sc h e m e s . f o r exa m ple , if th e ga i n is t o b e co n t r o l l e d b y a d a c p r o v id in g a p o si t i v e onl y g r o u n d -r efer en c e d o u t p u t , t h e g a in c o n t r o l l o w (gneg) p i n sho u ld be b i as e d to a f i xed o f fs et o f 500 mv t o s e t th e ga in t o ?10 db w h en gain c o n t r o l h i g h ( g pos) is a t zero , a nd t o 30 db w h en a t 1.00 v . i t is a sim p le ma t t er t o i n cl ude a v o l t a g e di v i der t o achie v e o t her s c a l in g f a c t o r s. w h en usin g an 8-b i t d a c ha vi n g a n fs o u t p u t o f 2.55 v (10 mv/b i t ), a divider ra tio o f 2 (g en era t in g 5 mv/b i t ) w o u l d r e su l t in a ga in -s e t t i ng r e s o l u t i o n o f 0.2 db/ b i t . t h e us e o f s u c h o f fs ets is val u a b le w h en tw o ad603 s a r e cas c ade d , w h en v a r i ou s opt i ons e x i s t f o r opt i m i z i ng t h e s / n pr of i l e, a s w i l l b e shown l a te r . programm ing the fi x e d-gai n am plifier using pin s t rapping a c ces s t o t h e f e ed back n e tw o r k is p r o v ided a t pin 5 (fd b k). the us er ma y p r og ra m th e ga in o f th e ad603 s o u t p u t a m p l if ier usin g this p i n, as s h own in f i gu r e 30, f i gur e 31, a nd f i gur e 32. ther e a r e t h r e e m o de s: in t h e defa u l t m o de , fd bk is un co nn ec t e d , p r o v idin g t h e ra ng e +9 db/+51 db; wh en v ou t a nd f d bk a r e sh o r te d , t h e ga i n is lo w e r e d to ?11 db/+31 db; a nd w h en an ext e r n al r e sis t o r is p l aced betw e e n v ou t a nd f d bk a n y in t e r m edia t e ga i n ca n be a c h i ev e d , f o r e x a m p l e , ?1 d b / + 41 db . f i gur e 33 sho w s t h e n o min a l max i m u m ga i n vs. ex ter n a l re s i stor f o r t h i s mo d e . gpos gneg vinp comm vpos vout vneg fdbk ad603 vc1 vc2 vin vpos vout vneg 00539-030 8 7 6 5 1 2 3 4 f i g u re 30. ? 10 d b t o + 3 0 db; 9 0 m h z b a ndw i dt h
ad603 rev. g | page 13 of 20 2.15k ? 5.6pf gpos gneg vinp comm vpos vout vneg fdbk ad603 vc1 vc2 v in vpos vout vneg 00539-031 8 7 6 5 1 2 3 4 f i gure 31. 0 d b to 40 db; 30 mh z b a nd width 18pf gpos gneg vinp comm vpos vout vneg fdbk ad603 vc1 vc2 vin vpos vout vneg 00539-032 8 7 6 1 2 3 4 5 f i g u re 32. 1 0 db to 50 db ; 9 m h z t o s e t g a i n 00539-033 r ext ( ? ) 1m 10 100 1k 10k 100k de cibe ls 52 48 50 44 42 46 40 38 36 34 32 30 ?1:vdb (out) ?2:vdb (out) vdb (out) f i g u re 33. g a in v s . r ex t , sho w i n g w o r s t- c a se li mi ts a ssum i ng int e r n a l resi st o r s ha v e a m a xim u m t o l e r a nce o f 20 % o p t i on a l ly , w h e n a re s i stor i s pl a c e d f r om f d b k to c o m m , hig h er ga in s c a n b e achi e v e d . t h is fo ur t h m o de is o f limi t e d val u e b e c a us e of t h e lo w b a n d w i d t h and t h e e l e v a t e d o u t p u t o f fs ets; i t is th us n o t in cl uded in f i gur e 30, f i gur e 31, o r f i gur e 32. the ga i n o f t h is a m plif ier in t h e f i rst tw o mo des is s e t b y t h e ra ti o o f o n - c hi p la se r - tri m m e d r e si s t o r s. w h ile th e ra ti o o f th e s e r e sis t o r s is v e r y acc u ra t e , t h e a b s o l u t e val u e o f t h es e r e sis t o r s ca n va r y b y as m u c h as 20 % . th us, w h en a n ext e r n al r e sis t o r is co nnec t e d in p a ral l e l wi t h t h e no minal 6.44 k? 20% in t e r n al re s i stor , t h e ove r a l l g a i n a c c u r a c y i s s o me w h a t p o ore r . t h e w o rs t-c a s e er r o r o c c u rs a t a b o u t 2 k? (s ee f i gu r e 34). 00539-034 r ext ( ? ) 1m 10 100 1k 10k 100k de cibe ls 1.2 1.0 0.8 0.6 0.4 0.2 0 ?0.2 ?0.4 ?0.6 ?0.8 ?1.0 ?1:vdb (out) ? (?1):vdb (o ref ) vdb (out) ? vdb (o ref ) f i gur e 3 4 . w o r s t- c a se g a i n er r o r , a ssum i ng inter n a l resi st o r s ha v e a m a x i m u m t o ler a n c e of ?2 0% ( t op cu r v e) or =2 0% (b ot t o m cur v e) w h i l e t h e ga i n b a ndwi d t h p r o d uc t o f t h e f i xe d- ga in am plif ier is a b o u t 4 g h z, t h e ac t u a l b a n d w i d t h is n o t exac t l y r e la t e d t o t h e maxim u m ga in. this is b e c a us e t h er e is a slig h t en han c in g o f t h e ac r e sp o n s e ma g n i t ude on t h e maxim u m b a ndwid t h ra n g e , d u e t o hig h er o r der p o les in t h e o p en-lo o p gain f u n c t i on; t h is mi ld p e a k in g is n o t p r es e n t on t h e hig h er ga in r a n g es. f i gur e 30, f i g u re 3 1 , an d fi g u re 3 2 show h o w an opt i on a l c a p a c i tor m a y b e a dde d to ex tend t h e f r e q uenc y r e sp o n s e in h i g h ga i n m o des.
ad603 rev. g | page 14 of 20 using the ad603 in cascade t w o o r m o r e ad603s can b e c o nn ec t e d in s e r i es t o ac hieve h i gh e r ga i n . i n va ri a b l y , a c co u p li n g m u s t b e use d t o p r ev en t t h e d c of f s e t vo lt age a t t h e output of e a ch a m pl i f i e r f r om o v erlo adin g t h e fol l o w in g am pli f ier a t maxim u m ga in. t h e r e q u ir e d hig h -p ass co u p li n g n e t w o r k wi l l usua l l y b e j u st a c a p a c i tor , cho s e n to s e t t h e de s i re d c o r n e r f r e q u e nc y i n co n j u n c t io n wi t h the w e l l -def ined 100 ? in p u t r e sis t a n c e o f th e fol l o w in g am pli f ier . f o r tw o ad603 s, th e t o t a l ga in co n t r o l ra n g e b e co m e s 84 db (2 42.14 db); th e o v eral l ?3 db ba ndwid th o f cas c ade d s t a g es w i l l b e s o m e w h a t re d u c e d. d e p e n d i ng o n t h e pi n st r a ppi ng , t h e ga in and b a n d w i d t h fo r tw o c a s c ade d am plif iers ca n ran g e f r o m ?22 db t o +62 db (wi t h a b a n d wid t h o f abo u t 70 mh z) t o +22 db t o +102 db (wi t h a bandwid th o f abo u t 6 mh z). ther e a r e s e v e ral wa ys o f co nne c t i n g t h e ga i n co n t r o l in pu ts i n cas c ade d o p er a t io n. th e ch o i ce dep e n d s on w h et h e r i t is im p o r t an t t o achie v e t h e hig h est p o s s i b le in st a n t a ne o u s sig n al- to - n oi s e r a t i o ( i snr ) , or , a l te r n a t iv ely , to mi ni mi z e t h e r i ppl e i n th e ga in er r o r . th e f o l l o w in g ex a m p l es f e a t ur e th e ad603 p r o g ra mm e d fo r max i m u m b a nd w i d t h; t h e ex pl a n a t ion s a p ply t o o t h e r ga in / b a n d w id t h co m b in a t i o n s wi th a p p r o p ri a t e c h a n g e s t o t h e ar ra n g em en ts f o r set t in g t h e max i m u m ga in . sequential mode (optimal s/n r a tio) i n t h e s e q u en t i a l m o de o f o p era t io n, t h e is nr i s ma in t a i n e d a t i t s hig h e s t le ve l f o r as m u c h o f th e ga in con t r o l ra n g e as p o s s i b le . f i gur e 35 s h o w s t h e snr o v er a ga in ra n g e o f ?22 db t o +62 db , as s u min g an o u t p u t o f 1 v r m s a n d a 1 mh z ba ndwid th; f i g u r e 36, f i gur e 37, a n d f i gur e 38 s h o w t h e g e n e r a l co n n e cti o n s t o a c co m p li sh th i s . h e r e , bo th t h e posi ti v e ga in co n t r o l in p u ts ( g pos) a r e dr i v en in p a ral l e l b y a p o si ti v e -o nl y , g r o u n d -r efer e n ce d s o ur ce w i t h a ra n g e o f 0 v to +2 v , w h i l e t h e n e g a ti ve ga in con t r o l in p u ts (g neg) a r e b i as e d b y s t a b le v o l t a g es t o p r o v ide t h e n e e d e d ga in o f fs ets. thes e v o l t a g es ma y b e prov i d e d by re s i st ive d i v i d e r s op e r a t i n g f r om a c o m m on vo lt age re f e re nc e. 00539-035 v c (v) 2.2 ?0.2 0.6 0.2 1.4 1.0 1.8 s / n ratio (db) 90 80 85 70 75 65 55 60 50 f i gure 35. snr v s . control v o lt age C s e q u ential cont r o l (1 mhz b a ndw i dth) the ga in s a r e o f fs et (f igur e 39) s u ch t h a t a2 s g a in is in cr e a s e d o n ly a f t e r a1 s ga in ha s r e ache d i t s max i m u m va l u e. n o t e t h a t f o r a dif f er en tial in p u t o f C600 mv o r les s , th e ga in o f a sin g le a m p l if ier (a1 or a2) wil l be a t i t s minim u m val u e o f ?11.07 db; f o r a dif f er en tial in p u t o f +600 mv o r m o r e , the ga in wil l be a t i t s maxim u m va l u e o f 31.07 db . c o n t r o l in p u ts b e yo nd th es e limi ts w i l l n o t af fe c t t h e ga i n and can b e t o lera te d wi t h o u t da m a ge o r foldo v er in t h e r e sp o n s e . this is a n im p o r t an t a s p e c t o f th e ad603 s ga in con t r o l r e s p o n s e . (s e e the s p ecif ic a t io n s s e c t io n fo r m o re det a i l s on t h e al lo wa b l e v o l t a g e ra n g e . ) th e ga in is n o w ga i n (db) = 40 v g + g o ( 3 ) w h er e v g is t h e a p plie d co n t r o l vol t a g e and g o is det e r m i n e d b y t h e ga i n ra n g e ch os en. i n t h e ex pla n a t o r y n o t e s t h a t fol l o w , i t is assu m e d t h e ma x i m u m b a n d wi d t h conne c t io n s a r e us e d , fo r wh i c h g o is ?20 db . 31.07db ? 42.14db gpos gneg 31.07db ?42.14db gpos gneg ?40.00db ?51.07db ? 8.93db input 0db v c = 0v v g1 v g2 v o1 = 0.473v v o2 = 1.526v output ?20db a2 a1 00539-036 f i g u re 36. a d 6 0 3 g a in cont r o l inp u t cal c ul at i o ns f o r s e q u ent i al cont r o l o p er at ion v c = 0 v 31.07db ? 42.14db gpos gneg 31.07db 0db gpos gneg 0db ? 11.07db 31.07db input 0db v c = 1.0v v g1 v g2 v o1 = 0.473v v o2 = 1.526v output 20db 00539-037 f i g u re 37. a d 6 0 3 g a in cont r o l cal c ul a t ions f o r s e que n t i a l co nt rol o p er at i o n v c = 1. 0 v
ad603 rev. g | page 15 of 20 31.07db ?2.14db gpos gneg 31.07db 0db gpos gneg 0db ? 28.93db 31.07db input 0db v c = 2.0v v g1 v g2 v o1 = 0.473v v o2 = 1.526v output 60db 00539-038 f i g u re 38. a d 6 0 3 g a in cont r o l inp u t cal c ul at i o ns f o r s e q u ent i al o p er at i o n v c = 2.0 v +31.07db +10db a1 a2 +31.07db +28.96db ?11.07db ?11.07db 0.473 1.526 ?8.93db 1 1 0 ?2 0 0.5 0 1.0 20 1.50 40 2.0 60 v c (v) 62.14 ?22.14 gain (db) 1 gain offset of 1.07db, or 26.75mv. 00539-039 f i g u re 39. e x plan at ion of o f f s et ca lib r a t i o n f o r s e quent i a l co nt rol w i t h re f e re nc e t o f i g u re 3 6 , f i g u re 3 7 , and fi g u re 3 8 , note t h a t v g1 r e fers t o t h e dif f er en t i al gai n co n t r o l i n p u t t o a1, an d v g2 r e fers t o t h e dif f er en t i a l ga in con t r o l in p u t t o a 2 . w h e n v g is 0, v g1 = ?473 mv a nd th us t h e ga in o f a1 is ?8.93 db (r ecal l tha t t h e ga i n o f e a ch indivi d u a l am plif ier in t h e max i m u m b a ndwi d t h m o d e is C10 db fo r v g = ?500 mv a nd 10 db f o r v g = 0 v); m e an w h ile , v g2 = ?1.908 v s o th e ga in of a2 is p i nn e d a t ?11.07 db . th e o v eral l ga in is th us C20 db . s e e f i gur e 36. wh e n v g = +1.00 v , v g1 = 1.00 v ? 0.473 v = +0.526 v , which s e ts t h e ga in o f a1 t o a t n e a r l y i t s maxim u m val u e o f 31.07 db , wh ile v g2 = 1.00 v ? 1.526 v = 0.526 v , which s e ts a2 s ga in a t n e a r l y i t s minim u m va l u e o f ?11.07 db . clos e a n al ysis s h o w s tha t t h e deg r ee to whic h n e i t h e r ad603 is co m p l e t e l y p u sh ed t o i t s max i m u m no r mini m u m gain ex ac t l y can c els in t h e o v era l l ga in, w h ich is no w +20 db . s e e f i gur e 37. wh e n v g = 2.0 v , th e ga in o f a1 is p i nn e d a t 3 1 .07 db an d tha t o f a2 is n e a r i t s maxim u m val u e o f 28.93 db , r e s u l t in g in an ove r a l l g a i n of 6 0 d b ( s e e f i g u re 3 8 ) . t h i s mo d e of op e r a t i o n i s f u r t h e r cla r if ie d b y f i gur e 40, w h ich is a plo t o f t h e s e p a r a t e ga in s o f a1 and a2 an d t h e o v eral l ga in vs. t h e c o n t r o l v o l t a g e . f i gur e 41 is a p l o t o f th e s n r o f th e c a s c ade d am p l if iers vs. th e c o n t ro l vo lt age. f i g u re 4 2 i s a p l ot of t h e g a i n e r ror of t h e cas c ade d s t a g es vs. t h e con t r o l v o l t a g es. 00539-040 v c 2.0 ?0.2 0.2 0.6 1.0 1.4 1.8 ov e rall gain (db) 70 50 60 40 30 20 10 0 ?10 ?20 ?30 combined a1 a2 f i g u re 40. pl ot of s e par a t e a n d o v e r a l l g a ins in s e que n t i al cont r o l 00539-041 v c 2.0 ?0.2 0.2 0.6 1.0 1.4 1.8 s / n ratio (db) 90 80 70 60 50 40 30 20 10 f i gure 41. snr fo r casc ad ed stag ess equ e nti a l cont rol
ad603 rev. g | page 16 of 20 00539-042 v c 2.2 ?0.2 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 gain e rror (db) 2.0 1.5 1.0 0.5 0 ?0.5 ?1.0 ?1.5 ?2.0 f i gure 42. g a in e r ror f o r cas c ad ed st agesC s equ e nti a l control parallel mode ( s implest gain contr o l interface) i n t h is m o d e , t h e ga in con t r o l o f v o l t a g e is a p pli e d t o b o t h in p u ts in p a ral l e lthe gpos p i n s o f bo t h a1 and a2 a r e co nne c t e d t o t h e co n t r o l v o l t a g e a nd t h e g n ew i n p u ts a r e g r o u n d e d . t h e ga in s c a l in g is t h e n do ub le d t o 80 db/ v , r e q u ir in g o n l y a 1.00 v c h a n g e fo r a n 80 db c h an g e o f ga in: ga i n = (db) = 80 v g + g o ( 4 ) wh e r e , a s be f o r e , g o dep e n d s on t h e ra n g e s e le c t e d ; fo r exa m ple , in t h e max i m u m b a nd w i d t h mo de, g o is +20 db . al t e r n a t i v e l y , t h e g n eg p i n s ma y b e co nn e c te d t o an o f fs et v o l t a g e o f 0.500 v , in w h ich cas e g o is ?20 db . the am p l i t ude o f th e ga in r i p p le in this cas e is als o do u b led , as s h own i n f i gur e 43, w h i l e t h e ins t a n t a n e o u s sig n al-t o- n o is e r a t i o a t t h e output of a 2 now d e c r e a s e s l i n e ar ly a s t h e g a i n in cr eas e s, as sh o w n in f i gur e 44. 00539-043 v c 2.2 ?0.2 0 0.2 0.4 0.6 1.0 1.2 0.8 1.6 1.8 2.0 1.4 gain e rror (db) 2.0 1.5 1.0 0.5 0 ?0.5 ?1.0 ?1.5 ?2.0 f i gure 43. g a in e r ror f o r cas c ad ed st ages p a r a l lel co n t r o l 00539-044 v c 1.2 ?0.2 0 0.2 0.4 0.6 0.8 1.0 is/n ratio ( d b) 90 80 85 75 65 70 55 60 50 f i g u re 44. isnr f o r cas c ad ed st ag es p ar al le l cont rol low gain r i pple mode (minimum gain error ) a s c a n be s e en in f i gur e 42 an d f i gur e 43, th e er r o r in th e ga in is p e r i o d ic, t h a t is, i t sh o w s a sm a l l r i p p le . (n o t e t h a t t h er e is a l s o a va r i a t ion in t h e o u t p u t o f fs et v o l t a g e , w h i c h is d u e t o t h e ga in i n t e r p ol a t i o n, b u t t h i s is no t exac t in am pl i t ude . ) by o f fs et t i n g t h e gain s o f a1 and a 2 b y half t h e p e r i o d o f t h e r i p p l e , t h a t i s , b y 3 d b , t h e r e s i d u a l g a i n e r r o r s o f t h e t w o a m plif iers can b e made t o can c el. f i gur e 45 sh o w s m u ch lo w e r ga in r i p p le w h e n co nf igur e d i n t h is ma n n er . f i gur e 46 plo t s t h e is nr as a f u n c t i o n o f ga in; i t is v e r y simila r t o tha t in t h e pa r a ll e l m o de . 00539-045 v c 1.1 ?0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 gain e rror (db) 3.0 2.5 2.0 1.5 1.0 0.5 ?0.5 ?1.0 0 ?1.5 ?2.0 ?2.5 ?3.0 f i gure 45. g a in e r ror f o r cas c ad ed st agesl o w r i p p le mode
ad603 rev. g | page 17 of 20 00539-046 v c 1.2 ?0.2 0 0.2 0.4 0.6 0.8 1.0 is/n ratio ( d b) 90 85 80 75 70 65 60 55 50 f i gure 46. isnr v s . control v o ltage l o w r i p p l e m o de
ad603 rev. g | page 18 of 20 appli c ations a low nois e agc ampl ifier f i gur e 47 s h o w s th e eas e wi t h w h ic h t h e ad603 ca n be co nne c t e d as an a g c am plif ier . the cir c ui t i l l u st ra t e s man y o f th e p o in ts p r e v io us l y dis c us s e d: i t us es f e w p a r t s, has lin e a r -in- db ga in, o p era t es f r o m a sin g le su p p l y , us es tw o cas c ade d a m plif iers in s e q u en t i a l ga i n mo de fo r max i m u m s/ n ra t i o , and a n ext e r n al r e sis t o r p r og ra m s eac h a m p l if ier s ga in. i t als o us es a s i mp l e t e mp e r at u r e - c o mp e n s a t e d d e t e c t o r . the cir c ui t op era t es f r o m a si n g le 10 v s u p p l y . resis t o r s r1, r2 , r3, a nd r4 b i as th e comm o n p i n s o f a1 an d a2 a t 5 v . this p i n i s a l o w i m peda n c e po in t a n d m u s t ha v e a l o w i m peda n c e pa th t o g r o u n d , p r o v ided h e r e b y the 100 f ta n t al u m c a p a ci t o rs and th e 0.1 f ceramic ca p a c i t o rs. the ca s c a d e d am plif iers o p era t e in s e q u en t i a l g a in. h e r e , t h e o f fs et v o l t a g e b e tw een t h e p i n 2 (gneg) o f a1 a nd a2 is 1.05 v (42.14 db 25 mv/db), p r o v ided b y a v o l t a g e divider c o n s i s t i n g o f r e s i s t o r s r 5 , r 6 , a n d r 7 . u s i n g s t a n d a r d v a l u e s , t h e o f fs et is n o t exa c t, b u t i t is n o t c r i t ica l fo r t h is a p plica t io n. the ga in o f bo t h a1 and a2 is p r og ra mm e d b y r e sis t o r s r13 a nd r14, r e s p ec ti v e l y , t o b e abou t 42 db; th us t h e maxim u m ga in o f t h e cir c ui t is t w ic e t h a t , o r 84 db . the ga in con t r o l ra n g e ca n be s h if ted u p b y as m u c h as 20 db b y a p p r op r i a t e c h o i ces of r13 a nd r14. the cir c ui t op era t es as fol l o w s. a1 an d a2 a r e c a s c ade d . c a p a ci t o r c1 and the 100 ? o f r e sis t a n ce a t t h e in p u t o f a1 f o r m a time co ns ta n t o f 10 s. c2 b l o c ks t h e smal l dc o f fs et v o l t a g e a t t h e ou t p ut o f a1 (w hich mig h t o t h e r w is e s a t u ra te a 2 a t i t s maxi m u m ga in) an d in t r o d uces a h i g h -p a s s co r n er a t a b o u t 16 khz, e l imin a t in g lo w f r e q uen c y n o is e . a ha lf-w a v e de te c t o r is us e d , b a s e d on q1 an d r8. the c u r r en t i n to c a p a c i tor c av is j u s t t h e dif f er en ce b e tw e e n t h e col l e c t o r c u r r en t o f q2 ( b ias e d t o be 300 a a t 300 k, 27c) an d t h e co ll ect o r curr e n t o f q1, wh i c h in cr ea se s wi th t h e a m p l i t ude o f th e o u t p u t si gn a l . the a u t o ma tic ga in con t r o l v o l t a g e , v ag c , i s th e ti m e i n t e gral o f this er r o r cu r r e n t. i n o r d e r f o r v ag c (a n d th us t h e ga in) t o r e ma in i n s e n s i t iv e t o sh o r t-t e r m am pli t ude f l uc t u a t io n s in t h e o u t p ut sig n al , t h e r e c t if ie d c u r r en t i n q1 m u st, o n a v er a g e , exac t l y b a la n c e t h e c u r r en t i n q 2 . i f t h e o u t p u t o f a2 is t o o smal l t o do this, v ag c wi l l i n cr e a s e , ca usin g t h e g a in t o i n cr e a s e , un til q1 co n d uct s s u f f i ci e n tl y . c o n s ider t h e cas e w h er e r8 is zer o a n d t h e o u t p u t v o l t a g e v ou t is a s q u a r e wa ve a t , s a y , 455 kh z, whic h is w e l l abo v e the co r n er f r e q uen c y o f the co n t r o l lo o p . c2 0. 1 f r1 3 2. 49 k ? 10 v 10 v 5. 5 v 6. 5v + + r2 2.49 k ? c3 2 10 0 f c4 0. 1 f r1 2. 4 9 k ? rt 1 10 0 ? j1 r6 1. 05 k ? r5 5. 4 9 k ? r7 3. 48 k ? 10 v agc line c av 0. 1 f this capacitor sets agc time constant v agc r9 1.54 k ? r8 806 ? q1 2n3 904 q2 2n3 906 r1 0 1. 24 k ? r1 1 3. 83 k ? 5v r1 2 4. 99 k ? c1 1 0. 1 f c9 0. 1 f 10 v j2 c10 0. 1 f 10v c7 0.1 f c1 0.1 f a1 ad603 1 2 3 7 4 8 6 5 r1 4 2. 49 k ? r4 2. 49 k ? c5 2 10 0 f c6 0. 1 f r3 2.49 k ? 10v c8 0.1 f a2 ad603 1 2 3 7 4 8 6 5 1 v o f f set f o r s e q ue nt i a l g ai n 1 r t pr o v i d es a 5 0 ? i n pu t i mped a n c e . 2 c 3 an d c 5 a re t a nt al um . 00539-047 f i g u re 47. a l o w n o is e a g c a m p lif i e r
ad603 rev. g | page 19 of 20 during the time v out is negative with respect to the base voltage of q1, q1 conducts; when v out is positive, it is cut off. since the average collector current of q1 is forced to be 300 a, and the square wave has a duty cycle of 1:1, q1s collector current when conducting must be 600 a. with r8 omitted, the peak amplitude of v out is forced to be just the v be of q1 at 600 a, typically about 700 mv, or 2 v be peak-to-peak. this voltage, the amplitude at which the output stabilizes, has a strong negative temperature coefficient (tc), typically ?1.7 mv/c. although this may not be troublesome in some applications, the correct value of r8 will render the output stable with temperature. to understand this, note that the current in q2 is made to be proportional to absolute temperature (ptat). for the moment, continue to assume that the signal is a square wave. when q1 is conducting, v out is now the sum of v be and a voltage that is ptat and that can be chosen to have an equal but opposite tc to that of the v be . this is actually nothing more than an application of the band gap voltage reference principle. when r8 is chosen such that the sum of the voltage across it and the v be of q1 is close to the band gap voltage of about 1.2 v, v out will be stable over a wide range of temperatures, provided, of course, that q1 and q2 share the same thermal environment. since the average emitter current is 600 a during each half cycle of the square wave, a resistor of 833 ? would add a ptat voltage of 500 mv at 300 k, increasing by 1.66 mv/c. in practice, the optimum value will depend on the type of transistor used and, to a lesser extent, on the waveform for which the temperature stability is to be optimized; for the inexpensive 2n3904/2n3906 pair and sine wave signals, the recommended value is 806 ?. this resistor also serves to lower the peak current in q1 when more typical signals (usually sinusoidal) are involved, and the 1.8 khz lp filter it forms with c av helps to minimize distortion due to ripple in v agc . note that the output amplitude under sine wave conditions will be higher than for a square wave, since the average value of the current for an ideal rectifier would be 0.637 times as large, causing the output amplitude to be 1.88 (= 1.2/0.637) v, or 1.33 v rms. in practice, the somewhat nonideal rectifier results in the sine wave output being regulated to about 1.4 v rms, or 3.6 v p-p. the bandwidth of the circuit exceeds 40 mhz. at 10.7 mhz, the agc threshold is 100 v (?67 dbm) and its maximum gain is 83 db (20 log 1.4 v/100 v). the circuit holds its output at 1.4 v rms for inputs as low as ?67 dbm to +15 dbm (82 db), where the input signal exceeds the ad603s maximum input rating. for a 30 dbm input at 10.7 mhz, the second harmonic is 34 db down from the fundamental and the third harmonic is 35 db down. caution careful component selection, circuit layout, power supply decoupling, and shielding are n eeded to minimize the ad603s susceptibility to interference from signals such as those from radio and tv stations. in bench evaluation, it is recommended to place all of the components into a shielded box and using feedthrough decoupling networks for the supply voltage. circuit layout and construction are also critical, since stray capacitances and lead inductances can form resonant circuits and are a potential source of circuit peaking, oscillation, or both.
ad603 rev. g | page 20 of 20 outline dimensions 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. 1 4 5 8 0.310 (7.87) 0.220 (5.59) 0.005 (0.13) min 0.055 (1.40) max 0.100 (2.54) bsc 15 0 0.320 (8.13) 0.290 (7.37) 0.015 (0.38) 0.008 (0.20) seating plane 0.200 (5.08) max 0.405 (10.29) max 0.150 (3.81) min 0.200 (5.08) 0.125 (3.18) 0.023 (0.58) 0.014 (0.36) 0.070 (1.78) 0.030 (0.76) 0.060 (1.52) 0.015 (0.38) pin 1 f i gure 48. 8-l e ad cer a m i c d u a l in-l in e p a ckage [ce r di p ] (q - 8 ) di me nsio ns sho w n i n i n che s a n d ( m il lim e t e r s) 0.25 (0.0098) 0.17 (0.0067) 1.27 (0.0500) 0.40 (0.0157) 0.50 (0.0196) 0.25 (0.0099) 45 8 0 1.75 (0.0688) 1.35 (0.0532) seating plane 0.25 (0.0098) 0.10 (0.0040) 4 1 85 5. 00 ( 0 . 1 968) 4. 80 ( 0 . 1 890) 4.00 (0.1574) 3.80 (0.1497) 1.27 (0.0500) bsc 6.20 (0.2440) 5.80 (0.2284) 0.51 (0.0201) 0.31 (0.0122) coplanarity 0.10 controlling dimensions are in millimeters; inch dimensions (in parentheses) are rounded-off millimeter equivalents for reference only and are not appropriate for use in design compliant to jedec standards ms-012-aa f i g u re 49. 8-l e ad s t anda r d s m a l l o u t l ine p a ckag e [soic- n] nar r o w b o dy (r -8) di me nsio ns sho w n i n mi ll im e t e r s a n d (i nc he s) ordering guide part number temperature r a nge package descri ption package option ad603ar ?40c to +85c 8-lead soic r-8 ad603ar-re el ?40c to +85c 8-lead soic, 13 " reel r-8 ad603ar-re el7 ?40c to +85c 8-lead soic, 7" reel r-8 ad603arz 1 ?40c to +85c 8-lead soic r-8 AD603ARZ-REEL 1 ?40c to +85c 8-lead soic, 13 " reel r-8 AD603ARZ-REEL 7 1 ?40c to +85c 8-lead soic, 7" reel r-8 ad603aq ?40c to +85c 8-lead cerdip q-8 ad603sq/883b 2 ?55c to +125c 8-lead cerdip q-8 a d 6 0 3 - e b e v a l u a t i o n boar d a d 6 0 3 a c h i p s d i e 1 z = pb-free part. 2 ref e r to ad603 mil i tary data sh eet. al s o available as 5962-9457203mpa. ? 2005 anal og d e v i ces, inc. all ri ghts reserve d . tr ade m a r ks and registered trade m ar ks are the pr op erty of their respective owne rs. c00539C0 C 3/05(g)

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