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lt6604-2.5 1 660425f typical application features applications description dual very low noise, differential ampli er and 2.5mhz lowpass filter the lt ? 6604-2.5 consists of two matched, fully differential ampli? ers, each with a 4th order, 2.5mhz lowpass ? lter. the ? xed frequency lowpass ? lter approximates a chebyshev response. by integrating a ? lter and a differential ampli- ? er, distortion and noise are made exceptionally low. at unity gain, the measured in-band signal-to-noise ratio is an impressive 86db. at higher gains, the input referred noise decreases, allowing the part to process smaller input differential signals without signi? cantly degrading the signal-to-noise ratio. gain and phase are well matched between the two chan- nels. gain for each channel is independently programmed using two external resistors. the lt6604-2.5 enables level shifting by providing an adjustable output common mode voltage, making it ideal for directly interfacing to adcs. the lt6604-2.5 is fully speci? ed for 3v operation. the differential design enables outstanding performance up to a 4v p-p signal level for a single 3v supply. see the back page of this datasheet for a complete list of related single and dual differential ampli? ers with integrated 2.5mhz to 20mhz lowpass ? lters. l , lt, ltc and ltm are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners.v n dual differential ampli? er with 2.5mhz lowpass filters 4th order filters approximates chebyshev response guaranteed phase and gain matching resistor-programmable differential gain n >86db signal-to-noise (3v supply, 1v rms output) n low distortion (1mhz, 1v rms output, 800 load) hd2: 92dbc hd3: 88dbc n speci? ed for operation with 3v, 5v and 5v supplies n fully differential inputs and outputs n adjustable output common mode voltage n small 4mm 7mm 0.75mm qfn package n dual differential adc driver and filter n single-ended to differential converter n matched, dual, differential gain or filter stage n common mode translation of differential signals n high speed adc antialiasing and dac smoothing in wireless infrastructure or networking applications n high speed test and measurement equipment n medical imaging channel to channel gain matching gain match (db) number of units 18 16 14 12 10 8 6 4 2 660425 ta01b 0 50 typical units t a = 25c gain = 1 f in = 2.5mhz C0.25 0.05 C0.05 0.15 C0.15 0.25 0 1580 50 50 1580 3v 3v + C + C 660425 ta01 + C dout dual adc ain lt6604-2.5 ltc22xx + C 1580 1580 3v v + a Couta +outa Coutb +outb v + b v + b +ina +inb v mida v midb v ocma v ocmb Cina Cinb v C + C + C + C dout ain + C 50 50 18pf 0.01f 0.01f 18pf
lt6604-2.5 2 660425f pin configuration absolute maximum ratings total supply voltage .................................................11v operating temperature range (note 6).... ?40c to 85c speci? ed temperature range (note 7) .... ?40c to 85c junction temperature ........................................... 150c storage temperature range ................... ?65c to 150c input voltage +in, ?in, v ocm , v mid (note 8) .............................. v s input current +in, ?in, v ocm , v mid (note 8) ........................10ma (note 1) 31 v ? 32 v ? 33 nc 34 v mida v + b 17 nc 16 nc 15 v ocmb 14 35 30 nc 29 ?outa 28 nc 27 +outa 26 nc 25 v + a 24 v ? 23 nc 22 nc 21 ?outb 20 nc 19 +outb 18 nc nc 1 +ina 2 nc 3 ?ina 4 nc 5 v ocma 6 v ? 7 v midb 8 nc 9 +inb 10 nc 11 ?inb 12 nc 13 top view uff package 34-lead (4mm s 7mm) plastic qfn t jmax = 150c, ja = 43c/w, jc = 4c/w exposed pad (pin 35) is v ? , must be soldered to pcb order information lead free finish tape and reel part marking* package description specified temperature range lt6604cuff-2.5#pbf lt6604cuff-2.5#trpbf 60425 34-lead (4mm 7mm) plastic qfn 0c to 70c lt6604iuff-2.5#pbf lt6604iuff-2.5#trpbf 60425 34-lead (4mm 7mm) plastic qfn ?40c to 85c consult ltc marketing for parts speci? ed with wider operating temperature ranges. *the temperature grade is identi? ed by a label on the shipping container. consult ltc marketing for information on non-standard lead based ? nish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel speci? cations, go to: http://www.linear.com/tapeandreel/ electrical characteristics the l denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. unless otherwise speci? ed v s = 5v (v + = 5v, v ? = 0v), r in = 1580, and r load = 1k. parameter conditions min typ max units filter gain either channel, v s = 3v v in = 2v p-p , f in = dc to 260khz v in = 2v p-p , f in = 700khz (gain relative to 260khz) v in = 2v p-p , f in = 1.9mhz (gain relative to 260khz) v in = 2v p-p , f in = 2.2mhz (gain relative to 260khz) v in = 2v p-p , f in = 2.5mhz (gain relative to 260khz) v in = 2v p-p , f in = 7.5mhz (gain relative to 260khz) v in = 2v p-p , f in = 12.5mhz (gain relative to 260khz) l l l l l l ?0.5 ?0.15 ?0.2 ?0.6 ?2.1 0.1 0 0.2 0.1 ?0.9 ?34 ?51 0.4 0.1 0.6 0.5 0 ? 31 db db db db db db db
lt6604-2.5 3 660425f electrical characteristics the l denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. unless otherwise speci? ed v s = 5v (v + = 5v, v C = 0v), r in = 1580, and r load = 1k. parameter conditions min typ max units matching of filter gain, v s = 3v v in = 2v p-p , f in = dc to 260khz v in = 2v p-p , f in = 700khz (gain relative to 260khz) v in = 2v p-p , f in = 1.9mhz (gain relative to 260khz) v in = 2v p-p , f in = 2.2mhz (gain relative to 260khz) v in = 2v p-p , f in = 2.5mhz (gain relative to 260khz) v in = 2v p-p , f in = 7.5mhz (gain relative to 260khz) v in = 2v p-p , f in = 12.5mhz (gain relative to 260khz) l l l l l l 0.04 0.005 0.02 0.03 0.05 0.15 0.05 0.4 0.1 0.3 0.4 0.6 1.1 2.8 db db db db db db db matching of filter phase, v s = 3v v in = 2v p-p , f in = 700khz v in = 2v p-p , f in = 1.9mhz v in = 2v p-p , f in = 2.2mhz l l l 0.2 0.6 0.8 1.5 3.5 4.5 deg deg deg filter gain either channel, v s = 5v v in = 2v p-p , f in = dc to 260khz v in = 2v p-p , f in = 700khz (gain relative to 260khz) v in = 2v p-p , f in = 1.9mhz (gain relative to 260khz) v in = 2v p-p , f in = 2.2mhz (gain relative to 260khz) v in = 2v p-p , f in = 2.5mhz (gain relative to 260khz) v in = 2v p-p , f in = 7.5mhz (gain relative to 260khz) v in = 2v p-p , f in = 12.5mhz (gain relative to 260khz) l l l l l l C0.5 C0.15 C0.2 C0.6 C2.1 C0.1 0 0.2 0.1 C0.9 C34 C51 0.4 0.1 0.6 0.5 0 C 31 db db db db db db db matching of filter gain, v s = 5v v in = 2v p-p , f in = dc to 260khz v in = 2v p-p , f in = 700khz (gain relative to 260khz) v in = 2v p-p , f in = 1.9mhz (gain relative to 260khz) v in = 2v p-p , f in = 2.2mhz (gain relative to 260khz) v in = 2v p-p , f in = 2.5mhz (gain relative to 260khz) v in = 2v p-p , f in = 7.5mhz (gain relative to 260khz) v in = 2v p-p , f in = 12.5mhz (gain relative to 260khz) l l l l l l 0.04 0.005 0.02 0.03 0.05 0.15 0.05 0.4 0.1 0.3 0.4 0.6 1.1 2.8 db db db db db db db matching of filter phase, v s = 5v v in = 2v p-p , f in = 700khz v in = 2v p-p , f in = 1.9mhz v in = 2v p-p , f in = 2.2mhz l l l 0.2 0.6 0.8 1.5 3.5 4.5 deg deg deg filter gain either channel, v s = 5v v in = 2v p-p , f in = dc to 260khz C0.6 C0.1 0.4 db filter gain, r in = 402 v out = 2v p-p , f in = dc to 260khz v s = 3v v s = 5v v s = 5v 11.3 11.3 11.2 11.8 11.8 11.7 12.3 12.3 12.2 db db db filter gain temperature coef? cient (note 2) f in = 260khz, v in = 2v p-p 780 ppm/c noise noise bw = 10khz to 2.5mhz, r in = 1580 51 v rms distortion (note 4) v in = 1v rms , f in = 1mhz, r l = 800 2nd harmonic 3rd harmonic 92 88 dbc dbc channel separation (note 9) v in = 2v p-p , f in = 1mhz C119 db differential output swing measured between out+ and outC, v ocm shorted to v mid v s = 5v v s = 3v l l 8.8 5.1 9.3 5.5 v p-p_diff v p-p_diff
lt6604-2.5 4 660425f electrical characteristics the l denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. unless otherwise speci? ed v s = 5v (v + = 5v, v C = 0v), r in = 1580, and r load = 1k. parameter conditions min typ max units input bias current average of in+ and inC l C35 C15 a input referred differential offset r in = 1580, differential gain = 1v/v v s = 3v v s = 5v v s = 5v l l l 5 5 5 25 30 35 mv mv mv r in = 402, differential gain = 4v/v v s = 3v v s = 5v v s = 5v l l l 3 3 3 13 16 20 mv mv mv differential offset drift 10 v/c input common mode voltage (note 3) differential input = 500mv p-p , r in 402 v s = 3v v s = 5v v s = 5v l l l 0 0 C2.5 1.5 3 1 v v v output common mode voltage (note 5) differential output = 2v p-p , v mid at mid supply v s = 3v v s = 5v v s = 5v l l l 1 1.5 C 1 1.5 3 2 v v v output common mode offset (with respect to v ocm ) v s = 3v v s = 5v v s = 5v l l l C25 C30 C55 10 5 C10 45 45 35 mv mv mv common mode rejection ratio 63 db voltage at v mid v s = 5v v s = 3v l 2.45 2.51 1.5 2.56 v v v mid input resistance l 4.3 5.7 7.7 k v ocm bias current v ocm = v mid = v s /2 v s = 5v v s = 3v l l C15 C10 C3 C3 a a power supply current (per channel) v s = 3v, v s = 5v v s = 3v, v s = 5v v s = 5v l l 26 28 30 33 36 ma ma ma note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. note 2: this is the temperature coef? cient of the internal feedback resistors assuming a temperature independent external resistor (r in ). note 3: the input common mode voltage is the average of the voltages applied to the external resistors (r in ). note 4: distortion is measured differentially using a differential stimulus. the input common mode voltage, the voltage at v ocm , and the voltage at v mid are equal to one half of the total power supply voltage. note 5: output common mode voltage is the average of the +out and Cout voltages. the output common mode voltage is equal to v ocm . note 6: the lt6604c-2.5 is guaranteed functional over the operating temperature range C40c to 85c. note 7: the lt6604c-2.5 is guaranteed to meet 0c to 70c speci? cations and is designed, characterized and expected to meet the extended temperature limits, but is not tested at C40c to 85c. the lt6604i-2.5 is guaranteed to meet speci? ed performance from C40c to 85c. note 8: input pins (+in, Cin, v ocm and v mid ) are protected by steering diodes to either supply. if the inputs should exceed either supply voltage, the input current should be limited to less than 10ma. in addition, the inputs +in, Cin are protected by a pair of back-to-back diodes. if the differential input voltage exceeds 1.4v, the input current should be limited to less than 10ma note 9: channel separation (the inverse of crosstalk) is measured by driving a signal into one input while terminating the other input. channel separation is the ratio of the resulting output signal at the driven channel to the output at the channel that is not driven.
lt6604-2.5 5 660425f frequency (hz) 1k 40 cmrr (db) 80 90 110 100 100k 10k 1m 10m 100m 660425 g05 70 60 50 v in = 1v p-p v s = 5v r in = 1580 gain = 1 typical performance characteristics frequency response passband gain and group delay passband gain and group delay output impedance vs frequency (out+ or outC) common mode rejection ratio power supply rejection ratio distortion vs frequency distortion vs frequency distortion vs frequency frequency (hz) 100k C36 gain (db) C24 C12 0 12 1m 10m 50m 660425 g01 C48 C60 C84 C96 C72 v s = p 2.5v r in = 1580 gain = 1 frequency (mhz) 0.5 gain (db) group delay (ns) C3 C1 1 2.5 660425 g02 C5 C7 C4 C2 0 C6 C8 C9 240 280 320 200 160 220 260 300 180 140 120 1.0 1.5 2.0 0.75 2.75 1.25 1.75 2.25 3.0 v s = 5v r in = 1580 gain = 1 t a = 25 o c gain group delay gain (db) 8 10 12 660425 g03 6 4 7 9 11 5 3 2 frequency (mhz) 0.5 group delay (ns) 2.5 240 280 320 200 160 220 260 300 180 140 120 1.0 1.5 2.0 0.75 2.75 1.25 1.75 2.25 3.0 v s = 5v r in = 402 gain = 4 t a = 25 o c gain group delay frequency (hz) 1 output impedance () 10 100k 10m 100m 660425 g04 0.1 1m 100 frequency (hz) 1k psrr (db) 100k 10k 1m 10m 100m 660425 g06 40 50 60 70 80 30 20 10 0 90 v + to differential out v s = 3v frequency (mhz) 0.1 C110 distortion (dbc) C70 C60 110 660425 g07 C80 C90 C100 differential input, 2nd harmonic differential input, 3rd harmonic single-ended input, 2nd harmonic single-ended input, 3rd harmonic v in = 2v p-p v s = 3v r l = 800 at each output gain = 1 frequency (mhz) 0.1 C110 distortion (dbc) C70 C60 110 660425 g08 C80 C90 C100 differential input, 2nd harmonic differential input, 3rd harmonic single-ended input, 2nd harmonic single-ended input, 3rd harmonic v in = 2v p-p v s = 5v r l = 800 at each output gain = 1 frequency (mhz) 0.1 C110 distortion (dbc) C70 C60 110 660425 g09 C80 C90 C100 differential input, 2nd harmonic differential input, 3rd harmonic single-ended input, 2nd harmonic single-ended input, 3rd harmonic v in = 2v p-p v s = p 5v r l = 800 at each output gain = 1
lt6604-2.5 6 660425f input level (v p-p ) 0 C110 C100 distortion (dbc) C90 C80 C70 C60 C50 C40 1234 660425 g11 9 8 7 6 5 v s = 5v f = 1mhz r l = 800 at each output gain = 1 2nd harmonic, differential input 3rd harmonic, differential input 2nd harmonic, single-ended input 3rd harmonic, single-ended input typical performance characteristics distortion vs signal level distortion vs signal level distortion vs signal level distortion vs input common mode level distortion vs input common mode level distortion vs output common mode level single channel supply current vs total supply voltage input level (v p-p ) 0 C110 C100 distortion (dbc) C90 C80 C70 C60 C50 C40 1234 660425 g10 6 5 v s = 3v f = 1mhz r l = 800 at each output gain = 1 2nd harmonic, differential input 3rd harmonic, differential input 2nd harmonic, single-ended input 3rd harmonic, single-ended input input level (v p-p ) 0 C110 C100 distortion (dbc) C90 C80 C70 C60 C50 C40 1234 660425 g12 9 8 7 6 5 2nd harmonic, differential input 3rd harmonic, differential input 2nd harmonic, single-ended input 3rd harmonic, single-ended input v s = p 5v f = 1mhz r l = 800 at each output gain = 1 input common mode voltage relative to v mid (v) C3 C110 C100 distortion component (dbc) C90 C80 C70 C60 C50 C40 C2 C1 0 1 2 660425 g13 3 2v p-p 1mhz input r in = 1580 gain = 1 2nd harmonic, v s = 3v 3rd harmonic, v s = 3v 2nd harmonic, v s = 5v 3rd harmonic, v s = 5v input common mode voltage relative to v mid (v) C3 C110 C100 distortion component (dbc) C90 C80 C70 C60 C50 C40 C2 C1 0 1 2 660425 g14 3 2v p-p 1mhz input, r in = 402, gain = 4 2nd harmonic, v s = 3v 3rd harmonic, v s = 3v 2nd harmonic, v s = 5v 3rd harmonic, v s = 5v voltage v ocm to v mid (v) distortion component (dbc) C70 C60 C50 0.5 1.0 1.5 660425 g15 C80 C90 C1.5 C1.0 C0.5 0 2.5 2.0 C100 C110 C40 2v p-p 1mhz input, r in = 1580, gain = 1 2nd harmonic, v s = 3v 3rd harmonic, v s = 3v 2nd harmonic, v s = 5v 3rd harmonic, v s = 5v 2nd harmonic, v s = p 5v 3rd harmonic, v s = p 5v total supply voltage (v) 16 supply current (ma) 24 22 20 28 32 18 26 30 2468 660425 g16 10 3579 t a = C40c t a = 25c t a = 85c
lt6604-2.5 7 660425f frequency (hz) channel separation (db) 100k 10m 100m 660425 g18 C130 C110 1m C10 C30 C50 C70 C90 v in = 2v p-p v s = 5v r l = 800 at each output gain = 1 pin functions +ina, Cina (pins 2, 4): channel a input pins. signals can be applied to either or both input pins through identical external resistors, r in . the dc gain from the differential inputs to the differential outputs is 1580/r in . v ocma (pin 6): dc common mode reference voltage for the 2nd filter stage in channel a. its value programs the common mode voltage of the differential output of the ? lter. pin 6 is a high impedance input, which can be driven from an external voltage reference, or pin 6 can be tied to pin 34 on the pc board. pin 6 should be bypassed with a 0.01f ceramic capacitor unless it is connected to a ground plane. v C (pins 7, 24, 31, 32, 35): negative power supply pin (can be ground). v midb (pin 8): the v midb pin is internally biased at mid- supply, see block diagram. for single supply operation the v midb pin should be bypassed with a quality 0.01f ceramic capacitor to v C . for dual supply operation, pin 8 can be bypassed or connected to a high quality dc ground. a ground plane should be used. a poor ground will increase noise and distortion. pin 8 sets the output common mode voltage of the 1st stage of the ? lter in channel b. it has a 5.5k impedance, and it can be overridden with an external low impedance voltage source. +inb, Cinb (pins 10, 12): channel b input pins. signals can be applied to either or both input pins through identi- cal external resistors, r in . the dc gain from differential inputs to the differential outputs is 1580/r in . v ocmb (pin 14): dc common mode reference voltage for the 2nd filter stage in channel b. its value programs the common mode voltage of the differential output of the ? lter. pin 14 is a high impedance input, which can be driven from an external voltage reference, or pin 14 can be tied to pin 8 on the pc board. pin 14 should be bypassed with a 0.01f ceramic capacitor unless it is connected to a ground plane. v + a, v + b (pins 25, 17): positive power supply pins for channels a and b. for a single 3.3v or 5v supply (v C grounded) a quality 0.1f ceramic bypass capacitor is required from each positive supply pin (v + a, v + b) to the negative supply pin (v C ). the bypass should be as close as possible to the ic. for dual supply applications, bypass the negative supply pins to ground and each of the positive supply pins (v + a, v + b) to ground with a quality 0.1f ceramic capacitor. +outb, Coutb (pins 19, 21): output pins. pins 19 and 21 are the ? lter differential outputs for channel b. with a typical short-circuit current limit greater than 40ma, each pin can drive a 100 and/or 50pf load to ac ground. channel separation vs frequency (note 9) transient response gain = 1 v out + 50mv/div differential input 200mv/div 500ns/div 660425 g17 typical performance characteristics
lt6604-2.5 8 660425f block diagram +outa, C outa (pins 27, 29): output pins. pins 27 and 29 are the ? lter differential outputs for channel a. with a typical short-circuit current greater than 40ma, each pin can drive a 100 and/or 50pf load to ac ground. v mida (pin 34): the v mida pin is internally biased at mid- supply, see block diagram. for single supply operation the v mida pin should be bypassed with a quality 0.01f ceramic capacitor to v C . for dual supply operation, pin 34 can be bypassed or connected to a high quality dc ground. a ground plane should be used. a poor ground will increase noise and distortion. pin 34 sets the output common mode voltage of the 1st stage of the ? lter in chan- nel a. it has a 5.5k impedance, and it can be overridden with an external low impedance voltage source. exposed pad (pin 35): v C . the exposed pad must be soldered to the pcb. if v C is separate from ground, tie the exposed pad to v C . pin functions C + C + v ocm C C + + v ocm 1580 1580 800 800 800 800 v + a v C 11k 11k op amp lowpass filter stage v in C a v in + a r in r in 660025 bd v ocmb v ocma v C nc nc nc nc nc nc v + b nc nc Couta +outa nc nc nc v C v + a Coutb +outb nc nc nc nc v C nc v C v mida Cina +ina C + C + v ocm C C + + v ocm 1580 1580 800 800 800 800 v + b v C 11k 11k op amp lowpass filter stage v midb v in + b r in +inb v in C b r in Cinb
lt6604-2.5 9 660425f C + 1580 1580 0.01f 0.1f 3.3v C + v in C v in + 660425 f01 v out + v out C v t 3 2 1 v in + v in C v t 3 2 1 v out + 1/2 lt6604-2.5 v out C 0 0 25 27 4 34 6 2 29 7 C + 1580 1580 0.01f 0.1f 0.1f 0.1f 3.3v C + v in + 660425 f02 v out + v out C v 3 2 1 t 0 v out + v out C 2 v t 1 0 C1 v in + 25 27 4 34 6 2 29 7 1/2 lt6604-2.5 C + 402 402 0.1f 0.01f 5v C + v in C v in + 25 27 4 34 6 2 29 7 660425 f03 v out + v out C + C 2v v t 3 2 1 0 v out + v out C v t 3 2 1 0 v in + v in C 500mv p-p (diff) 1/2 lt6604-2.5 applications information figure 1 figure 2 figure 3 interfacing to the lt6604-2.5 note: the lt6604-2.5 contains two identical lowpass ? lters. the following applications information only refers to one ? lter. the two ? lters are independent except that they share the same negative supply voltage v C . the two ? lters can be used simultaneously by replicating the ex- ample circuits. the referenced pin numbers correspond to the a channel ? lter. each lt6604-2.5 channel requires two equal external re- sistors, r in , to set the differential gain to 1580 /r in . the inputs to the ? lter are the voltages v in + and v in C presented to these external components, figure 1. the difference between v in + and v in C is the differential input voltage. the average of v in + and v in C is the common mode input voltage. similarly, the voltages v out + and v out C appearing at pins 27 and 29 of the lt6604-2.5 are the ? lter outputs. the dif- ference between v out + and v out C is the differential output voltage. the average of v out + and v out C is the common mode output voltage. figure 1 illustrates the lt6604-2.5 operating with a single 3.3v supply and unity passband gain; the input signal is dc-coupled. the common mode input voltage is 0.5v, and the differential input voltage is 2v p-p . the common mode output voltage is 1.65v, and the differential output voltage is 2v p-p for frequencies below 2.5mhz. the common mode output voltage is determined by the voltage at v ocm . since v ocm is shorted to v mid , the output common mode is the mid-supply voltage. in addition, the common mode input voltage can be equal to the mid-supply voltage of v mid . figure 2 shows how to ac couple signals into the lt6604- 2.5. in this instance, the input is a single-ended signal. ac-coupling allows the processing of single-ended or differential signals with arbitrary common mode levels. the 0.1f coupling capacitor and the 1580 gain setting resistor form a high pass ? lter, attenuating signals below 1khz. larger values of coupling capacitors will proportion- ally reduce this highpass 3db frequency. in figure 3 the lt6604-2.5 channel is providing 12db of gain. the common mode output voltage is set to 2v.
lt6604-2.5 10 660425f applications information use figure 4 to determine the interface between the lt6604-2.5 and a current output dac. the gain, or tran- simpedance, is de? ned as a = v out /i in . to compute the transimpedance, use the following equation: a r rr = + () () 1580 1 12 ? by setting r1 + r2 = 1580, the gain equation reduces to a = r1(). the voltage at the pins of the dac is determined by r1, r2, the voltage on v mid and the dac output current. consider figure 4 with r1 = 49.9 and r2 = 1540. the voltage at v mid , for v s = 3.3v, is 1.65v. the voltage at the dac pins is given by: vv r rr i rr rr mv dac mid in = ++ + + =+ ?? ? 1 1 2 1580 12 12 26 i i in ?. 48 3 53.6 and 392 resistors satisfy the two constraints above. the transformer converts the single-ended source into a differential stimulus. similarly, the output of the lt6604-2.5 will have lower distortion with larger load resistance yet the analyzer input is typically 50. the 4:1 turns (16:1 impedance) transformer and the two 402 resistors of figure 5, present the output of the lt6604-2.5 with a 1600 differential load, or the equivalent of 800 to ground at each output. the impedance seen by the network analyzer input is still 50, reducing re? ections in the cabling between the transformer and analyzer input. differential and common mode voltage ranges the rail-to-rail output stage of the lt6604-2.5 can process large differential signal levels. on a 3v supply, the output signal can be 5.1v p-p . similarly, a 5v supply can support signals as large as 8.8v p-p . to prevent excessive power dissipation in the internal circuitry, the user must limit differential signal levels to 9v p-p . the two ampli? ers inside the lt6604-2.5 channel have independent control of their output common mode voltage (see the block diagram section). the following guidelines will optimize the performance of the ? lter. v mid can be allowed to ? oat, but it must be bypassed to an ac ground with a 0.01f capacitor or instability may be observed. v mid can be driven from a low impedance source, provided it remains at least 1.5v above v C and at least 1.5v below v + . an internal resistor divider sets the voltage of v mid . while the internal 11k resistors are well matched, their absolute value can vary by 20%. this should be taken into consideration when connecting an external resistor network to alter the voltage of v mid . C + 0.1f 3.3v C + 0.01f current output dac v out + v out C 660425 f04 r2 r1 i in C i in + r2 r1 = v out + C v out C i in + C i in C 1580 ? r1 r1 + r2 25 27 4 34 6 2 29 7 1/2 lt6604-2.5 figure 4 evaluating the lt6604-2.5 the low impedance levels and high frequency operation of the lt6604-2.5 require some attention to the imped- ance matching networks between the lt6604-2.5 and other devices. the previous examples assume an ideal (0) source impedance and a large (1k) load resistance. among practical examples where impedance must be considered is the evaluation of the lt6604-2.5 with a network analyzer. figure 5 is a laboratory setup that can be used to char- acterize the lt6604-2.5 using single-ended instruments with 50 source impedance and 50 input impedance. for a 12db gain con? guration the lt6604-2.5 requires a 402 source resistance yet the network analyzer output is calibrated for a 50 load resistance. the 1:1 transformer, C + 0.1f 0.1f 2.5v C2.5v C + 660425 f05 402 402 network analyzer input 50 coilcraft ttwb-16a 4:1 network analyzer source coilcraft ttwb-1010 1:1 50 53.6 392 392 25 27 4 34 6 2 29 7 1/2 lt6604-2.5 figure 5
lt6604-2.5 11 660425f applications information v ocm can be shorted to v mid for simplicity. if a different common mode output voltage is required, connect v ocm to a voltage source or resistor network. for 3v and 3.3v supplies the voltage at v ocm must be less than or equal to the mid supply level. for example, voltage (v ocm ) 1.65v on a single 3.3v supply. for power supply voltages higher than 3.3v the voltage at v ocm can be set above mid supply, as shown in table 1. the voltage on v ocm should not be more than 1v below the voltage on v mid . v ocm is a high impedance input. table 1. output common mode range for various supplies supply voltage differential out voltage swing output common mode range for low distortion 3v 4v p-p 2v p-p 1v p-p 1.4v v ocm 1.6v 1v v ocm 1.6v 0.75v v ocm 1.6v 5v 8v p-p 4v p-p 2v p-p 1v p-p 2.4v v ocm 2.6v 1.5v v ocm 3.5v 1v v ocm 3.75v 0.75v v ocm 3.75v 5v 9v p-p 4v p-p 2v p-p 1v p-p C2v v ocm 2v C3.5v v ocm 3.5v C3.75v v ocm 3.75v C4.25v v ocm 3.75v note: the voltage at v ocm should not be more than 1v below the voltage at v mid . to achieve some of the output common mode ranges shown in the table, the voltage at v mid must be set externally to a value below mid supply. the lt6604-2.5 was designed to process a variety of input signals including signals centered on the mid-sup- ply voltage and signals that swing between ground and a positive voltage in a single supply system (figure 1). the allowable range of the input common mode voltage (the average of v in + and v in C in figure 1) is determined by the power supply level and gain setting (see electrical characteristics). common mode dc currents in applications like figure 1 and figure 3 where the lt6604- 2.5 not only provides lowpass ? ltering but also level shifts the common mode voltage of the input signal, dc currents will be generated through the dc path between input and output terminals. minimize these currents to decrease power dissipation and distortion. consider the application in figure 3. v mid sets the output common mode voltage of the 1st differential ampli? er inside the lt6604-2.5 channel (see the block diagram section) at 2.5v. since the input common mode voltage is near 0v, there will be approximately a total of 2.5v drop across the series combination of the internal 1580 feedback resistor and the external 402 input resistor. the result- ing 1.25ma common mode dc current in each input path, must be absorbed by the sources v in + and v in C . v ocm sets the common mode output voltage of the 2nd differential ampli? er inside the lt6604-2.5 channel, and therefore sets the common mode output voltage of the ? lter. since, in the example of figure 3, v ocm differs from v mid by 0.5v, an additional 625a (312a per side) of dc current will ? ow in the resistors coupling the 1st differential ampli? er output stage to the ? lter output. thus, a total of 3.125ma is used to translate the common mode voltages. a simple modi? cation to figure 3 will reduce the dc com- mon mode currents by 36%. if v mid is shorted to v ocm the common mode output voltage of both op amp stages will be 2v and the resulting dc current will be 2ma. of course, by ac-coupling the inputs of figure 3, the common mode dc current can be reduced to 625a. noise the noise performance of the lt6604-2.5 channel can be evaluated with the circuit of figure 6. given the low noise output of the lt6604-2.5 and the 6db attenuation of the transformer coupling network, it is necessary to measure the noise ? oor of the spectrum analyzer and subtract the instrument noise from the ? lter noise measurement. example: with the ic removed and the 25 resistors grounded, figure 6, measure the total integrated noise (e s ) of the spectrum analyzer from 10khz to 2.5mhz. with the ic inserted, the signal source (v in ) disconnected, and the figure 6 C + 0.1f 0.1f 2.5v C2.5v C + 1/2 lt6604-2.5 r in r in 25 25 660425 f06 spectrum analyzer input 50 v in coilcraft ttwb-1010 1:1 25 27 4 34 6 2 29 7
lt6604-2.5 12 660425f applications information input resistors grounded, measure the total integrated noise out of the ? lter (e o ). with the signal source connected, set the frequency to 100khz and adjust the amplitude until v in measures 100mv p-p . measure the output amplitude, v out , and compute the passband gain a = v out /v in . now compute the input referred integrated noise (e in ) as: e ee a in os = ()?() 22 table 2 lists the typical input referred integrated noise for various values of r in . table 2. noise performance passband gain r in input referred integrated noise 10khz to 2.5mhz input referred integrated noise 10khz to 5mhz 4 402 18v rms 23v rms 2 806 29v rms 39v rms 1 1580 51v rms 73v rms figure 7 is plot of the noise spectral density as a function of frequency for an lt6604-2.5 channel with r in = 1580 using the ? xture of figure 6 (the instrument noise has been subtracted from the results). the noise at each output is comprised of a differential component and a common mode component. using a transformer or combiner to convert the differential outputs to single-ended signal rejects the common mode noise and gives a true measure of the s/n achievable in the system. conversely, if each output is measured individually and the noise power added together, the resulting calculated noise level will be higher than the true differential noise. power dissipation the lt6604-2.5 ampli? ers combine high speed with large signal currents in a small package. there is a need to en- sure that the dies junction temperature does not exceed 150c. the lt6604-2.5 has an exposed pad (pin 35) which is connected to the negative supply (v C ). connecting the pad to a ground plane helps to dissipate the heat generated by the chip. metal trace and plated through-holes can be used to spread the heat generated by the device to the backside of the pc board. junction temperature, t j , is calculated from the ambient temperature, t a , and power dissipation, p d . the power dissipation is the product of supply voltage, v s , and total supply current, i s . therefore, the junction temperature is given by: t j = t a + (p d ? ja ) = t a + (v s ? i s ? ja ) where the supply current, i s , is a function of signal level, load impedance, temperature and common mode voltages. for a given supply voltage, the worst-case power dissipation occurs when the differential input signal is maximum, the common mode currents are maximum (see applications information regarding common mode dc currents), the load impedance is small and the ambient temperature is maximum. to compute the junction temperature, measure the supply current under these worst-case conditions, use 43c/w as the package thermal resistance, then apply the equation for t j . for example, using the circuit in figure 3 with dc differential input voltage of 1v, a differential output voltage of 4v, no load resistance and an ambient temperature of 85c, the supply current (current into v + ) measures 37.6ma per channel. the resulting junction temperature is: t j = t a + (p d ? ja ) = 85 + (5 ? 2 ? 0.0376 ? 43) = 101c. the thermal resistance can be affected by the amount of copper on the pcb that is connected to v C . the thermal resistance of the circuit can increase if the exposed pad is not connected to a large ground plane with a number of vias. frequency (mhz) 0.01 0 30 40 50 0.1 1 10 660425 f07 20 10 0 60 80 100 40 20 noise spectral density (nv rms / hz ) integrated noise (v rms ) spectral density integrated figure 7. input referred noise, gain = 1
lt6604-2.5 13 660425f frequency (mhz) (dbm) 0 C10 C20 C30 C40 C50 C60 C70 C80 C90 0 120 108 96 84 72 60 48 36 24 12 660425 ta02b dac output image baseband signal typical applications dac output spectrum lt6604-2.5 output spectrum iq dac output filter frequency (mhz) (dbm) 0 C10 C20 C30 C40 C50 C60 C70 C80 C90 0 120 108 96 84 72 60 48 36 24 12 660025 ta02c 1580 1580 0.1f 5v 5v C5v C5v 0.1f C + C + q out 25 27 4 50mhz 34 6 2 29 7 52.3 52.3 ladcom i out a i out b clk ltc1668 16 bit 4khz to 2.5mhz discrete multi-tone signal @ 50msps 1/2 lt6604-2.5 1580 1580 0.1f 5v 5v C5v C5v 0.1f C + C + 660425 ta02a i out 17 19 12 50mhz 8 14 10 21 24 52.3 52.3 ladcom i out a i out b clk ltc1668 1/2 lt6604-2.5 56pf 56pf
lt6604-2.5 14 660425f typical applications frequency response transient response gain = 1 dual, matched 5th order, 2.5mhz lowpass filter, gain = 1 v out + 50mv/div differential input 200mv/div 500ns/div 660425 ta03c frequency (hz) 100k C30 gain (db) C20 C10 0 10 1m 10m 20m 660425 ta03b C40 C50 C60 C80 C90 C70 v s = 2.5v gain = 1 r = 787 t a = 25c r 787 r 787 r 787 r 787 c 82pf c = 0.1f 1/2 lt6604-2.5 v + v C 0.1f C + C + v in + v in C q out 25 27 4 1 2 ? r ? 2.5mhz 34 6 2 29 7 gain = 1580 2r r 787 r 787 r 787 r 787 c 82pf 0.1f 1/2 lt6604-2.5 v + v C 0.1f C + C + v in + v in C 660425 ta03a q out 17 19 12 8 14 10 21 24
lt6604-2.5 15 660425f information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. 4.00 0.10 1.50 ref 7.00 0.10 note: 1. drawing is not a jedec package outline 2. drawing not to scale 3. all dimensions are in millimeters 4. dimensions of exposed pad on bottom of package do not include mold flash. mold flash, if present, shall not exceed 0.20mm on any side 5. exposed pad shall be solder plated 6. shaded area is only a reference for pin 1 location on the top and bottom of package pin 1 top mark (note 6) 0.40 0.10 34 33 1 2 bottom view?exposed pad 6.00 ref 0.75 0.05 r = 0.125 typ r = 0.10 typ pin 1 notch r = 0.30 or 0.25 45 chamfer 0.25 0.05 0.50 bsc 0.200 ref 0.00 ? 0.05 (uff34) qfn 0807 rev ? recommended solder pad pitch and dimensions apply solder mask to areas that are not soldered 0.70 0.05 0.25 0.05 0.50 bsc 1.50 ref 6.00 ref 6.10 0.05 7.50 0.05 3.10 0.05 4.50 0.05 package outline 2.64 0.10 1.47 0.10 1.90 0.10 1.83 0.10 1.90 0.10 0.99 0.10 2.64 0.05 1.47 0.05 1.90 0.05 1.90 0.05 1.29 0.05 1.83 0.05 package description uff package 34-lead plastic qfn (4mm 7mm) (reference ltc dwg # 05-08-1758 rev ?)
lt6604-2.5 16 660425f linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2008 lt 0708 ? printed in usa related parts part number description comments integrated filters ltc1562-2 very low noise, 8th order filter building block lowpass and bandpass filters up to 300khz ltc1565-31 650khz linear phase lowpass filter continuous time, 7th order, differential ltc1566-1 low noise, 2.3mhz lowpass filter continuous time, 7th order, differential lt1568 very low noise, 4th order filter building block lowpass and bandpass filters up to 10mhz ltc1569-7 linear phase, tunable 10th order lowpass filter single-resistor programmable cut-off to 300khz lt6600-2.5 very low noise differential 2.5mhz lowpass filter snr = 86db at 3v supply, 4th order filter lt6600-5 very low noise differential 5mhz lowpass filter snr = 82db at 3v supply, 4th order filter lt6600-10 very low noise differential 10mhz lowpass filter snr = 82db at 3v supply, 4th order filter lt6600-15 very low noise differential 15mhz lowpass filter snr = 76db at 3v supply, 4th order filter lt6600-20 very low noise differential 20mhz lowpass filter snr = 76db at 3v supply, 4th order filter ltc6601 low noise, fully differential, pin con? gurable ampli? er/driver/2nd order filter building block ltc6602 dual adjustable lowpass filter for rfid ltc6603 dual adjustable lowpass filter for communications lt6604-5 dual very low noise, differential ampli? er and 5mhz lowpass filter snr = 82db at 3v supply, 4th order filter lt6604-10 dual very low noise, differential ampli? er and 10mhz lowpass filter snr = 82db at 3v supply, 4th order filter lt6604-15 dual very low noise, differential ampli? er and 15mhz lowpass filter snr = 76db at 3v supply, 4th order filter

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