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lt6600-2.5 1 660025fb typical application description very low noise, differential ampli er and 2.5mhz lowpass filter the lt ? 6600-2.5 combines a fully differential ampli? er with a 4th order 2.5mhz lowpass ? lter approximating a chebyshev frequency response. most differential ampli- ? ers require many precision external components to tail or gain and bandwidth. in contrast, with the lt6600-2.5, two external resistors program differential gain, and the ? lters 2.5mhz cutoff frequency and passband ripple are internally set. the lt6600-2.5 also provides the necessary level shifting to set its output common mode voltage to ac- commodate the reference voltage requirements of a/ds. using a proprietary internal architecture, the lt6600-2.5 integrates an antialiasing ? lter and a differential ampli- ? er/driver without compromising distortion or low noise performance. at unity gain the measured in band signal- to-noise ratio is an impressive 86db. at higher gains the input referred noise decreases so the part can process smaller input differential signals without signi? cantly degrading the output signal-to-noise ratio. the lt6600-2.5 also features low voltage operation. the differential design provides outstanding performance for a 4v p-p signal level while the part operates with a single 3v supply. the lt6600-2.5 is available in so-8 and dfn-12 packages. for similar devices with higher cutoff frequency, refer to the lt6600-5, lt6600-10, lt6600-15 and lt6600-20 data sheets. dac output filter l , lt, ltc and ltm are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. features applications n 0.6db (max) ripple 4th order lowpass filter with 2.5mhz cutoff n programmable differential gain via two external resistors n adjustable output common mode voltage n operates and speci? ed with 3v, 5v, 5v supplies n 86db s/n with 3v supply and 1v rms output n low distortion, 1v rms , 800 load 1mhz: 95dbc 2nd, 88dbc 3rd n fully differential inputs and outputs n compatible with popular differential ampli? er pinouts n so-8 and dfn-12 packages n high speed adc antialiasing and dac smoothing in networking or cellular base station applications n high speed test and measurement equipment n medical imaging n drop-in replacement for differential ampli? ers dac output spectrum lt6600-2.5 output spectrum (s8 pin numbers shown) 1580 1580 0.1f 5v 5v C5v C5v 0.1f C + C + v out C 660025 ta01a v out + 3 4 1 50mhz 7 2 8 5 6 52.3 52.3 ladcom i out a i out b clk ltc1668 16 bit 4khz to 2.5mhz discrete multi-tone signal at 50msps lt6600-2.5 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 ta01b dac output image baseband signal frequency (mhz) (dbm) 0 ?10 ?20 ?30 ?40 ?50 ?60 ?70 ?80 ?90 0 120 108 96 84 72 60 48 36 24 12 660025 ta01c
lt6600-2.5 2 660025fb absolute maximum ratings total supply voltage ...................................................1v input voltage (note 8) ............................................... v s input current (note 8) ..........................................10ma operating temperature range (note 6).... C40c to 85c (note 1) top view df package 12-lead (4mm 4mm) plastic dfn 12 12 11 8 9 10 4 5 3 2 1 in + nc v mid v C v C out C in C nc v ocm v + nc out + 6 7 t jmax = 150c, ja = 43c/w, jc = 4c/w exposed pad (pin 13) is vC, must be soldered to pcb top view in + v mid v C out C in C v ocm v + out + s8 package 8-lead plastic so 1 2 3 4 8 7 6 5 t jmax = 150c, ja = 100c/w pin configuration parameter conditions min typ max units filter gain, v s = 3v v in = 2v p-p , f in = dc to 260khz C0.5 0.1 0.4 db r in = 1580 v in = 2v p-p , f in = 700khz (gain relative to 260khz) l C0.15 0 0.1 db v in = 2v p-p , f in = 1.9mhz (gain relative to 260khz) l C0.2 0.2 0.6 db v in = 2v p-p , f in = 2.2mhz (gain relative to 260khz) l C0.6 0.1 0.5 db v in = 2v p-p , f in = 2.5mhz (gain relative to 260khz) l C2.1 C0.9 0 db order information lead free finish tape and reel part marking package description temperature range lt6600cs8-2.5#pbf lt6600cs8-2.5#trpbf 660025 8-lead plastic so 0c to 70c lt6600is8-2.5#pbf lt6600is8-2.5#trpbf 6600i25 8-lead plastic so C40c to 85c lt6600cdf-2.5#pbf lt6600cdf-2.5#trpbf 60025 12-lead (4mm 4mm) plastic dfn 0c to 70c lt6600idf-2.5#pbf lt6600idf-2.5#trpbf 60025 12-lead (4mm 4mm) plastic dfn C40c to 85c lead based finish tape and reel part marking package description temperature range lt6600cs8-2.5 lt6600cs8-2.5#tr 660025 8-lead plastic so 0c to 70c lt6600is8-2.5 lt6600is8-2.5#tr 600i25 8-lead plastic so C40c to 85c consult ltc marketing for parts speci? ed with wider operating temperature ranges. consult ltc marketing for information on nonstandard 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/ speci? ed temperature range (note 7) .... C40c to 85c junction temperature ........................................... 150c storage temperature range ................... C65c to 150c lead temperature (soldering, 10 sec) .................. 300c 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.
lt6600-2.5 3 660025fb 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 v in = 2v p-p , f in = 7.5mhz (gain relative to 260khz) l C34 C31 db v in = 2v p-p , f in = 12.5mhz (gain relative to 260khz) l C51 db filter gain, v s = 5v v in = 2v p-p , f in = dc to 260khz C0.5 C0.1 0.4 db r in = 1580 v in = 2v p-p , f in = 700khz (gain relative to 260khz) l C0.15 0 0.1 db v in = 2v p-p , f in = 2.2mhz (gain relative to 260khz) l C0.2 0.2 0.6 db v in = 2v p-p , f in = 2.2mhz (gain relative to 260khz) l C0.6 0.1 0.5 db v in = 2v p-p , f in = 2.5mhz (gain relative to 260khz) l C2.1 C0.9 0 db v in = 2v p-p , f in = 7.5mhz (gain relative to 260khz) l C34 C31 db v in = 2v p-p , f in = 12.5mhz (gain relative to 260khz) l C51 db filter gain, 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 in = 2v p-p , f in = dc to 260khz, v s = 3v v in = 2v p-p , f in = dc to 260khz, v s = 5v v in = 2v p-p , f in = dc to 260khz, 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 51 v rms distortion (note 4) 1mhz, 1v rms , r l = 800 2nd harmonic 3rd harmonic 95 88 dbc dbc differential output swing measured between pins 4 and 5 v s = 5v v s = 3v l l 8.8 5.1 9.3 5.5 v p-p diff v p-p diff input bias current average of pin 1 and pin 8 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 , v s = 3v r in 402 v s = 5v v s = 5v l l l 0.0 0.0 C2.5 1.5 3.0 1.0 v v v output common mode voltage (note 5) differential input = 2v p-p , v s = 3v pin 7 at mid-supply v s = 5v v s = 5v l l l 1.0 1.5 C1.0 1.5 3.0 2.0 v v v output common mode offset (with respect to pin 2) 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 (pin 7) v s = 5v (s8) v s = 5v (dfn) v s = 3v l l 2.46 2.45 2.51 2.51 1.5 2.55 2.56 v 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 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
lt6600-2.5 4 660025fb electrical characteristics amplitude response passband gain and group delay passband gain and group delay output impedance vs frequency (out + or out C ) cmrr psrr typical performance characteristics 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 ). speci? cation guaranteed for r in 402. for 5v supplies, the minimum input common mode voltage is guaranteed by design to reach C5v. note 4: distortion is measured differentially using a single-ended 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 voltages at pins 4 and 5. the output common mode voltage is equal to the voltage applied to pin 2. note 6: the lt6600c-2.5 is guaranteed functional over the operating temperature range of C40c to 85c. note 7: the lt6600c-2.5 is guaranteed to meet speci? ed performance from 0c to 70c and is designed, characterized and expected to meet speci? ed performance from C40c and 85c, but is not tested or qa sampled at these temperatures. the lt6600i-2.5 is guaranteed to meet speci? ed performance from C40c to 85c. note 8: the inputs are protected by back-to-back diodes. if the differential input voltage exceeds 1.4v, the input current should be limited to less than 10ma. frequency (hz) 100k C36 gain (db) C24 C12 0 12 1m 10m 50m 660025 g01 C48 C60 C84 C96 C72 v s = 2.5v r in = 1580 gain = 1 frequency (mhz) 0.5 gain (db) group delay (ns) C3 C1 1 2.5 660025 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 = 25c gain (db) 8 10 12 660025 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 c frequency (hz) 1 output impedance () 10 100k 10m 100m 660025 g04 0.1 1m 100 frequency (hz) 1k 40 cmrr (db) 80 90 110 100 100k 10k 1m 10m 100m 660025 g05 70 60 50 v in = 1v p-p v s = 5v r in = 1580 gain = 1 frequency (hz) 1k psrr (db) 100k 10k 1m 10m 100m 660025 g06 40 50 60 70 80 30 20 10 0 90 v + to differential out v s = 3v
lt6600-2.5 5 660025fb typical performance characteristics distortion vs signal level distortion vs signal level distortion vs signal level distortion vs frequency distortion vs frequency distortion vs frequency frequency (mhz) 0.1 C110 distortion (db) C70 C60 110 660025 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 frequency (mhz) 0.1 C110 distortion (db) C70 C60 110 660025 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 frequency (mhz) 0.1 C110 distortion (db) C70 C60 110 660025 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 = 5v r l = 800 at each output input level (v p-p ) 0 C110 C100 distortion (db) C90 C80 C70 C60 C50 C40 1234 660025 g10 6 5 v s = 3v f = 1mhz r l = 800 at each output 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 (db) C90 C80 C70 C60 C50 C40 1234 660025 g11 9 8 7 6 5 v s = 5v f = 1mhz r l = 800 at each output 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 (db) C90 C80 C70 C60 C50 C40 1234 660025 g12 9 8 7 6 5 v s = 5v f = 1mhz r l = 800 at each output 2nd harmonic, differential input 3rd harmonic, differential input 2nd harmonic, single-ended input 3rd harmonic, single-ended input
lt6600-2.5 6 660025fb typical performance characteristics distortion vs output common mode level supply current vs total supply voltage transient response gain = 1 distortion vs input common mode level distortion vs input common mode level input common mode voltage relative to v mid (v) C3 C110 C100 distortion component (db) C90 C80 C70 C60 C50 C40 C2 C1 0 1 2 660025 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 (db) C90 C80 C70 C60 C50 C40 C2 C1 0 1 2 660025 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 (db) C70 C60 C50 0.5 1.0 1.5 660025 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 = 5v 3rd harmonic, v s = 5v total supply voltage (v) 16 total supply current (ma) 24 22 20 28 32 18 26 30 2468 660025 g16 10 3579 t a = C40c t a = 25c t a = 85c v out + 50mv/div 500ns/div 660025 g17 differential input 200mv/div
lt6600-2.5 7 660025fb pin functions in C and in + (pins 1, 12/pins 1, 8): input pins. signals can be applied to either or both input pins through identical external resistors, r in . the dc gain from differential inputs to the differential outputs is 1580/r in . nc (pins 2, 5, 11/na): no connection v ocm (pin 3/pin 2): dc common mode reference voltage- for the 2nd filter stage. its value programs the common mode voltage of the differential output of the ? lter. this is a high impedance input, which can be driven from an external voltage reference, or it can be tied to v mid on the pc board. v ocm should be bypassed with a 0.01f ceramic capacitor unless it is connected to a ground plane. v + and v C (pins 4, 8, 9/pins 3, 6): power supply pins. for a single 3.3v or 5v supply (v C grounded) a quality 0.1f ceramic bypass capacitor is required from the positive supply pin (v + ) to the negative supply pin (v C ). the bypass should be as close as possible to the ic. for dual supply applications, bypass v + to ground and v C to ground with a quality 0.1f ceramic capacitor. out + and out C (pins 6, 7/pins 4, 5): output pins. these are the ? lter differential outputs. each pin can drive a 100 and/or 50pf load to ac ground. v mid (pin 10/pin 7): the v mid pin is internally biased at mid-supply, see block diagram. for single supply op- eration, the v mid pin should be bypassed with a quality 0.01f ceramic capacitor to v C . for dual supply operation, v mid 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. v mid sets the output common mode voltage of the 1st stage of the ? lter. it has a 5.5k impedance, and it can be overridden with an external low impedance voltage source. (dfn/so)
lt6600-2.5 8 660025fb block diagram C + C + v ocm C C + + v ocm 1580 1580 800 800 800 800 v + v C 11k 11k op amp proprietary lowpass filter stage v in C v in + r in r in 660025 bd in + v ocm v + out + out C v C v mid in C
lt6600-2.5 9 660025fb applications information figure 1. (s8 pin numbers) interfacing to the lt6600-2.5 note: the referenced pin numbers correspond to the s8 package. see the pin functions for the equivalent dfn-12 package pin numbers. the lt6600-2.5 requires two equal external resistors, 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 the see external components, figure 1. the difference between v in + and v in C is the differential input voltage. the aver- age 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 4 and 5 of the lt6600-2.5 are the ? lter outputs. the differ- ence 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 lt6600-2.5 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 lt6600-2.5. in this instance, the input is a single-ended signal. ac cou- pling 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 proportionally reduce this highpass 3db frequency. in figure 3 the lt6600-2.5 is providing 12db of gain. the common mode output voltage is set to 2v. figure 2. (s8 pin numbers) figure 3. (s8 pin numbers) C + 1580 1580 0.01f 0.1f 3.3v C + v in C v in + 3 4 1 7 2 8 5 6 660025 f01 v out + v out C v t 3 2 1 v in + v in C v t 3 2 1 v out + lt6600-2.5 v out C 0 0 C + 1580 1580 0.01f 0.1f 0.1f 0.1f 3.3v C + v in + 3 4 1 7 2 8 5 6 660025 f02 v out + v out C v 3 2 1 t 0 lt6600-2.5 v out + v out C 2 v t 1 0 C1 v in + C + 402 402 0.1f 0.01f 5v C + v in C v in + 3 4 1 7 2 8 5 6 660025 f03 v out + v out C + C 2v v t 3 2 1 0 v out + v out C lt6600-2.5 v t 3 2 1 0 v in + v in C 500mv p-p (diff)
lt6600-2.5 10 660025fb applications information use figure 4 to determine the interface between the lt6600-2.5 and a current output dac. the gain, or trans- impedance, is de? ned as a = v out /i in . to compute the transimpedance, use the following equation: a = 15 8 0?r1 r1 + r2 () () 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: v dac = v pin7 ? r1 r1 + r2 + 15 8 0 + i in ? r1? r2 r1 + r2 = 26mv + i in ?4 8 .3 i in is i in + or i in C . the transimpedance in this example is 49.6. evaluating the lt6600-2.5 the low impedance levels and high frequency operation of the lt6600-2.5 require some attention to the matching networks between the lt6600-2.5 and other devices. the previous examples assume an ideal (0) source imped- ance and a large (1k) load resistance. among practical examples where impedance must be considered is the evaluation of the lt6600-2.5 with a network analyzer. figure 5 is a laboratory setup that can be used to charac- terize the lt6600-2.5 using single-ended instruments with 50 source impedance and 50 input impedance. for a 12db gain con? guration the lt6600-2.5 requires a402 source resistance yet the network analyzer output is calibrated for a 50 load resistance. the 1:1 transformer, 53.6 and 388 resistors satisfy the two constraints above. the transformer converts the single-ended source into a differential stimulus. similarly, the output of the lt6600-2.5 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 lt6600-2.5 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 lt6600-2.5 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 lt6600-2.5 have indepen- dent control of their output common mode voltage (see the block diagram section). the following guidelines will optimize the performance of the ? lter. figure 4. (s8 pin numbers) figure 5. (s8 pin numbers) C + 0.1f 3.3v C + lt6600-2.5 3 4 1 0.01f current output dac 7 2 8 5 v out + v out C 660025 f04 6 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 C + 0.1f 0.1f 2.5v C2.5v C + lt6600-2.5 3 4 1 7 2 8 5 6 660025 f05 402 402 network analyzer input 50 coilcraft ttwb-16a 4:1 network analyzer source coilcraft ttwb-1010 1:1 50 53.6 388 388
lt6600-2.5 11 660025fb applications information v mid can be allowed to ? oat, but it must be bypassed to an ac ground with a 0.01f capacitor or some instability maybe 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 . 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 exceed 1v below the voltage on v mid . v ocm is a high impedance input. table 1. output common range for various supplies supply voltage differential out voltage swing output common mode range for low distortion 3v 4v p-p 1.4v v ocm 1.6v 2v p-p 1v v ocm 1.6v 1v p-p 0.75v v ocm 1.6v 5v 8v p-p 2.4v v ocm 2.6v 4v p-p 1.5v v ocm 3.5v 2v p-p 1v v ocm 3.75v 1v p-p 0.75v v ocm 3.75v 5v 9v p-p C2v v ocm 2v 4v p-p C3.5v v ocm 3.5v 2v p-p C3.75v v ocm 3.75v 1v p-p C4.25v v ocm 3.75v note: v ocm is set by the voltage at this r in . the voltage at v ocm should not exceed 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 lt6600-2.5 was designed to process a variety of input signals including signals centered around the mid-sup- ply voltage and signals that swing between ground and a positive voltage in a single supply system (figure 1). the range of allowable 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 lt6600-2.5 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 lt6600-2.5 (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 resulting 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 lt6600-2.5, 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 ? 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 tov 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.
lt6600-2.5 12 660025fb applications information noise the noise performance of the lt6600-2.5 can be evaluated with the circuit of figure 6. given the low noise output of the lt6600-2.5 and the 6db attenuation of the transformer coupling network, it will be necessary to measure the noise ? oor of the spectrum analyzer and subtract the instrument noise from the ? lter noise measurement. figure 7 is plot of the noise spectral density as a function of frequency for an lt6600-2.5 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 lt6600-2.5 ampli? ers combine high speed with large- signal currents in a small package. there is a need to ensure that the dies junction temperature does not exceed 150c. the lt6600-2.5 s8 package has pin 6 fused to the lead frame to enhance thermal conduction when connect- ing to a ground plane or a large metal trace. metal trace and plated through-holes can be used to spread the heat generated by the device to the backside of the pc board. for example, on a 3/32 fr-4 board with 2oz copper, a totalof 660 square millimeters connected to pin 6 of thelt6600-2.5 s8 (330 square millimeters on each side of the pc board) will result in a thermal resistance, ja , of about 85c/w. without the extra metal trace connected figure 7. input referred noise, gain = 1 frequency (mhz) 0.01 0 30 40 50 0.1 1 10 660025 f07 20 10 0 60 80 100 40 20 noise spectral density (nv rms / hz ) integrated noise (v rms ) spectral density integrated figure 6. (s8 pin numbers) C + 0.1f 0.1f 2.5v C2.5v C + lt6600-2.5 3 4 1 7 2 8 5 6 r in r in 25 25 660025 f06 spectrum analyzer input 50 v in coilcraft ttwb-1010 1:1 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 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 in = (e o ) 2 ?(e s ) 2 a table 2 lists the typical input referred integrated noise for various values of r in . table 2. noise performance passband gain (v/v) 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
lt6600-2.5 13 660025fb applications information to the v C pin to provide a heat sink, the thermal resistance will be around 105c/w. table 3 can be used as a guide when considering thermal resistance. table 3. lt6600-2.5 so-8 package thermal resistance copper area topside (mm2) backside (mm2) board area (mm2) thermal resistance (junction-to-ambient) 1100 1100 2500 65c/w 330 330 2500 85c/w 35 35 2500 95c/w 35 0 2500 100c/w 0 0 2500 105c/w 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 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, es- timate the thermal resistance from table 2, 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. assuming a pc board layout with a 35mm 2 copper trace, the ja is 100c/w. the resulting junction temperature is: t j = t a + (p d ? ja ) = 85 + (5 ? 0.0376 ? 100) = 104c when using higher supply voltages or when driving small impedances, more copper may be necessary to keep t j below 150c.
lt6600-2.5 14 660025fb package description df package 12-lead plastic dfn (4mm 4mm) (reference ltc dwg # 05-08-1733 rev ?) 4.00 0.10 (4 sides) note: 1. drawing is proposed to be made a jedec package outline mo-220 variation (wggd-x)?to be approved 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 0.15mm 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 1 6 12 7 bottom view?exposed pad 2.65 0.10 0.75 0.05 r = 0.115 typ 0.25 0.05 0.50 bsc 2.50 ref 3.3 8 0.10 0.200 ref 0.00 ? 0.05 (df12) dfn 0 8 06 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 3.10 0.05 4.50 0.05 package outline pin 1 notch r = 0.20 typ or 0.35 45 chamfer 2.65 0.05 3.3 8 0.05 2.50 ref
lt6600-2.5 15 660025fb 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. package description s8 package 8-lead plastic small outline (narrow .150 inch) (reference ltc dwg # 05-08-1610) .016 ? .050 (0.406 ? 1.270) .010 ? .020 (0.254 ? 0.50 8 ) 45 0 ? 8 typ .00 8 ? .010 (0.203 ? 0.254) so 8 0303 .053 ? .069 (1.346 ? 1.752) .014 ? .019 (0.355 ? 0.4 8 3) typ .004 ? .010 (0.101 ? 0.254) .050 (1.270) bsc 1 2 3 4 .150 ? .157 (3. 8 10 ? 3.9 88 ) note 3 8 7 6 5 .1 8 9 ? .197 (4. 8 01 ? 5.004) note 3 .22 8 ? .244 (5.791 ? 6.197) .245 min .160 .005 recommended solder pad layout .045 .005 .050 bsc .030 .005 typ inches (millimeters) note: 1. dimensions in 2. drawing not to scale 3. these dimensions do not include mold flash or protrusions. mold flash or protrusions shall not exceed .006" (0.15mm)
lt6600-2.5 16 660025fb linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2003 lt 0408 rev b ? printed in usa related parts typical application part number description comments lt c ? 1565-31 650khz linear phase lowpass filter continuous time, so8 package, fully differential ltc1566-1 low noise, 2.3mhz lowpass filter continuous time, so8 package lt1567 very low noise, high frequency filter building block 1.4nv/ hz op amp, msop package, fully differential lt1568 very low noise, 4th order building block lowpass and bandpass filter designs up to 10mhz, differential outputs ltc1992 low-power differential in/out ampli? er adjustable gain, msop package ltc1992-1 low-power differential in/out ampli? er fixed gain of 1, matching 0.3% ltc1992-2 low-power differential in/out ampli? er fixed gain of 2, matching 0.3% ltc1992-5 low-power differential in/out ampli? er fixed gain of 5, matching 0.3% ltc1992-10 low-power differential in/out ampli? er fixed gain of 10, matching 0.3% lt6600-10 very low noise differential ampli? er and 10mhz lowpass filter 82db s/n with 3v supply, so-8 package lt6600-20 very low noise differential ampli? er and 20mhz lowpass filter 76db s/n with 3v supply, so-8 package 5th order lowpass filter (s8 pin numbers shown) amplitude response transient response gain = 1 r r r r c c = 0.1f lt6600 v + v C 0.1f C + C + v in + v in C v out C 660025 ta02a v out + 3 4 1 1 2 ? r ? 2.5mhz 7 2 8 5 6 gain = , maximum gain = 4 1580 2r frequency (hz) 100k C30 gain (db) C20 C10 0 10 1m 10m 20m 660025 ta02b C40 C50 C60 C80 C90 C70 v s = 2.5v gain = 1 r = 787 t a = 25c v out + 50mv/div 500ns/div 660025 ta02c differential input 200mv/div

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