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LT6600-20 1 66002f , ltc and lt are registered trademarks of linear technology corporation. n programmable differential gain via two external resistors n adjustable output common mode voltage n operates and specified with 3v, 5v, 5v supplies n 0.5db ripple 4th order lowpass filter with 20mhz cutoff n 76db s/n with 3v supply and 2v p-p output n low distortion, 2v p-p , 800 w load 2.5mhz: 83dbc 2nd, 88dbc 3rd 20mhz: 63dbc 2nd, 64dbc 3rd n fully differential inputs and outputs n so-8 package n compatible with popular differential amplifier pinouts very low noise, differential amplifier and 20mhz lowpass filter 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 amplifiers the lt ? 6600-20 combines a fully differential amplifier with a 4th order 20mhz lowpass filter approximating a chebyshev frequency response. most differential amplifi- ers require many precision external components to tailor gain and bandwidth. in contrast, with the LT6600-20, two external resistors program differential gain, and the filters 20mhz cutoff frequency and passband ripple are internally set. the LT6600-20 also provides the necessary level shifting to set its output common mode voltage to accom- modate the reference voltage requirements of a/ds. using a proprietary internal architecture, the LT6600-20 integrates an antialiasing filter and a differential amplifier/ driver without compromising distortion or low noise performance. at unity gain the measured in band signal-to-noise ratio is an impressive 76db. at higher gains the input referred noise decreases so the part can process smaller input differential signals without signifi- cantly degrading the output signal-to-noise ratio. the LT6600-20 also features low voltage operation. the differential design provides outstanding performance for a 2v p-p signal level while the part operates with a single 3v supply. the LT6600-20 is packaged in an so-8 and is pin compat- ible with stand alone differential amplifiers. + + r in 402 r in 402 0.01 f 0.1 f 49.9 49.9 18pf 5v 5v + v mid v ocm v in v cm a in v + v d out LT6600-20 a/d ltc1748 3 4 1 7 2 8 5 6 66002 ta01a gain = 402 /r in 1 f frequency (mhz) 0 amplitude (db) ?0 ?0 ?0 40 66002 ta01b ?0 ?0 ?20 10 20 30 ?00 0 ?0 ?0 ?0 ?0 ?10 input 5.9mhz 2v p-p f sample = 80mhz an 8192 point fft spectrum descriptio u features applicatio s u typical applicatio u
LT6600-20 2 66002f parameter conditions min typ max units filter gain, v s = 3v v in = 2v p-p , f in = dc to 260khz C 0.4 0.1 0.5 db v in = 2v p-p , f in = 2mhz (gain relative to 260khz) l C 0.1 0 0.1 db v in = 2v p-p , f in = 10mhz (gain relative to 260khz) l C 0.2 0.1 0.5 db v in = 2v p-p , f in = 16mhz (gain relative to 260khz) l C 0.1 0.5 1.9 db v in = 2v p-p , f in = 20mhz (gain relative to 260khz) l C 0.8 0 1 db v in = 2v p-p , f in = 60mhz (gain relative to 260khz) l C33 C28 db v in = 2v p-p , f in = 100mhz (gain relative to 260khz) l C50 db filter gain, v s = 5v v in = 2v p-p , f in = dc to 260khz C 0.5 0 0.5 db v in = 2v p-p , f in = 2mhz (gain relative to 260khz) l C 0.1 0 0.1 db v in = 2v p-p , f in = 10mhz (gain relative to 260khz) l C 0.2 0.1 0.4 db v in = 2v p-p , f in = 16mhz (gain relative to 260khz) l C 0.3 0.4 1.6 db v in = 2v p-p , f in = 20mhz (gain relative to 260khz) l C 1.3 C0.4 0.6 db v in = 2v p-p , f in = 60mhz (gain relative to 260khz) l C33 C28 db v in = 2v p-p , f in = 100mhz (gain relative to 260khz) l C50 db filter gain, v s = 5v v in = 2v p-p , f in = dc to 260khz C 0.6 C 0.1 0.4 db filter gain, r in = 100 w v in = 2v p-p , f in = dc to 260khz, v s = 3v 11.6 12.1 12.6 db v in = 2v p-p , f in = dc to 260khz, v s = 5v 11.5 12.0 12.5 db v in = 2v p-p , f in = dc to 260khz, v s = 5v 11.4 11.9 12.4 db filter gain temperature coefficient (note 2) f in = 250khz, v in = 2v p-p 780 ppm/c noise noise bw = 10khz to 20mhz 118 m v rms distortion (note 4) 2.5mhz, 2v p-p , r l = 800 w 2nd harmonic 83 dbc 3rd harmonic 88 dbc 20mhz, 2v p-p , r l = 800 w 2nd harmonic 63 dbc 3rd harmonic 64 dbc differential output swing measured between pins 4 and 5 v s = 5v l 3.80 4.75 v p-p diff v s = 3v l 3.75 4.50 v p-p diff input bias current average of pin 1 and pin 8 l C95 C50 m a total supply voltage ................................................ 11v operating temperature range (note 6) ...C40 c to 85 c specified temperature range (note 7) ....C40 c to 85 c junction temperature ........................................... 150 c storage temperature range ................. C 65 c to 150 c lead temperature (soldering, 10 sec).................. 300 c order part number s8 part marking t jmax = 150 c, q ja = 100 c/w 660020 600i20 absolute axi u rati gs w ww u package/order i for atio uu w (note 1) electrical characteristics consult ltc marketing for parts specified with wider operating temperature ranges. the l denotes specifications that apply over the full operating temperature range, otherwise specifications are at t a = 25 c. unless otherwise specified v s = 5v (v + = 5v, v C = 0v), r in = 402 w , and r load = 1k. lt6600cs8-20 lt6600is8-20 top view in + v mid v out in v ocm v + out + s8 package 8-lead plastic so 1 2 3 4 8 7 6 5
LT6600-20 3 66002f note 1: absolute maximum ratings are those values beyond which the life of a device may be impaired. note 2: this is the temperature coefficient 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 ). specification guaranteed for r in 3 100 w . note 4: distortion is measured differentially using a differential stimulus, the input common mode voltage, the voltage at pin 2, and the voltage at pin 7 are equal to one half of the total power supply voltage. parameter conditions min typ max units input referred differential offset r in = 402 w v s = 3v l 525 mv v s = 5v l 10 30 mv v s = 5v l 10 35 mv r in = 100 w v s = 3v l 515 mv v s = 5v l 517 mv v s = 5v l 520 mv differential offset drift 10 m v/ c input common mode voltage (note 3) differential input = 500mv p-p ,v s = 3v l 0.0 1.5 v r in = 100 w v s = 5v l 0.0 3.0 v v s = 5v l C2.5 1.0 v output common mode voltage (note 5) differential input = 2v p-p ,v s = 3v l 1.0 1.5 v pin 7 at mid-supply v s = 5v l 1.5 3.0 v common mode voltage at pin 2 v s = 5v l C1.0 2.0 v output common mode offset v s = 3v l C35 5 40 mv (with respect to pin 2) v s = 5v l C40 0 40 mv v s = 5v l C55 C5 35 mv common mode rejection ratio 66 db voltage at v mid (pin 7) v s = 5 l 2.46 2.51 2.55 v v s = 3 1.5 v v mid input resistance l 4.35 5.7 7.65 k w v ocm bias current v ocm = v mid = v s /2 v s = 5v l C15 C3 m a v s = 3v l C10 C3 m a power supply current v s = 3v, v s = 5 42 46 ma v s = 3v, v s = 5 l 53 ma v s = 5v l 46 56 ma electrical characteristics 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-20 is guaranteed functional over the operating temperature range C40 c to 85 c. note 7: the lt6600c-20 is guaranteed to meet 0 c to 70 c specifications and is designed, characterized and expected to meet the extended temperature limits, but is not tested at C40 c and 85 c. the lt6600i-20 is guaranteed to meet specified performance from C40 c to 85 c. the l denotes specifications that apply over the full operating temperature range, otherwise specifications are at t a = 25 c. unless otherwise specified v s = 5v (v + = 5v, v C = 0v), r in = 402 w , and r load = 1k.
LT6600-20 4 66002f typical perfor a ce characteristics uw amplitude response passband gain and phase passband gain and group delay frequency (mhz) ?0 gain (db) 0 10 ?0 ?0 ?0 ?0 ?0 ?0 ?0 0.1 10 100 66002 g01 ?0 1 v s = 5v gain = 1 t a = 25 c frequency (mhz) 0.5 gain (db) phase (deg) ? ? 2 24.5 66002 g02 ?0 ?4 ? ? 0 ?2 ?6 ?8 ?5 5 95 ?75 265 ?30 ?0 50 220 310 355 6.5 12.5 18.5 30.5 v s = 5v gain = 1 t a = 25 c gain phase frequency (mhz) 0.5 gain (db) group delay (ns) ? ? 2 24.5 66002 g03 ?0 ?4 ? ? 0 ?2 ?6 ?8 30 40 50 20 10 25 35 45 15 5 0 6.5 12.5 18.5 30.5 v s = 5v gain = 1 t a = 25 c gain group delay passband gain and group delay output impedance common mode rejection ratio frequency (mhz) 0.5 gain (db) group delay (ns) 6 10 14 24.5 66002 g04 2 ? 4 8 12 0 ? ? 30 40 50 20 10 25 35 45 15 5 0 6.5 12.5 18.5 30.5 v s = 5v gain = 4 t a = 25 c gain group delay frequency (mhz) 1 output impedance ( ) 10 0.1 10 100 66002 g05 0.1 1 100 v s = 5v gain = 1 t a = 25 c frequency (mhz) 0.1 cmrr (db) 60 70 80 1 10 100 66002 g06 50 40 30 65 75 55 45 35 input = 1v p-p v s = 5v gain = 1 t a = 25 c power supply rejection ratio distortion vs frequency frequency (mhz) 0.001 40 psrr (db) 50 60 70 80 0.01 0.1 1 10 100 66002 g07 30 20 10 0 90 100 v + to diffout v s = 3v t a = 25 c frequency (mhz) 0.1 ?00 distortion (db) ?0 ?0 ?0 1 10 100 66002 g08 ?0 ?0 ?0 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 t a = 25 c input level (v p-p ) 0 ?00 distortion (db) ?0 ?0 ?0 ?0 ?0 ?0 1234 66002 g09 5 v s = 3v r l = 800 at each output gain = 1 t a = 25 c 2nd harmonic 10mhz input 2nd harmonic 1mhz input 3rd harmonic 10mhz input 3rd harmonic 1mhz input distortion vs signal level, v s = 3v
LT6600-20 5 66002f typical perfor a ce characteristics uw distortion vs signal level, v s = 5v distortion vs output common mode total supply current vs total supply voltage transient response, gain = 1 input level (v p-p ) 0 ?00 distortion (db) ?0 ?0 ?0 ?0 ?0 ?0 1234 66002 g10 5 v s = 5v r l = 800 at each output gain = 1 t a = 25 c 2nd harmonic, 10mhz input 3rd harmonic, 10mhz input 2nd harmonic, 1mhz input 3rd harmonic, 1mhz input input common mode votlage relative to pin 7 (v) ? ?00 distortion component (db) ?0 ?0 ?0 ?0 ?0 ?0 2 ? 0 1 2 66002 g11 3 2v p-p 1mhz input r l = 800 at each output gain = 1 t a = 25 c 2nd harmonic, v s = 3v 3rd harmonic, v s = 3v 2nd harmonic, v s = 5v 3rd harmonic, v s = 5v input common mode votlage relative to pin 7 (v) ? ?00 distortion component (db) ?0 ?0 ?0 ?0 ?0 ?0 2 ? 0 1 2 66002 g12 3 500mv p-p 1mhz input, gain = 4, r l = 800 at each output 2nd harmonic, v s = 3v 3rd harmonic, v s = 3v 2nd harmonic, v s = 5v 3rd harmonic, v s = 5v distortion vs input common mode level distortion vs input common mode level voltage pin 2 to pin 7 (v) ? distortion component (db) ?0 ?0 ?0 0.5 1 1.5 66002 g13 ?0 ?0 ?.5 ? 0.5 0 2 ?00 ?10 ?0 2v p-p 1mhz input, gain = 1, r l = 800 at each output 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) 0 total supply current (ma) 20 40 60 10 30 50 2468 66002 g14 10 1 03579 t a = ?0 c t a = 25 c t a = 85 c v out + 50mv/div differential input 200mv/div 100ns/div 66002 g15
LT6600-20 6 66002f block diagra w + + v ocm + + v ocm 402 402 200 200 200 200 1 2 3 4 v + v 11k 11k 8 7 6 5 op amp proprietary lowpass filter stage v in v in + r in r in 66002 bd in + v ocm v + out + out v v mid in uu u pi fu ctio s in C and in + (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 402 w /r in . v ocm (pin 2): is the dc common mode reference voltage for the 2nd filter stage. its value programs the common mode voltage of the differential output of the filter. pin 2 is a high impedance input, which can be driven from an external voltage reference, or pin 2 can be tied to pin 7 on the pc board. pin 2 should be bypassed with a 0.01 m f ceramic capacitor unless it is connected to a ground plane. v + and v C (pins 3, 6): power supply pins . for a single 3.3v or 5v supply (pin 6 grounded) a quality 0.1 m f ceramic bypass capacitor is required from the positive supply pin (pin 3) to the negative supply pin (pin 6). the bypass should be as close as possible to the ic. for dual supply applications, bypass pin 3 to ground and pin 6 to ground with a quality 0.1 m f ceramic capacitor. out + and out C (pins 4, 5): output pins . pins 4 and 5 are the filter differential outputs. each pin can drive a 100 w and/or 50pf load. v mid (pin 7): the v mid pin is internally biased at mid- supply, see block diagram. for single supply operation, the v mid pin should be bypassed with a quality 0.01 m f ceramic capacitor to pin 6. for dual supply operation, pin 7 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 7 sets the output common mode voltage of the 1st stage of the filter. it has a 5.5k w impedance, and it can be overridden with an external low impedance voltage source.
LT6600-20 7 66002f applicatio s i for atio wu uu + 402 402 0.01 f 0.1 f 3.3v + v in v in + 3 4 1 7 2 8 5 6 66002 f01 v out + v out v t 3 2 1 v in + v in v t 3 2 1 v out + LT6600-20 v out 0 0 + 402 402 0.01 f 0.1 f 0.1 f 0.1 f 3.3v + v in + 3 4 1 7 2 8 5 6 66002 f02 v out + v out v 3 2 2 1 v t 1 0 0 ? v in + LT6600-20 v out + v out + 100 100 0.1 f 0.01 f 5v + v in v in + 3 4 1 7 2 8 5 6 66002 f03 v out + v out 62pf 62pf + 2v v t 3 2 1 0 v in + v in v t 3 2 1 0 v out + v out LT6600-20 500mv p-p (diff) figure 1 figure 2 figure 3 interfacing to the LT6600-20 the LT6600-20 requires two equal external resistors, r in , to set the differential gain to 402 w /r in . the inputs to the filter 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 4 and 5 of the LT6600-20 are the filter outputs. the difference 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 LT6600-20 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 20mhz. the common mode output voltage is determined by the voltage at pin 2. since pin 2 is shorted to pin 7, 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 pin 7 (see the distortion vs input common mode level graphs in the typical performance characteristics). figure 2 shows how to ac couple signals into the LT6600-20. 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 0.1 m f coupling capacitor and the 402 w gain setting resistor form a high pass filter, attenuating signals below 4khz. larger values of coupling capacitors will propor- tionally reduce this highpass 3db frequency.
LT6600-20 8 66002f applicatio s i for atio wu u u in figure 3 the LT6600-20 is providing 12db of gain. the gain resistor has an optional 62pf in parallel to improve the passband flatness near 20mhz. the common mode output voltage is set to 2v. use figure 4 to determine the interface between the LT6600-20 and a current output dac. the gain, or trans- impedance, is defined as a = v out /i in . to compute the transimpedance, use the following equation: a r rr = + () w () 402 1 12 by setting r1 + r2 = 402 w , the gain equation reduces to a = r1( w ). the voltage at the pins of the dac is determined by r1, r2, the voltage on pin 7 and the dac output current. consider figure 4 with r1 = 49.9 w and r2 = 348 w . the voltage at pin 7 is 1.65v. the voltage at the dac pins is given by: vv r rr i rr rr mv i dac pin in in = ++ + + =+ w 7 1 1 2 402 12 12 26 48 3 . i in is i in + or i in C . the transimpedance in this example is 50.4 w . evaluating the LT6600-20 the low impedance levels and high frequency operation of the LT6600-20 require some attention to the matching networks between the LT6600-20 and other devices. the previous examples assume an ideal (0 w ) source imped- ance and a large (1k w ) load resistance. among practical figure 5 + 0.1 f 0.1 f 2.5v 2.5v + LT6600-20 3 4 1 7 2 8 5 6 66002 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 examples where impedance must be considered is the evaluation of the LT6600-20 with a network analyzer. figure 5 is a laboratory setup that can be used to charac- terize the LT6600-20 using single-ended instruments with 50 w source impedance and 50 w input impedance. for a unity gain configuration the LT6600-20 requires a 402 w source resistance yet the network analyzer output is calibrated for a 50 w load resistance. the 1:1 transformer, 53.6 w and 388 w resistors satisfy the two constraints above. the transformer converts the single-ended source into a differential stimulus. similarly, the output of the LT6600-20 will have lower distortion with larger load resistance yet the analyzer input is typically 50 w . the 4:1 turns (16:1 impedance) transformer and the two 402 w resistors of figure 5, present the output of the LT6600-20 with a 1600 w differential load, or the equivalent of 800 w to ground at each output. the impedance seen by the network analyzer input is still 50 w , reducing reflections in the cabling between the transformer and analyzer input. differential and common mode voltage ranges the differential amplifiers inside the LT6600-20 contain circuitry to limit the maximum peak-to-peak differential voltage through the filter. this limiting function prevents excessive power dissipation in the internal circuitry and provides output short-circuit protection. the limiting function begins to take effect at output signal levels above 2v p-p and it becomes noticeable above 3.5v p-p . this is illustrated in figure 6; the LT6600-20 was configured with unity passband gain and the input of the filter was driven with a 1mhz signal. because this voltage limiting takes place well before the output stage of the filter reaches the + 0.1 f 3.3v + LT6600-20 3 4 1 0.01 f current output dac 7 2 8 5 v out + v out 66002 f04 6 r2 r1 i in i in + r2 r1 figure 4
LT6600-20 9 66002f applicatio s i for atio wu u u supply rails, the input/output behavior of the ic shown in figure 6 is relatively independent of the power supply voltage. the two amplifiers inside the LT6600-20 have indepen- dent control of their output common mode voltage (see the block diagram section). the following guidelines will optimize the performance of the filter. pin 7 must be bypassed to an ac ground with a 0.01 m f or larger capacitor. pin 7 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 pin 7. 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 pin 7. pin 2 can be shorted to pin 7 for simplicity. if a different common mode output voltage is required, connect pin 2 to a voltage source or resistor network. for 3v and 3.3v supplies the voltage at pin 2 must be less than or equal to the mid supply level. for example, voltage (pin 2) 1.65v on a single 3.3v supply. for power supply voltages higher than 3.3v the voltage at pin 2 should be within the voltage of pin 7 C 1v to the voltage of pin 7 + 2v. pin 2 is a high impedance input. the LT6600-20 was designed to process a variety of input signals including signals centered around the mid-supply 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 distortion vs input common mode level in the typical performance characteristics). common mode dc currents in applications like figure 1 and figure 3 where the LT6600-20 not only provides lowpass filtering but also level shifts the common mode voltage of the input signal, dc currents will be generated through the dc path be- tween input and output terminals. minimize these currents to decrease power dissipation and distortion. consider the application in figure 3. pin 7 sets the output common mode voltage of the 1st differential amplifier inside the LT6600-20 (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 402 w feedback resistor and the external 100 w input resistor. the resulting 5ma common mode dc current in each input path, must be absorbed by the sources v in + and v in C . pin 2 sets the common mode output voltage of the 2nd differential amplifier inside the LT6600-20, and therefore sets the common mode output voltage of the filter. since, in the example of figure 3, pin 2 differs from pin 7 by 0.5v, an additional 2.5ma (1.25ma per side) of dc current will flow in the resistors coupling the 1st differential amplifier output stage to filter output. thus, a total of 12.5ma is used to translate the common mode voltages. a simple modification to figure 3 will reduce the dc common mode currents by 36%. if pin 7 is shorted to pin 2 the common mode output voltage of both op amp stages will be 2v and the resulting dc current will be 8ma. of course, by ac coupling the inputs of figure 3, the common mode dc current can be reduced to 2.5ma. figure 6. output level vs input level, differential 1mhz input, gain = 1 1mhz input level (v p-p ) 0 20 0 ?0 ?0 ?0 ?0 100 120 35 66002 f06 12 467 output level (dbv) 3rd harmonic 85 c 1db passband gain compression points 1mhz 25 c 1mhz 85 c 3rd harmonic 25 c 2nd harmonic 25 c 2nd harmonic 85 c
LT6600-20 10 66002f applicatio s i for atio wu u u figure 7 + 0.1 f 0.1 f 2.5v 2.5v + LT6600-20 3 4 1 7 2 8 5 6 r in r in 25 25 66002 f07 spectrum analyzer input 50 v in coilcraft ttwb-1010 1:1 noise the noise performance of the LT6600-20 can be evaluated with the circuit of figure 7. given the low noise output of the LT6600-20 and the 6db attenuation of the transformer coupling network, it is necessary to measure the noise floor of the spectrum analyzer and subtract the instrument noise from the filter noise measurement. example: with the ic removed and the 25 w resistors grounded, figure 7, measure the total integrated noise (e s ) of the spectrum analyzer from 10khz to 20mhz. with the ic inserted, the signal source (v in ) disconnected, and the input resistors grounded, measure the total integrated noise out of the filter (e o ). with the signal source con- nected, set the frequency to 1mhz and adjust the ampli- tude 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 1 lists the typical input referred integrated noise for various values of r in . figure 8 is plot of the noise spectral density as a function of frequency for an LT6600-20 with r in = 402 w using the fixture of figure 7 (the instrument noise has been sub- tracted from the results). table 1. noise performance input referred passband integrated noise input referred gain (v/v) r in 10khz to 20mhz noise dbm/hz 4 100 w 42 m v rms C148 2 200 w 67 m v rms C143 1 402 w 118 m v rms C139 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 figure 8. input referred noise, gain = 1 frequency (mhz) 0.1 0 30 40 50 1 10 100 66002 f08 20 10 0 150 200 250 100 50 noise spectral density (nv rms / hz) integrated noise ( v rms ) spectral density integrated v s = 5v noise power added together, the resulting calculated noise level will be higher than the true differential noise. power dissipation the LT6600-20 amplifiers combine high speed with large- signal currents in a small package. there is a need to ensure that the die junction temperature does not exceed 150 c. the LT6600-20 package has pin 6 fused to the lead frame to enhance thermal conduction when connecting 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 total of 660 square millimeters connected to pin 6 of the LT6600-20 (330 square millimeters on each side of the pc board) will result in a thermal resistance, q ja , of about 85 c/w. without the extra metal trace connected to the v C pin to provide a heat sink, the thermal resistance will be around 105 c/w. table 2 can be used as a guide when considering thermal resistance.
LT6600-20 11 66002f 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 ? q ja ) = t a + (v s ? i s ? q ja ) where the supply current, i s , is a function of signal level, load impedance, temperature and common mode voltages. table 2. LT6600-20 so-8 package thermal resistance copper area topside backside board area thermal resistance (mm 2 ) (mm 2 ) (mm 2 ) (junction-to-ambient) 1100 1100 2500 65 c/w 330 330 2500 85 c/w 35 35 2500 95 c/w 35 0 2500 100 c/w 0 0 2500 105 c/w for a given supply voltage, the worst-case power dissi- pation occurs when the differential input signal is maxi- mum, 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 tem- perature, measure the supply current under these worst- case conditions, estimate the thermal resistance from table 2, then apply the equation for t j . for example, using the circuit in figure 3 with a dc differential input voltage of 250mv, a differential output voltage of 1v, no load resistance and an ambient temperature of 85 c, the supply current (current into pin 3) measures 55.5ma. assuming a pc board layout with a 35mm 2 copper trace, the q ja is 100 c/w. the resulting junction temperature is: t j = t a + (p d ? q ja ) = 85 + (5 ? 0.0555 ? 100) = 113 c when using higher supply voltages or when driving small impedances, more copper may be necessary to keep t j below 150 c. s8 package 8-lead plastic small outline (narrow .150 inch) (reference ltc dwg # 05-08-1610) u package descriptio applicatio s i for atio wu u u .016 ?.050 (0.406 ?1.270) .010 ?.020 (0.254 ?0.508) 45 0 ?8 typ .008 ?.010 (0.203 ?0.254) so8 0303 .053 ?.069 (1.346 ?1.752) .014 ?.019 (0.355 ?0.483) typ .004 ?.010 (0.101 ?0.254) .050 (1.270) bsc 1 2 3 4 .150 ?.157 (3.810 ?3.988) note 3 8 7 6 5 .189 ?.197 (4.801 ?5.004) note 3 .228 ?.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) 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 represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
LT6600-20 12 66002f related parts part number description comments ltc ? 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 lt6600-2.5 very low noise differential amplifier and 2.5mhz 86db s/n with 3v supply, so-8 lowpass filter lt6600-10 very low noise differential amplifier and 10mhz 82db s/n with 3v supply, so-8 lowpass filter linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 l fax: (408) 434-0507 l www.linear.com ? linear technology corporation 2003 lt/tp 0503 1k ? printed in usa typical applicatio u a 5th order, 20mhz lowpass filter + r c r r r 0.1 f 0.1 f v + v + 3 4 1 7 2 8 5 6 v out + v out LT6600-20 v out + v out 66002 ta02a v in v in + c = 1 2 ?r ?20mhz gain = , maximum gain = 4 402 2r frequency (mhz) gain (db) 0.1 10 ?0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 0 10 100 66002 ta04 1 v s = 2.5v gain = 1 c = 39pf r = 200 t a = 25 c amplitude response v out + 50mv/div differential input 200mv/div 100ns/div 66002 ta03 transient response, gain = 1

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