LT Dual Very Low Noise, Differential Amplifi er and 5MHz Lowpass Filter DESCRIPTION FEATURES APPLICATIONS TYPICAL APPLICATION

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1 FEATURES n Dua Differentia Ampifi er with MHz Lowpass Fiters 4th Order Fiters Approximates Chebyshev Response Guaranteed Phase and Gain Matching Resistor-Programmabe Differentia Gain n >8 Signa-to-Noise ( Suppy, P-P Output) n Low Distortion (MHz, P-P Output, 8Ω Load) HD: 9c HD: 9c n Specifi ed for Operation with, and ± Suppies n Fuy Differentia Inputs and Outputs n Adjustabe Output Common Mode otage n Sma 4mm mm.mm QFN Package APPLICATIONS n Dua Differentia ADC Driver and Fiter n Singe-Ended to Differentia Converter n Matched, Dua, Differentia Gain or Fiter Stage n Common Mode Transation of Differentia Signas n High Speed ADC Antiaiasing and DAC Smoothing in Wireess Infrastructure or Networking Appications n High Speed Test and Measurement Equipment n Medica Imaging LT4- Dua ery Low Noise, Differentia Ampifi er and MHz Lowpass Fiter DESCRIPTION The LT 4- consists of two matched, fuy differentia ampifiers, each with a 4th order, MHz owpass fiter. The fixed frequency owpass fiter approximates a Chebyshev response. By integrating a fiter and a differentia ampifier, distortion and noise are made exceptionay ow. At unity gain, the measured in-band signa-to-noise ratio is an impressive 8. At higher gains, the input referred noise decreases, aowing the part to process smaer input differentia signas without significanty degrading the signa-to-noise ratio. Gain and phase are we matched between the two channes. Gain for each channe is independenty programmed using two externa resistors. The LT4- enabes eve shifting by providing an adjustabe output common mode votage, making it idea for directy interfacing to ADCs. The LT4- is fuy specified for operation. The differentia design enabes outstanding performance at a P-P signa eve for a singe suppy. See the back page of this datasheet for a compete ist of reated singe and dua differentia ampifiers with integrated.mhz to MHz owpass fiters. L, LT, LTC and LTM are registered trademarks of Linear Technoogy Corporation. A other trademarks are the property of their respective owners. TYPICAL APPLICATION 8Ω.μF 8Ω 8Ω.μF 8Ω LT4- INA MIDA OCMA INA INB MIDB OCMB INB A OUTA OUTA B OUTB OUTB Ω Ω Ω Ω 8pF 8pF LTCxx AIN AIN DUAL ADC DOUT DOUT 4 TA NUMBER OF UNITS Channe to Channe Gain Matching TYPICAL UNITS T A = C GAIN = f IN = MHz.... GAIN MATCH () 4 TAb 4fa

2 LT4- ABSOLUTE MAXIMUM RATINGS (Note ) Tota Suppy otage... Operating Temperature Range (Note )...4 C to 8 C Specifi ed Temperature Range (Note )...4 C to 8 C Junction Temperature... C Storage Temperature Range... C to C Input Current IN, IN, OCM, MID (Note 8)...±mA PIN CONFIGURATION TOP IEW 4 MIDA INA INA 4 OCMA MIDB 8 9 INB INB 9 OUTA 8 OUTA A 4 OUTB 9 OUTB 8 OCMB 4 B UFF PACKAGE 4-LEAD (4mm mm) PLASTIC QFN T JMAX = C, θ JA = 4 C/W, θ JC = 4 C/W EXPOSED PAD (PIN ) IS, MUST BE SOLDERED TO PCB ORDEFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION SPECIFIED TEMPERATURE RANGE LT4CUFF-#PBF LT4CUFF-#TRPBF 4 4-Lead (4mm mm) Pastic QFN C to C LT4IUFF-#PBF LT4IUFF-#TRPBF 4 4-Lead (4mm mm) Pastic QFN 4 C to 8 C Consut LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a abe on the shipping container. Consut LTC Marketing for information on non-standard ead based fi nish parts. For more information on ead free part marking, go to: For more information on tape and ree specifi cations, go to: 4fa

3 LT4- ELECTRICAL CHARACTERISTICS The denotes the specifi cations which appy over the fu operating temperature range, otherwise specifi cations are at T A = C. Uness otherwise specifi ed S = ( =, = ), = 8Ω, and R LOAD = k. PARAMETER CONDITIONS MIN TYP MAX UNITS Fiter Gain Either Channe, S = IN = P-P, f IN = DC to khz.. IN = P-P, f IN = khz (Gain Reative to khz) IN = P-P, f IN =.MHz (Gain Reative to khz) IN = P-P, f IN = 4MHz (Gain Reative to khz) IN = P-P, f IN = MHz (Gain Reative to khz) IN = P-P, f IN = MHz (Gain Reative to khz) IN = P-P, f IN = MHz (Gain Reative to khz) Matching of Fiter Gain, S = Matching of Fiter Phase, S = Fiter Gain Either Channe, S = Matching of Fiter Gain, S = Matching of Fiter Phase, S = IN = P-P, f IN = DC to khz IN = P-P, f IN = khz (Gain Reative to khz) IN = P-P, f IN =.MHz (Gain Reative to khz) IN = P-P, f IN = 4MHz (Gain Reative to khz) IN = P-P, f IN = MHz (Gain Reative to khz) IN = P-P, f IN = MHz (Gain Reative to khz) IN = P-P, f IN = MHz (Gain Reative to khz) IN = P-P, f IN =.MHz IN = P-P, f IN = 4MHz IN = P-P, f IN = DC to khz IN = P-P, f IN = khz (Gain Reative to khz) IN = P-P, f IN =.MHz (Gain Reative to khz) IN = P-P, f IN = 4MHz (Gain Reative to khz) IN = P-P, f IN = MHz (Gain Reative to khz) IN = P-P, f IN = MHz (Gain Reative to khz) IN = P-P, f IN = MHz (Gain Reative to khz) IN = P-P, f IN = DC to khz IN = P-P, f IN = khz (Gain Reative to khz) IN = P-P, f IN =.MHz (Gain Reative to khz) IN = P-P, f IN = 4MHz (Gain Reative to khz) IN = P-P, f IN = MHz (Gain Reative to khz) IN = P-P, f IN = MHz (Gain Reative to khz) IN = P-P, f IN = MHz (Gain Reative to khz) IN = P-P, f IN =.MHz IN = P-P, f IN = 4MHz Fiter Gain Either Channe, S = ± IN = P-P, f IN = DC to khz...4 Fiter Gain, = 9Ω IN =. P-P, f IN = DC to khz S = S = S = ± Fiter Gain Temperature Coeffi cient (Note ) f IN = khz, IN = P-P 8 ppm/ C Noise Noise BW = khz to MHz, = 8Ω 4 μ RMS Distortion (Note 4) IN = P-P, f IN = MHz, R L = 8Ω nd Harmonic rd Harmonic IN = P-P, f IN = MHz, R L = 8Ω nd Harmonic rd Harmonic Channe Separation (Note 9) IN = P-P, f IN = MHz Differentia Output Swing Measured Between OUT and OUT, OCM Shorted to MID S = S = P-P_DIFF P-P_DIFF Input Bias Current Average of IN and IN μa deg deg deg deg c c c c 4fa

4 LT4- ELECTRICAL CHARACTERISTICS The denotes the specifi cations which appy over the fu operating temperature range, otherwise specifi cations are at T A = C. Uness otherwise specifi ed S = ( =, = ), = 8Ω, and R LOAD = k. PARAMETER CONDITIONS MIN TYP MAX UNITS Input Referred Differentia Offset = 8Ω S = S = S = ± 8 m m m = 9Ω S = S = S = ± Differentia Offset Drift μ/ C Input Common Mode otage (Note ) Differentia Input = m P-P, = 9Ω S = S = S = ±.. Output Common Mode otage (Note ) Output Common Mode Offset (with Respect to OCM ) Differentia Output = P-P, MID = OPEN S = S = S = ± S = S = S = ± Common Mode Rejection Ratio otage at MID S = S =.4... MID Input Resistance 4... kω OCM Bias Current OCM = MID = S / S = S = Power Suppy Current (Per Channe) S =, S = S =, S = S = ± m m m m m m μa μa ma ma ma Note : Stresses beyond those isted under Absoute Maximum Ratings may cause permanent damage to the device. Exposure to any Absoute Maximum Rating condition for extended periods may affect device reiabiity and ifetime. Note : This is the temperature coeffi cient of the interna feedback resistors assuming a temperature independent externa resistor ( ). Note : The input common mode votage is the average of the votages appied to the externa resistors ( ). Specifi cation guaranteed for 9Ω. Note 4: Distortion is measured differentiay using a differentia stimuus. The input common mode votage, the votage at OCM, and the votage at MID are equa to one haf of the tota power suppy votage. Note : Output common mode votage is the average of the OUT and OUT votages. The output common mode votage is equa to OCM. Note : The LT4C- is guaranteed functiona over the operating temperature range 4 C to 8 C. Note : The LT4C- is guaranteed to meet C to C specifications and is designed, characterized and expected to meet the extended temperature imits, but is not tested at 4 C to 8 C. The LT4I- is guaranteed to meet specified performance from 4 C to 8 C. Note 8: Input pins (IN, IN, OCM and MID ) are protected by steering diodes to either suppy. If the inputs shoud exceed either suppy votage, the input current shoud be imited to ess than ma. In addition, the inputs IN, IN are protected by a pair of back-to-back diodes. If the differentia input votage exceeds.4, the input current shoud be imited to ess than ma Note 9: Channe separation (the inverse of crosstak) is measured by driving a signa into one input whie terminating the other input. Channe separation is the ratio of the resuting output signa at the driven channe to the output at the channe that is not driven. 4 4fa

5 TYPICAL PERFORMAE CHARACTERISTICS LT4- GAIN () 4 8. Frequency Response FREQUEY (MHz) S = GAIN = T A = C 4 G GAIN () 4 Passband Gain and Group Deay GAIN DELAY 4 8 GAIN = T A = C FREQUEY (MHz) 4 G 9 8 DELAY (ns) GAIN () 9 8 Passband Gain and Group Deay Output Impedance vs Frequency Common Mode Rejection Ratio GAIN DELAY 4 4 GAIN = 4 T A = C FREQUEY (MHz) 9 8 DELAY (ns) OUTPUT IMPEDAE (Ω).. S = GAIN = T A = C FREQUEY (MHz) CMRR () S = GAIN = IN = P-P T A = C.. FREQUEY (MHz) 4 G 4 G4 4 G PSRR () 8 4 Power Suppy Rejection Ratio Distortion vs Frequency Distortion vs Frequency S = IN = m P-P T A = C TO DIFFOUT.. FREQUEY (MHz) DISTORTION (c) 8 9. DIFFERENTIAL INPUT, ND HARMONIC DIFFERENTIAL INPUT, RD HARMONIC SINGLE-ENDED INPUT, ND HARMONIC SINGLE-ENDED INPUT, RD HARMONIC S =, IN = P-P R L = 8Ω, T A = C, GAIN = FREQUEY (MHz) DISTORTION (c) 8 9. DIFFERENTIAL INPUT, ND HARMONIC DIFFERENTIAL INPUT, RD HARMONIC SINGLE-ENDED INPUT, ND HARMONIC SINGLE-ENDED INPUT, RD HARMONIC S =, IN = P-P R L = 8Ω, T A = C, GAIN = FREQUEY (MHz) 4 G 4 G 4 G8 4fa

6 LT4- TYPICAL PERFORMAE CHARACTERISTICS DISTORTION (c) Distortion vs Signa Leve S =, R L = 8Ω T A = C, GAIN = RD HARMONIC, MHz INPUT ND HARMONIC, MHz INPUT RD HARMONIC, MHz INPUT ND HARMONIC, MHz INPUT 4 INPUT LEEL ( P-P ) 4 G9 DISTORTION (c) Distortion vs Signa Leve ND HARMONIC MHz INPUT RD HARMONIC MHz INPUT RD HARMONIC MHz INPUT ND HARMONIC MHz INPUT S = R L = 8Ω, T A = C, GAIN = 4 INPUT LEEL ( P-P ) 4 G DISTORTION COMPONENT (c) Distortion vs Input Common Mode otage GAIN =, MID = S / P-P MHz INPUT R L = 8Ω, T A = C ND HARMONIC, S = RD HARMONIC, S = ND HARMONIC, S = RD HARMONIC, S = INPUT COMMON MODE OLTAGE RELATIE TO MID () 4 G DISTORTION COMPONENT (c) Distortion vs Input Common Mode otage GAIN = 4, MID = S / P-P MHz INPUT R L = 8Ω, T A = C ND HARMONIC, S = RD HARMONIC, S = ND HARMONIC, S = RD HARMONIC, S = INPUT COMMON MODE OLTAGE RELATIE TO MID () 4 G SUPPLY CURRENT (ma) Singe Channe Suppy Current vs Tota Suppy otage T A = 8 C T A = C T A = 4 C 4 8 TOTAL SUPPLY OLTAGE () 4 G OUT m/di OUT m/di IN m/di IN Transient Response, Differentia Gain =, Singe-Ended Input, Differentia Output ns/di 4 G4 OUTPUT LEEL () 8 Distortion vs Temperature PASSBAND GAIN COMPRESSION POINTS MHz T A = C MHz T A = 8 C 4 RD HARMONIC T A = 8 C RD HARMONIC T A = C ND HARMONIC T A = 8 C ND HARMONIC T A = C 4 MHz INPUT LEEL ( P-P ) DISTORTION COMPONENT (c) Distortion vs Output Common Mode otage GAIN = 4 MID = S / T A = C. P-P MHz INPUT R L = 8Ω OLTAGE OCM TO MID () ND HARMONIC, S = RD HARMONIC, S = ND HARMONIC, S = RD HARMONIC, S = ND HARMONIC, S = RD HARMONIC, S = 4 G 4 G 4fa

7 TYPICAL PERFORMAE CHARACTERISTICS LT4- NOISE DENSITY (n/ Hz) 4 4 Input Referred Noise INTEGRATED NOISE, GAIN = X INTEGRATED NOISE, GAIN = 4X NOISE DENSITY, GAIN = X NOISE DENSITY, GAIN = 4X INTEGRATED NOISE (μ) CHANNEL SEPARATION () 9 Channe Separation vs Frequency (Note 9) IN = P-P S = R L = 8Ω AT EACH OUTPUT GAIN =.. FREQUEY (MHz) k M M M FREQUEY (Hz) 4 G8 4 G PIN FUTIONS INA, INA (Pins, 4): Channe A Input Pins. Signas can be appied to either or both input pins through identica externa resistors,. The DC gain from the differentia inputs to the differentia outputs is 8Ω/. OCMA (Pin ): DC Common Mode Reference otage for the nd Fiter Stage in channe A. Its vaue programs the common mode votage of the differentia output of the fi ter. Pin is a high impedance input, which can be driven from an externa votage reference, or Pin can be tied to Pin 4 on the PC board. Pin shoud be bypassed with a.μf ceramic capacitor uness it is connected to a ground pane. (Pins, 4,,, ): Negative Power Suppy Pin. Can be ground. MIDB (Pin 8): The MIDB pin is internay biased at mid suppy, see Bock Diagram. For singe suppy operation the MIDB pin shoud be bypassed with a quaity.μf ceramic capacitor to. For dua suppy operation, Pin 8 can be bypassed or connected to a high quaity DC ground. A ground pane shoud be used. A poor ground wi increase noise and distortion. Pin 8 sets the output common mode votage of the st stage of the fi ter in channe B. It has a.kω impedance, and it can be overridden with an externa ow impedance votage source. INB, INB (Pins, ): Channe B Input Pins. Signas can be appied to either or both input pins through identica externa resistors,. The DC gain from differentia inputs to the differentia outputs is 8Ω/. OCMB (Pin 4): DC Common Mode Reference otage for the nd Fiter Stage in Channe B. Its vaue programs the common mode votage of the differentia output of the fiter. Pin 4 is a high impedance input, which can be driven from an externa votage reference, or Pin 4 can be tied to Pin 8 on the PC board. Pin 4 shoud be bypassed with a.μf ceramic or greater capacitor uness it is connected to a ground pane. A, B (Pins, ): Positive Power Suppy Pins for Channes A and B. For a singe. or suppy ( grounded) a quaity.μf ceramic bypass capacitor is required from each positive suppy pin ( A, B) to the negative suppy pin ( ). The bypass shoud be as cose as possibe to the IC. For dua suppy appications, bypass the negative suppy pins to ground and each of the positive suppy pins ( A, B) to ground with a quaity.μf ceramic capacitor. OUTB, OUTB (Pins 9, ): Output Pins. Pins 9 and are the fiter differentia outputs for channe B. With a typica short-circuit current imit greater than ±4mA, each pin can drive a Ω and/or pf oad to AC ground. 4fa

8 LT4- PIN FUTIONS OUTA, OUTA (Pins, 9): Output Pins. Pins and 9 are the fiter differentia outputs for channe A. With a typica short-circuit current imit greater than ±4mA, each pin can drive a Ω and/or pf oad to AC ground. MIDA (Pin 4): The MIDA pin is internay biased at mid suppy, see Bock Diagram. For singe suppy operation the MIDA pin shoud be bypassed with a quaity.μf ceramic capacitor to Pins. For dua suppy operation, Pin 4 can be bypassed or connected to a high quaity DC ground. A ground pane shoud be used. A poor ground wi increase noise and distortion. Pin 4 sets the output common mode votage of the st stage of the fi ter in channe A. It has a.kω impedance, and it can be overridden with an externa ow impedance votage source. Exposed Pad (Pin ):. The Exposed Pad must be sodered to PCB. If is separate from ground, tie the Exposed Pad to. BLOCK DIAGRAM MIDA A IN A IN A INA INA 8Ω OCM OP AMP k k 4Ω 4Ω OCM LOWPASS FILTER STAGE 4Ω OUTA OUTA 4Ω OCMA MIDB 8Ω 8Ω B k k LOWPASS FILTER STAGE 4Ω A IN B IN B INB INB OCM OP AMP 4Ω 4Ω OCM 4Ω OUTB OUTB 8Ω OCMB B 4 BD 8 4fa

9 APPLICATIONS INFORMATION Interfacing to the LT4- Note: The LT4- contains two identica fiters. The foowing appications information ony refers to one fi ter. The two fiters are independent except that they share the same negative suppy votage. The two fiters can be used simutaneousy by repicating the exampe circuits. The referenced pin numbers correspond to the A channe fiter. Each LT4- channe requires two equa externa resistors,, to set the differentia gain to 8Ω/. The inputs to the fiter are the votages IN and IN presented to these externa components, Figure. The difference between IN and IN is the differentia input votage. The average of IN and IN is the common mode input votage. Simiary, the votages OUT and OUT appearing at Pins and 9 of the LT4- are the fiter outputs. The difference between OUT and OUT is the differentia output votage. The average of OUT and OUT is the common mode output votage. Figure iustrates the LT4- operating with a singe. suppy and unity passband LT4- gain; the input signa is DC-couped. The common mode input votage is., and the differentia input votage is P-P. The common mode output votage is., and the differentia output votage is P-P for frequencies beow MHz. The common mode output votage is determined by the votage at OCM. Since OCM is shorted to MID, the output common mode is the mid suppy votage. In addition, the common mode input votage can be equa to the mid suppy votage of MID. Figure shows how to AC coupe signas into the LT4-. In this instance, the input is a singe-ended signa. AC-couping aows the processing of singe-ended or differentia signas with arbitrary common mode eves. The.μF couping capacitor and the 8Ω gain setting resistor form a high pass fiter, attenuating signas beow khz. Larger vaues of couping capacitors wi proportionay reduce this highpass frequency. In Figure the LT4- is providing of gain. The gain resistor has an optiona pf in parae to improve the passband f atness near MHz. The common mode output votage is set to...μf 8Ω 4 IN 4 / OUT LT4- IN.μF IN OUT 9 8Ω t IN Figure OUT OUT 4 F t IN t IN.μF.μF 8Ω.μF 8Ω 4 4..μF / LT4-9 OUT OUT OUT OUT 4 F t Figure m P-P (DIFF) IN IN t IN IN pf Ω Ω pf.μf.μf 4 4 / LT4-9 OUT OUT OUT OUT 4 F t Figure 4fa 9

10 LT4- APPLICATIONS INFORMATION CURRENT OUTPUT DAC 4 F4 I IN R I IN R.μF R R 4 4..μF / OUT LT4-9 OUT OUT OUT I IN IIN = 8 R R R NETWORK ANALYZER SOURCE Ω COILCRAFT TTWB- : 8Ω 4 4.Ω 8Ω..μF / LT4-9.μF. COILCRAFT TTWB-A 4: 4Ω 4Ω NETWORK ANALYZER INPUT Ω 4 F Use Figure 4 to determine the interface between the LT4- and a current output DAC. The gain, or transimpedance, is defined as A = OUT /I IN. To compute the transimpedance, use the foowing equation: A = 8 R RR Ω Figure 4 By setting R R = 8Ω, the gain equation reduces to A = R(Ω). The votage at the pins of the DAC is determined by R, R, the votage on MID and the DAC output current. Consider Figure 4 with R = 49.9Ω and R = Ω. The votage at MID, for S =., is.. The votage at the DAC pins is given by: R DAC = MID RR 8 I R R IN RR = m I IN 4.8Ω Evauating the LT4- The ow impedance eves and high frequency operation of the LT4- require some attention to the matching networks between the LT4- and other devices. The previous exampes assume an idea (Ω) source impedance and a arge (k) oad resistance. Among practica exampes where impedance must be considered is the evauation of the LT4- with a network anayzer. Figure is a aboratory setup that can be used to characterize the LT4- using singe-ended instruments with Ω source impedance and Ω input impedance. For a unity Figure gain configuration the LT4- requires an 8Ω source resistance yet the network anayzer output is caibrated for a Ω oad resistance. The : transformer,.ω and 8Ω resistors satisfy the two constraints above. The transformer converts the singe-ended source into a differentia stimuus. Simiary, the output of the LT4- wi have ower distortion with arger oad resistance yet the anayzer input is typicay Ω. The 4: turns (: impedance) transformer and the two 4Ω resistors of Figure, present the output of the LT4- with a Ω differentia oad, or the equivaent of 8Ω to ground at each output. The impedance seen by the network anayzer input is sti Ω, reducing refections in the cabing between the transformer and anayzer input. Differentia and Common Mode otage Ranges The differentia ampifiers inside the LT4- contain circuitry to imit the maximum peak-to-peak differentia votage through the fi ter. This imiting function prevents excessive power dissipation in the interna circuitry and provides output short-circuit protection. The imiting function begins to take effect at output signa eves above P-P and it becomes noticeabe above. P-P. This is iustrated in Figure ; the LT4- channe was configured with unity passband gain and the input of the fiter was driven with a MHz signa. Because this votage imiting takes pace we before the output stage of the fiter reaches the suppy rais, the input/output behavior of the IC shown in Figure is reativey independent of the power suppy votage. 4fa

11 LT4- APPLICATIONS INFORMATION OUTPUT LEEL () PASSBAND GAIN COMPRESSION POINTS MHz T A = C MHz T A = 8 C 4 RD HARMONIC T A = 8 C RD HARMONIC T A = C 8 ND HARMONIC T A = 8 C ND HARMONIC T A = C, GAIN = 4 MHz INPUT LEEL ( P-P ) F Figure. Differentia otage Range The two ampifiers inside the LT4- channe have independent contro of their output common mode votage (see the Bock Diagram section). The foowing guideines wi optimize the performance of the fi ter. MID can be aowed to foat, but it must be bypassed to an AC ground with a.μf capacitor or some instabiity may be observed. MID can be driven from a ow impedance source, provided it remains at east. above and at east. beow. An interna resistor divider sets the votage of MID. Whie the interna k resistors are we matched, their absoute vaue can vary by ±%. This shoud be taken into consideration when connecting an externa resistor network to ater the votage of MID. OCM can be shorted to MID for simpicity. If a different common mode output votage is required, connect OCM to a votage source or resistor network. For and. suppies the votage at OCM must be ess than or equa to the mid suppy eve. For exampe, votage ( OCM ). on a singe. suppy. For power suppy votages higher than. the votage at OCM can be set above mid suppy. The votage on OCM shoud not be more than beow the votage on MID. The votage on OCM shoud not be more than above the votage on MID. OCM is a high impedance input. The LT4- was designed to process a variety of input signas incuding signas centered on the mid suppy votage and signas that swing between ground and a positive votage in a singe suppy system (Figure ). The aowabe range of the input common mode votage (the average of IN and IN in Figure ) is determined by the power suppy eve and gain setting (see the Eectrica Characteristics section). Common Mode DC Currents In appications ike Figure and Figure where the LT4- not ony provides owpass fitering but aso eve shifts the common mode votage of the input signa, DC currents wi be generated through the DC path between input and output terminas. Minimize these currents to decrease power dissipation and distortion. Consider the appication in Figure. MID sets the output common mode votage of the st differentia ampifi er inside the LT4- (see the Bock Diagram section) at.. Since the input common mode votage is near, there wi be approximatey a tota of. drop across the series combination of the interna 8Ω feedback resistor and the externa Ω input resistor. The resuting.ma common mode DC current in each input path, must be absorbed by the sources IN and IN. OCM sets the common mode output votage of the nd differentia ampifier inside the LT4- channe, and therefore sets the common mode output votage of the fiter. Since, in the exampe of Figure, OCM differs from MID by., an additiona.ma (μa per side) of DC current wi fow in the resistors couping the st differentia ampifier output stage to the fiter output. Thus, a tota of.ma is used to transate the common mode votages. A simpe modification to Figure wi reduce the DC common mode currents by %. If MID is shorted to OCM the common mode output votage of both op amp stages wi be and the resuting DC current wi be 4mA. Of course, by AC couping the inputs of Figure and shorting MID to OCM, the common mode DC current is eiminated. 4fa

12 LT4- APPLICATIONS INFORMATION Noise The noise performance of the LT4- channe can be evauated with the circuit of Figure. Given the ow noise output of the LT4- and the attenuation of the transformer couping network, it is necessary to measure the noise foor of the spectrum anayzer and subtract the instrument noise from the fi ter noise measurement. Exampe: With the IC removed and the Ω resistors grounded, Figure, measure the tota integrated noise (e S ) of the spectrum anayzer from khz to MHz. With the IC inserted, the signa source ( IN ) disconnected, and the input resistors grounded, measure the tota integrated noise out of the fiter (e O ). With the signa source connected, set the frequency to MHz and adjust the ampitude unti IN measures m P-P. Measure the output ampitude, OUT, and compute the passband gain A = OUT / IN. Now compute the input referred integrated noise (e IN ) as: e IN = (e O ) (e S ) A Tabe ists the typica input referred integrated noise for various vaues of. Tabe. Noise Performance INPUT REFERRED PASSBAND GAIN INTEGRATED NOISE khz TO MHz INPUT REFERRED NOISE m/hz 4 Ω 4μ RMS 49 4Ω 8μ RMS 4 8Ω 9μ RMS 4 IN 4 4..μF / LT4-9 Ω Ω COILCRAFT TTWB- : SPECTRUM ANALYZER INPUT Ω 4 F NOISE DENSITY (n/ Hz) 4 4. INTEGRATED NOISE, GAIN = X INTEGRATED NOISE, GAIN = 4X NOISE DENSITY, GAIN = X NOISE DENSITY, GAIN = 4X. FREQUEY (MHz) 4 F8 Figure 8. Input Referred Noise Figure 8 is pot of the noise spectra density as a function of frequency for an LT4- with = 8Ω using the fixture of Figure (the instrument noise has been subtracted from the resuts). The noise at each output is comprised of a differentia component and a common mode component. Using a transformer or combiner to convert the differentia outputs to singe-ended signa rejects the common mode noise and gives a true measure of the S/N achievabe in the system. Conversey, if each output is measured individuay and the noise power added together, the resuting cacuated noise eve wi be higher than the true differentia noise. Power Dissipation The LT4- ampifi ers combine high speed with arge signa currents in a sma package. There is a need to ensure that the die s junction temperature does not exceed C. The LT4- has an exposed pad (pin ) which is connected to the ower suppy ( ). Connecting the pad to a ground pane heps to dissipate the heat generated by the chip. Meta trace and pated through-hoes can be used to spread the heat generated by the device to the backside of the PC board INTEGRATED NOISE (μ).μf. Figure 4fa

13 APPLICATIONS INFORMATION Junction temperature, T J, is cacuated from the ambient temperature, T A, and power dissipation, P D. The power dissipation is the product of suppy votage, S, and tota suppy current, I S. Therefore, the junction temperature is given by: T J = T A (P D θ JA ) = T A ( S I S θ JA ) where the suppy current, I S, is a function of signa eve, oad impedance, temperature and common mode votages. For a given suppy votage, the worst-case power dissipation occurs when the differentia input signa is maximum, the common mode currents are maximum (see the Appications Information section regarding Common Mode DC Currents), the oad impedance is sma and LT4- the ambient temperature is maximum. To compute the junction temperature, measure the suppy current under these worst-case conditions, use 4 C/W as the package therma resistance, then appy the equation for T J. For exampe, using the circuit in Figure with DC differentia input votage of m, a differentia output votage of, k oad resistance and an ambient temperature of 8 C, the suppy current (current into ) measures.ma per channe. The resuting junction temperature is: T J = T A (P D θ JA ) = 8 (. 4) = 99 C. The therma resistance can be affected by the amount of copper on the PCB that is connected to. The therma resistance of the circuit can increase if the exposed pad is not connected to a arge ground pane with a number of vias. TYPICAL APPLICATIONS Dua, Matched, MHz Lowpass Fiter.μF MHz Phase Distribution ( Units) I IN OCM (-.) Q IN.μF.μF / LT4-4 9.μF / 9 LT4- Q OUT GAIN = 8Ω I OUT PERCENTAGE OF UNITS (%) MHz PHASE (DEG) 4 TA 4fa

14 LT4- TYPICAL APPLICATIONS Dua, Matched, th Order, MHz Lowpass Fiter Singe-Ended Input (I IN and Q IN ) and Differentia Output (I OUT and Q OUT ) I IN.μF.μF Q IN 49Ω 49Ω 49Ω.μF 4 8 LT8 INA INB SA OUTA OUTA GNDA SB OUTB OUTB GNDB EN 4 9 8Ω Ω 49Ω / LT4- I OUT 49Ω 9 8Ω.μF I GAIN = OUT Q OR OUT = I IN Q IN.μF 8Ω 9 8 / LT4-4 Q OUT 8Ω 4.μF 4 TA Frequency Response Transient Response GAIN () LOG (I OUT /I IN ) OR LOG (QOUT/Q IN ) OUTPUT (I OUT OR Q OUT ) m/di INPUT (I IN OR Q IN ) m/di FREQUEY (MHz) ns/di 4 TA4b 4 TA4a 4 4fa

15 PACKAGE DESCRIPTION UFF Package 4-Lead Pastic QFN (4mm mm) (Reference LTC DWG # -8-8 Rev Ø) LT4-. ±. 4. ±.. ±.. REF.9 ±..4 ±..8 ±..9 ±..4 ±. PACKAGE OUTLINE.9 ±.. ±.. BSC. REF. ±.. ±. PIN TOP MARK (NOTE ) 4. ±. RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED. ±. R =. TYP.9 ±.. REF 4 PIN NOTCH R =. OR. 4 CHAMFER.4 ±..4 ±.. ±.. REF.8 ±..9 ±..4 ±.. REF.. R =. TYP. BSC BOTTOM IEW EXPOSED PAD (UFF4) QFN 8 RE Ø. ±..99 ±. NOTE:. DRAWING IS NOT A JEDEC PACKAGE OUTLINE. DRAWING NOT TO SCALE. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT ILUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED.mm ON ANY SIDE. EXPOSED PAD SHALL BE SOLDER PLATED. SHADED AREA IS ONLY A REFEREE FOR PIN LOCATION ON THE TOP AND BOTTOM OF PACKAGE Information furnished by Linear Technoogy Corporation is beieved to be accurate and reiabe. However, no responsibiity is assumed for its use. Linear Technoogy Corporation makes no representation that the interconnection of its circuits as described herein wi not infringe on existing patent rights. 4fa

16 LT4- RELATED PARTS PART NUMBER DESCRIPTION COMMENTS Integrated Fiters LTC- ery Low Noise, 8th Order Fiter Buiding Bock Lowpass and Bandpass Fiters up to khz LTC- khz Linear Phase Lowpass Fiter Continuous Time, th Order, Differentia LTC- Low Noise,.MHz Lowpass Fiter Continuous Time, th Order, Differentia LT8 ery Low Noise, 4th Order Fiter Buiding Bock Lowpass and Bandpass Fiters up to MHz LTC9- Linear Phase, Tunabe th Order Lowpass Fiter Singe-Resistor Programmabe Cut-Off to khz LT-. ery Low Noise Differentia.MHz Lowpass Fiter SNR = 8 at Suppy, 4th Order Fiter LT- ery Low Noise Differentia MHz Lowpass Fiter SNR = 8 at Suppy, 4th Order Fiter LT- ery Low Noise Differentia MHz Lowpass Fiter SNR = 8 at Suppy, 4th Order Fiter LT- ery Low Noise Differentia MHz Lowpass Fiter SNR = at Suppy, 4th Order Fiter LT- ery Low Noise Differentia MHz Lowpass Fiter SNR = at Suppy, 4th Order Fiter LTC Low Noise, Fuy Differentia, Pin Confi gurabe Ampifi er/driver/nd Order Fiter Buiding Bock LTC Dua Adjustabe Lowpass Fiter for RFID LTC Dua Adjustabe Lowpass Fiter for Communications LT4-. Dua ery Low Noise, Differentia Ampifi er and.mhz Lowpass Fiter SNR = 8 at Suppy, 4th Order Fiter LT4- Dua ery Low Noise, Differentia Ampifi er and MHz Lowpass Fiter SNR = 8 at Suppy, 4th Order Fiter LT4- Dua ery Low Noise, Differentia Ampifi er and MHz Lowpass Fiter SNR = at Suppy, 4th Order Fiter LT 49 RE A PRINTED IN USA Linear Technoogy Corporation McCarthy Bvd., Mipitas, CA 9-4 (48) 4-9 FAX: (48) LINEAR TECHNOLOGY CORPORATION 8 4fa