LT MHz Low Distortion, Low Noise Differential Amplifi er/ ADC Driver (A V = 6dB) DESCRIPTION FEATURES APPLICATIONS TYPICAL APPLICATION

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1 FEATURES n 3 MHz 3dB Bandwidth n Fixed Gain of 6dB n Low Distortion: 49dBm OIP3, dbc HD3 (MHz, V P-P ) n Low Noise:.6dB NF, e n = 3.nV/ Hz (MHz) n Differential Inputs and Outputs n Additional Filtered Outputs n Adjustable Output Common Mode Voltage n DC- or AC-Coupled Operation n Minimal Support Circuitry Required n Small.7mm Profi le 6-Lead 3mm 3mm QFN Package APPLICATIONS n Differential ADC Driver for: Imaging Communications n Differential Driver/Receiver n Single Ended to Differential Conversion n Differential to Single Ended Conversion n Level Shifting n IF Sampling Receivers n SAW Filter Interfacing/Buffering L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. LT64-6 3MHz Low Distortion, Low Noise Differential Amplifi er/ ADC Driver (A V = 6dB) DESCRIPTION The LT 64-6 is a low distortion, low noise differential amplifier/adc driver for use in applications from DC to 3MHz. The LT64-6 has been designed for ease of use, with minimal support circuitry required. Exceptionally low input-referred noise and low distortion (with either single-ended or differential inputs) make the LT64-6 an excellent solution for driving high speed -bit and 4-bit ADCs. In addition to the normal unfi ltered outputs ( and ), the LT64-6 has a built-in 7MHz differential low pass filter and an additional pair of filtered outputs (FILTERED, FILTERED) to reduce external filtering components when driving high speed ADCs. The output common mode voltage is easily set via the V OCM pin, eliminating an output transformer or AC-coupling capacitors in many applications. The LT64-6 is designed to meet the demanding requirements of communications transceiver applications. It can be used as a differential ADC driver, a general-purpose differential gain block, or in other applications requiring differential drive. The LT64-6 can be used in data acquisition systems required to function at frequencies down to DC. The LT64-6 operates on a V supply and consumes 3mA. It comes in a compact 6-lead 3mm 3mm QFN package and operates over a 4 C to C temperature range. TYPICAL APPLICATION V Distortion vs Frequency, Differential Input, No R LOAD 4 V OUT = V P-P.μF.μF.μF IF IN INB V CC INA V OCM LT64-6 +INB +INA.μF Ω Ω V CM AIN + LTC 49 AIN 646 TAa DISTORTION (dbc) HD3 HD 646 TAb 646fa

2 LT64-6 ABSOLUTE MAXIMUM RATINGS (Note ) Total Supply Voltage (V CCA /V CCB /V CCC to A /B /C )...V Input Current (+INA, INA, +INB, INB, V OCM, ENABLE)...±mA Output Current (Continuous),...±mA FILTERED, FILTERED...±3mA Output Short-Circuit Duration (Note )... Indefinite Operating Temperature Range (Note 3)... 4 C to C Specifi ed Temperature Range (Note 4)... 4 C to C Storage Temperature Range... 6 C to C Junction Temperature... C PIN CONFIGURATION V CCC V OCM V CCA A 3 4 TOP VIEW +INA +INB INA INB FILTERED FILTERED C ENABLE V CCB 9 B UD PACKAGE 6-LEAD (3mm 3mm) PLASTIC QFN T JMAX = C, θ JA = 6 C/W, θ JC = 4. C/W EXPOSED PAD IS (PIN 7) MUST BE SOLDERED TO THE PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT64CUD-6#PBF LT64CUD-6#TRPBF LBZZ 6-Lead (3mm 3mm) Plastic QFN 4 C to C LT64IUD-6#PBF LT64IUD-6#TRPBF LBZZ 6-Lead (3mm 3mm) Plastic QFN 4 C to C LEAD BASED FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT64CUD-6 LT64CUD-6#TR LBZZ 6-Lead (3mm 3mm) Plastic QFN 4 C to C LT64IUD-6 LT64IUD-6#TR LBZZ 6-Lead (3mm 3mm) Plastic QFN 4 C to C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. For more information on lead free part marking, go to: For more information on tape and reel specifi cations, go to: DC ELECTRICAL CHARACTERISTICS The l denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at T A = C. V CCA = V CCB = V CCC = V, A = B = C = V, ENABLE =.V, +INA shorted to +INB (+IN), INA shorted to INB ( IN), V OCM =.V, Input common mode voltage =.V, no R LOAD unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Input/Output Characteristics (+INA, +INB, INA, INB,,, FILTERED, FILTERED) GDIFF Gain Differential (, ), V IN = ±mv Differential l db V SWINGMIN Single-Ended,, FILTERED, FILTERED, V IN = ±.V Differential l..3. V V V SWINGMAX Single-Ended,, FILTERED, FILTERED, V IN = ±.V Differential l V V V SWINGDIFF Output Voltage Swing Differential (, ), V IN = ±.V Differential 6. 7 V P-P l.6 V P-P I OUT Output Current Drive l ±3 ±3 ma V OS Input Offset Voltage mv l mv 646fa

3 DC ELECTRICAL CHARACTERISTICS LT64-6 The l denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at T A = C. V CCA = V CCB = V CCC = V, A = B = C = V, ENABLE =.V, +INA shorted to +INB (+IN), INA shorted to INB ( IN), V OCM =.V, Input common mode voltage =.V, no R LOAD unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS TCV OS Input Offset Voltage Drift T MIN to T MAX l. μv/ C I VRMIN Input Voltage Range, MIN Single-Ended l. V I VRMAX Input Voltage Range, MAX Single-Ended l. V R INDIFF Input Resistance l 7 4 Ω C INDIFF Input Capacitance pf CMRR Common Mode Rejection Ratio Input Common Mode.V to.v l 4 6 db R OUTDIFF Output Resistance.3 Ω C OUTDIFF Output Capacitance. pf Common Mode Voltage Control (V OCM Pin) GCM Common Mode Gain Differential (, ), V OCM =.V to 3.6V Differential (, ), V OCM =.4V to 3.4V V OCMMIN Output Common Mode Voltage Adjustment Range, MIN V OCMMAX Output Common Mode Voltage Single-Ended Adjustment Range, MAX V OSCM Output Common Mode Offset Voltage l l l Measured from V OCM to Average of and mv I BIASCM V OCM Input Bias Current l μa R INCM V OCM Input Resistance l. 3 MΩ C INCM V OCM Input Capacitance pf ENABLE Pin V IL ENABLE Input Low Voltage l. V V IH ENABLE Input High Voltage l V I IL ENABLE Input Low Current ENABLE =.V l. μa I IH ENABLE Input High Current ENABLE = V l 3 μa Power Supply V S Operating Range l 4. V I S Supply Current ENABLE =.V l ma I SDISABLED Supply Current (Disabled) ENABLE = V l μa PSRR Power Supply Rejection Ratio 4V to.v l 9 db V/V V/V V V V V 646fa 3

4 LT64-6 AC ELECTRICAL CHARACTERISTICS T A = C, V CCA = V CCB = V CCC = V, A = B = C = V, E N A B L E =.V, +INA shorted to +INB (+IN), INA shorted to INB ( IN), V OCM =.V, Input common mode voltage =.V, no R LOAD unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Input/Output Characteristics 3dBBW 3dB Bandwidth mv P-P Differential (, ) 3 MHz.dBBW Bandwidth for.db Flatness mv P-P Differential (, ) 3 MHz.dBBW Bandwidth for.db Flatness mv P-P Differential (, ) MHz SR Slew Rate 3.V P-P Differential (, ) 4 V/μs t s% % Settling % Settling for a V P-P Differential Step ns (, ) t ON Turn-On Time ns t OFF Turn-Off Time. μs Common Mode Voltage Control (V OCM Pin) 3dBBW CM Common Mode Small-Signal 3dB.V P-P at V OCM, Measured Single-Ended at MHz Bandwidth and SR CM Common Mode Slew Rate.3V to 3.4V Step at V OCM V/μs Noise/Harmonic Performance Input/Output Characteristics MHz Signal Second/Third Harmonic Distortion V P-P Differential (FILTERED, FILTERED) 6 dbc V P-P Differential (, ) 4 dbc Third-Order IMD V P-P Differential Composite (FILTERED, FILTERED), f = 9.MHz, f =.MHz dbc OIP3 M Output Third-Order Intercept Differential (FILTERED, FILTERED), 3 dbm f = 9.MHz, f =.MHz (Note ) NF Noise Figure Measured Using DC94A Demo Board.6 db e nm Input Referred Noise Voltage Density 3. nv/ Hz db Compression Point R L = Ω (Note ).7 dbm MHz Signal Second/Third Harmonic Distortion V P-P Differential (FILTERED, FILTERED) 4 dbc V P-P Differential (, ) 73 dbc Third-Order IMD V P-P Differential Composite (FILTERED, 9 dbc FILTERED), f = 9.MHz, f =.MHz V P-P Differential Composite (, ), 7 dbc R L = 4Ω, f = 9.MHz, f =.MHz OIP3 M Output Third-Order Intercept Differential (FILTERED, FILTERED), 49 dbm f = 9.MHz, f =.MHz NF Noise Figure Measured Using DC94A Demo Board (Note ).6 db e nm Input Referred Noise Voltage Density 3. nv/ Hz db Compression Point R L = Ω (Note ) 7.7 dbm 4 646fa

5 AC ELECTRICAL CHARACTERISTICS LT64-6 T A = C, V CCA = V CCB = V CCC = V,A = B = C = V, E N A B L E =.V, +INA shorted to +INB (+IN), INA shorted to INB ( IN), V OCM =.V, Input common mode voltage =.V, no R LOAD unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS MHz Signal Second/Third Harmonic Distortion V P-P Differential (FILTERED, FILTERED) 4 dbc V P-P Differential (, ) 69 dbc Third-Order IMD V P-P Differential Composite (FILTERED, dbc FILTERED), f = 4.MHz, f =.MHz V P-P Differential Composite (, ), 67 dbc R L = 4Ω, f = 4.MHz, f =.MHz OIP3 M Output Third-Order Intercept Differential (FILTERED, FILTERED), 47 dbm f = 4.MHz, f =.MHz (Note ) NF Noise Figure Measured Using DC94A Demo Board.6 db e nm Input Referred Noise Voltage Density 3.9 nv/ Hz db Compression Point R L = Ω (Note ) 7. dbm Note : 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 : As long as output current and junction temperature are kept below the Absolute Maximum Ratings, no damage to the part will occur. Note 3: The LT64 is guaranteed functional over the operating temperature range of 4 C to C. Note 4: The LT64C is guaranteed to meet specifi ed performance from C to 7 C. It is designed, characterized and expected to meet specified performance from 4 C and C but is not tested or QA sampled at these temperatures. The LT64I is guaranteed to meet specified performance from 4 C to C. Note : Since the LT64-6 is a feedback amplifi er with low output impedance, a resistive load is not required when driving an ADC. Therefore, typical output power is very small. In order to compare the LT64-6 with typical g m amplifi ers that require Ω output loading, the LT64-6 output voltage swing driving an ADC is converted to OIP3 and PdB as if it were driving a Ω load. TYPICAL PERFORMANCE CHARACTERISTICS GAIN (db) Frequency Response, R LOAD = 4Ω UNFILTERED FILTERED V IN = mv P-P UNFILTERED: R LOAD = 4Ω FILTERED: R LOAD = 3Ω (EXTERNAL) + Ω (INTERNAL, ) 646 G GAIN (db) 3 Frequency Response vs C LOAD, R LOAD = 4Ω pf.6pf pf pf 646 G GAIN (db) Frequency Response, R LOAD = Ω 9 UN V IN = mv P-P UNFILTERED: R LOAD = Ω 4 FILTERED: R LOAD = Ω 7 (INTERNAL, ) G3 646fa

6 LT64-6 TYPICAL PERFORMANCE CHARACTERISTICS THIRD ORDER IMD (dbc) Third Order Intermodulation Distortion vs Frequency, Differential Input, No R LOAD TONES V P-P COMPOSITE MHz TONE SPACING UNFILTERED OUTPUTS FILTERED OUTPUTS THIRD ORDER IMD (dbc) Third Order Intermodulation Distortion vs Frequency, Differential Input, R LOAD = 4Ω TONES V P-P COMPOSITE MHz TONE SPACING UNFILTERED OUTPUTS FILTERED OUTPUTS OUTPUT IP3 (dbm) Output Third Order Intercept vs Frequency, Differential Input, No R LOAD UN Output Third Order Intercept vs Frequency, Differential Input, R LOAD = 4Ω FILTERED OUTPUTS 646 G4 4 Distortion vs Frequency, Differential Input, No R LOAD V OUT = V P-P 646 G 4 Distortion vs Frequency, Differential Input, No R LOAD UN V OUT = V P-P 646 G6 OUTPUT IP3 (db) UNFILTERED OUTPUTS DISTORTION (dbc) 6 7 HD3 DISTORTION (dbc) 6 7 HD3 3 7 TONES V P-P COMPOSITE MHz TONE SPACING HD 9 HD 646 G7 646 G 646 G9 DISTORTION (dbc) Distortion vs Output Amplitude, MHz Differential Input, No R LOAD HD3 HD DISTORTION (dbc) 7 7 Distortion vs Output Amplitude, MHz Differential Input, No R LOAD UN HD3 HD OUTPUT db COMPRESSION (dbm) Output db Compression vs Frequency UN Ω LOAD 4Ω LOAD OUTPUT AMPLITUDE (dbm) OUTPUT AMPLITUDE (dbm) 646 G 646 G 646 G 6 646fa

7 LT64-6 TYPICAL PERFORMANCE CHARACTERISTICS NOISE FIGURE (db) Noise Figure vs Frequency MEASURED USING DC94 DEMO BOARD INPUT REFERRED NOISE VOLTAGE (nv/ Hz) 3 3 Input Referred Noise Voltage vs Frequency ISOLATION (db) Reverse Isolation vs Frequency UN G3 646 G4 646 G INPUT IMPEDANCE (MAGNITUDE Ω, PHASE) 4 3 Differential Input Impedance vs Frequency IMPEDANCE MAGNITUDE IMPEDANCE PHASE 646 G6 OUTPUT IMPEDANCE (Ω) Differential Output Impedance vs Frequency UN 646 G7 INPUT REFLECTION COEFFICIENT (S) 3 3 Input Refl ection Coeffi cient vs Frequency MEASURED USING DC94 DEMO BOARD 646 G OUTPUT REFLECTION COEFFICIENT (S) Output Refl ection Coeffi cient vs Frequency PSRR, CMRR vs Frequency Small-Signal Transient Response MEASURED USING DC94 DEMO BOARD 646 G9 PSRR, CMRR (db) UN PSRR CMRR 646 G VOLTAGE (V) TIME (ns/div) 646 G 646fa 7

8 LT64-6 TYPICAL PERFORMANCE CHARACTERISTICS Overdrive Recovery Time Distortion vs Output Common Mode Voltage, LT64-6 Driving an LTC49 4-Bit ADC Turn-On Time OUTPUT VOLTAGE (V) R LOAD = Ω PER OUTPUT DISTORTION (dbc) NO R LOAD V OUT = MHz V P-P HD HD3 VOLTAGE (V) R LOAD = Ω PER OUTPUT. TIME (ns/div) OUTPUT COMMON MODE VOLTAGE (V). ENABLE TIME (ns/div) 646 G 646 G3 646 G4 VOLTAGE (V) Turn-Off Time 4. R LOAD = Ω PER OUTPUT ENABLE. TIME (ns/div) AMPLITUDE (dbfs) MHz 9 Point FFT, LT64-6 Driving an LTC49 4-Bit ADC 9 POINT FFT f IN = MHz, dbfs AMPLITUDE (dbfs) MHz 9 Point FFT, LT64-6 Driving an LTC49 4-Bit ADC 9 POINT FFT f IN = MHz, dbfs G 646 G6 646 G7 AMPLITUDE (dbfs) MHz 9 Point FFT, LT64-6 Driving an LTC49 4-Bit ADC 9 POINT FFT f IN = MHz, dbfs AMPLITUDE (dbfs) MHz -Tone 376 Point FFT, LT64-6 Driving an LTC49 4-Bit ADC 376 POINT FFT TONE AT 9.MHz, 7dBFS TONE AT.MHz, 7dBFS G 646 G9 646fa

9 PIN FUNCTIONS V OCM (Pin ): This pin sets the output common mode voltage. Without additional biasing, both inputs bias to this voltage as well. This input is high impedance. V CCA, V CCB, V CCC (Pins 3,, ): Positive Power Supply (Normally Tied to V). All three pins must be tied to the same voltage. Bypass each pin with pf and.μf capacitors as close to the package as possible. Split supplies are possible as long as the voltage between V CC and is V. A, B, C (Pins 4, 9, ): Negative Power Supply (Normally Tied to Ground). All three pins must be tied to the same voltage. Split supplies are possible as long as the voltage between V CC and is V. If these pins are not tied to ground, bypass each pin with pf and.μf capacitors as close to the package as possible., (Pins, ): Outputs (Unfiltered). These pins are high bandwidth, low-impedance outputs. The DC output voltage at these pins is set to the voltage applied at V OCM. LT64-6 FILTERED, FILTERED (Pins 6, 7): Filtered Outputs. These pins add a series Ω resistor from the unfiltered outputs and three 4pF capacitors. Each output has 4pF to, plus an additional 4pF between each pin (See the Block Diagram). This filter has a 3dB bandwidth of 7MHz. ENABLE (Pin ): This pin is a TTL logic input referenced to the C pin. If low, the LT64-6 is enabled and draws typically 3mA of supply current. If high, the LT64-6 is disabled and draws typically μa. +INA, +INB (Pins, 6): Positive Inputs. These pins are normally tied together. These inputs may be DC- or ACcoupled. If the inputs are AC-coupled, they will self-bias to the voltage applied to the V OCM pin. INA, INB (Pins 4, 3): Negative Inputs. These pins are normally tied together. These inputs may be DC- or ACcoupled. If the inputs are AC-coupled, they will self-bias to the voltage applied to the V OCM pin. Exposed Pad (Pin 7): Tie the pad to C (Pin ). If split supplies are used, DO NOT tie the pad to ground. 646fa 9

10 LT64-6 BLOCK DIAGRAM Ω A INA 4 INB 3 Ω Ω + A V CCA 4pF FILTERED Ω 6 A Ω V CCC + V OCM C 4pF +INA 6 +INB Ω Ω Ω + B V CCB B C Ω 4pF FILTERED 7 Ω B BIAS 3 V CCA V CCB V CCC ENABLE 4 A 9 B C 646 BD APPLICATIONS INFORMATION Circuit Description The LT64-6 is a low noise, low distortion differential amplifi er/adc driver with: 3dB bandwidth DC to 3MHz Fixed gain independent of R LOAD V/V (6dB) Differential input impedance Ω Low output impedance Built-in, user adjustable output fi ltering Requires minimal support circuitry Referring to the block diagram, the LT64-6 uses a closed-loop topology which incorporates 3 internal amplifiers. Two of the amplifi ers (A and B) are identical and drive the differential outputs. The third amplifier is used to set the output common mode voltage. Gain and input impedance are set by the Ω resistors in the internal feedback network. Output impedance is low, determined by the inherent output impedance of amplifiers A and B, and further reduced by internal feedback. The LT64-6 also includes built-in single-pole output filtering. The user has the choice of using the unfiltered outputs, the fi ltered outputs (7MHz 3dB lowpass), or modifying the filtered outputs to alter frequency response by adding additional components. Many lowpass and bandpass fi lters are easily implemented with just one or two additional components. 646fa

11 APPLICATIONS INFORMATION The LT64-6 has been designed to minimize the need for external support components such as transformers or AC-coupling capacitors. As an ADC driver, the LT64-6 requires no external components except for power-supply bypass capacitors. This allows DC-coupled operation for applications that have frequency ranges including DC. At the outputs, the common mode voltage is set via the V OCM pin, allowing the LT64-6 to drive ADCs directly. No output AC-coupling capacitors or transformers are needed. At the inputs, signals can be differential or single-ended with virtually no difference in performance. Furthermore, DC levels at the inputs can be set independently of the output common mode voltage. These input characteristics often eliminate the need for an input transformer and/or AC-coupling capacitors. Input Impedance and Matching Networks Calculation of the input impedance of the LT64-6 is not straightforward from examination of the block diagram because of the internal feedback network. In addition, the input impedance when driven differentially is different than when driven single-ended. DIFFERENTIAL SINGLE-ENDED LT64-6 Ω 33Ω For single-ended Ω applications, an.6ω shunt matching resistor to ground will result in the proper input termination (Figure ). For differential inputs there are several termination options. If the input source is Ω differential, then the input matching can be accomplished by either a 67Ω shunt resistor across the inputs (Figure 3), or equivalent 33Ω shunt resistors on each of the inputs to ground (Figure ). LT64-6 Single-Ended to Differential Operation The LT64-6 s performance with single-ended inputs is comparable to its performance with differential inputs. This excellent single-ended performance is largely due to the internal topology of the LT64-6. Referring to the block diagram, if the +INA and +INB pins are driven with a single-ended signal (while INA and INB are tied to AC ground), then the and pins are driven differentially without any voltage swing needed from amplifier C. Single-ended to differential conversion using more conventional topologies suffers from performance limitations due to the common mode amplifier. Driving ADCs The LT64-6 has been specifically designed to interface directly with high speed Analog to Digital Converters (ADCs). In general, these ADCs have differential inputs, with an input impedance of kω or higher. In addition, there is generally some form of lowpass or bandpass fi ltering just prior to the ADC to limit input noise at the ADC, thereby improving system signal to noise ratio. Both the unfi ltered and filtered outputs of the LT64-6 can easily drive the IF IN Z IN = Ω DIFFERENTIAL IF IN + 33Ω 33Ω 3 4 INB INA +INB 6 +INA LT F Figure. Input Termination for Differential Ω Input Impedance IF IN.μF.6Ω Z IN = Ω SINGLE-ENDED INB INA LT64-6 +INB +INA 646 F IF IN Z IN = Ω DIFFERENTIAL IF IN Ω 6 INB INA LT64-6 +INB +INA 646 F3 Figure. Input Termination for Single-Ended Ω Input Impedance Figure 3. Alternate Input Termination for Differential Ω Input Impedance 646fa

12 LT64-6 APPLICATIONS INFORMATION high impedance inputs of these differential ADCs. If the filtered outputs are used, then cutoff frequency and the type of filter can be tailored for the specifi c application if needed. Wideband Applications (Using the and Pins) In applications where the full bandwidth of the LT64-6 is desired, the unfi ltered output pins ( and ) should be used. They have a low output impedance; therefore, gain is unaffected by output load. Capacitance in excess of pf placed directly on the unfiltered outputs results in additional peaking and reduced performance. When driving an ADC directly, a small series resistance is recommended between the LT64-6 s outputs and the ADC inputs (Figure 4). This resistance helps eliminate any resonances associated with bond wire inductances of either the ADC inputs or the LT64-6 s outputs. A value between Ω and Ω gives excellent results. resistor/capacitor combination creates filtered outputs that look like a series Ω resistor with a 4pF capacitor shunting each filtered output to AC ground, giving a 3dB bandwidth of 7MHz. The filter cutoff frequency is easily modifi ed with just a few external components. To increase the cutoff frequency, simply add equal value resistors, one between and FILTERED and the other between and FIL- TERED (Figure 6). These resistors are in parallel with the internal Ω resistor, lowering the overall resistance and increasing filter bandwidth. To double the fi lter bandwidth, for example, add two external Ω resistors to lower the series resistance to Ω. The 4pF of capacitance remains unchanged, so filter bandwidth doubles. To decrease filter bandwidth, add two external capacitors, one from FILTERED to ground, and the other from FILTERED to ground. A single differential capacitor connected between FILTERED and FILTERED LT64-6 LT64-6 Ω TO Ω Ω TO Ω 646 F4 Figure 4. Adding Small Series R at LT64-6 Output Filtered Applications (Using the FILTERED and FILTERED Pins) Filtering at the output of the LT64-6 is often desired to provide either anti-aliasing or improved signal to noise ratio. To simplify this filtering, the LT64-6 includes an additional pair of differential outputs (FILTERED and FILTERED) which incorporate an internal lowpass filter network with a 3dB bandwidth of 7MHz (Figure ). These pins each have an output impedance of Ω. Internal capacitances are 4pF to on each fi ltered output, plus an additional 4pF capacitor connected differentially between the two filtered outputs. This ADC 4pF Ω Ω 4pF 4pF FILTERED FILTERED 646 F Figure. LT64-6 Internal Filter Topology 3dB BW 7MHz LT64-6 4pF Ω Ω 4pF 4pF F6 Ω FILTERED FILTERED Ω FILTERED OUTPUT (7MHz) FILTERED OUTPUT (MHz) Figure 6. LT64-6 Internal Filter Topology Modifi ed for x Filter Bandwidth ( External Resistors) 646fa

13 APPLICATIONS INFORMATION can also be used, but since it is being driven differentially it will appear at each filtered output as a single-ended capacitance of twice the value. To halve the fi lter bandwidth, for example, two 4pF capacitors could be added (one from each filtered output to ground). Alternatively one pf capacitor could be added between the filtered outputs, again halving the fi lter bandwidth. Combinations of capacitors could be used as well; a three capacitor solution of 4pF from each fi ltered output to ground plus a 4pF capacitor between the filtered outputs would also halve the fi lter bandwidth (Figure 7). Bandpass filtering is also easily implemented with just a few external components. An additional 6pF and 6nH, each added differentially between FILTERED and FILTERED creates a bandpass fi lter with a 6MHz center frequency, 3dB points of 3MHz and 3MHz, and.6db of insertion loss (Figure ). LT64-6 4pF Ω Ω 4pF 4pF 7 6 FILTERED FILTERED 646 F7 4pF 4pF 4pF Figure 7. LT64-6 Internal Filter Topology Modifi ed for /x Filter Bandwidth (3 External Capacitors) LT64-6 4pF Ω Ω 4pF 7 6 FILTERED FILTERED FILTERED OUTPUT (37.MHz) FILTERED OUTPUT LT64-6 Output Common Mode Adjustment The LT64-6 s output common mode voltage is set by the V OCM pin. It is a high-impedance input, capable of setting the output common mode voltage anywhere in a range from.v to 3.6V. Bandwidth of the V OCM pin is typically MHz, so for applications where the V OCM pin is tied to a DC bias voltage, a.μf capacitor at this pin is recommended. For best distortion performance, the voltage at the V OCM pin should be between.v and.6v. When interfacing with most ADCs, there is generally a V OCM output pin that is at about half of the supply voltage of the ADC. For V ADCs such as the LTC7XX family, this V OCM output pin should be connected directly (with the addition of a.μf capacitor) to the input V OCM pin of the LT64-6. For 3V ADCs such as the LTCXX families, the LT64-6 will function properly using the.6v from the ADC s V CM reference pin, but improved Spurious Free Dynamic Range (SFDR) and distortion performance can be achieved by level-shifting the LTCXX s V CM reference voltage up to at least.v. This can be accomplished as shown in Figure 9 by using a resistor divider between the LTCXX s V CM output pin and V CC and then bypassing the LT64-6 s V OCM pin with a.μf capacitor. For a common mode voltage above.9v, AC coupling capacitors are recommended between the LT64-6 and LTCXX ADCs because of the input voltage range constraints of the ADC. IF IN.μF.6Ω 3 INB 4 INA V OCM FILTERED LT64-6 FILTERED +INB 6 +INA.μF 6 7 Ω Ω AIN + AIN k 4.k 3 3V V CM LTCxx.9V.V 646 F9 4pF 646 F Figure. LT64-6 Output Filter Modifi ed for Bandpass Filtering ( External Inductor, External Capacitor) Figure 9. Level Shifting 3V ADC V CM Voltage for Improved SFDR 646fa 3

14 LT64-6 APPLICATIONS INFORMATION Large Output Voltage Swings The LT64-6 has been designed to provide the 3.V P-P output swing needed by the LTC74 family of 4-bit low-noise ADCs. This additional output swing improves system SNR by up to 4dB. Input Bias Voltage and Bias Current The input pins of the LT64-6 are internally biased to the voltage applied to the V OCM pin. No external biasing resistors are needed, even for AC-coupled operation. The input bias current is determined by the voltage difference between the input common mode voltage and the V OCM pin (which sets the output common mode voltage). For example, if the inputs are tied to.v with the V OCM pin at.v, then a total input bias current of.ma will fl ow into the LT64-6 s +INA and +INB pins. Furthermore, an additional input bias current totaling.ma will flow into the INA and INB inputs. Application (Demo) Boards The DC94A Demo Board has been created for stand-alone evaluation of the LT64-6 with either single-ended or differential input and output signals. As shown, it accepts a single-ended input and produces a single-ended output so that the LT64-6 can be evaluated using standard laboratory test equipment. For more information on this Demo Board, please refer to the layout and schematic diagrams found later in this data sheet. There are also additional demo boards available that combine the LT64-6 with a variety of different Linear Technology ADCs. Please contact the factory for more information on these demo boards. TYPICAL APPLICATION 4 Top Silkscreen 646fa

15 PACKAGE DESCRIPTION UD Package 6-Lead Plastic QFN (3mm 3mm) (Reference LTC DWG # --69) LT ±. 3. ±.. ±..4 ±. (4 SIDES) PACKAGE OUTLINE RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3. ±. (4 SIDES) PIN TOP MARK (NOTE 6). ±.. BSC.7 ±..4 ±. (4-SIDES) BOTTOM VIEW EXPOSED PAD R =. TYP 6 PIN NOTCH R =. TYP OR. 4 CHAMFER.4 ±. (UD6) QFN 94. REF.. NOTE:. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO- VARIATION (WEED-). 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.mm ON ANY SIDE. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN LOCATION ON THE TOP AND BOTTOM OF PACKAGE. ±.. BSC 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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 646fa

16 LT64-6 TYPICAL APPLICATION Demo Circuit DC94A Schematic (AC Test Circuit) TP ENABLE R Ω R7 Ω GND V CC 3 SW V CC R6 Ω V CC C7 pf C.μF J IN J +IN R Ω db R [] T : Z-RATIO 4 M/A-COM ETC-T R4 33Ω R3 33Ω 3 C.μF C.μF C.μF R6 Ω R Ω db V CC C INB INA +INB +INA V CCC 9 ENABLE LT64-6 V CCB B FILTERED FILTERED V OCM V CCA A 3 4 V CC 7 6 R 4.9Ω R [] R7 [] R9 4.9Ω L [] C4.μF C [] C3.μF C [] 4.dB C6 [] R 7Ω R [] R 7Ω db T 3 4: Z-RATIO 4 C.μF R4 Ω MINI- CIRCUITS TCM 4-9 R3 [] 6dB J4 J J3 V OCM J6 TEST IN TP V CC R9 4k 4 V CC R k T3 :4 MINI- CIRCUITS TCM 4-9 V CC C.μF C9 pf C pf C7.μF C.μF C3.μF C9,.μF C,.μF R C6 R [].μf [] 3 3 T4 4: MINI- CIRCUITS TCM TA J7 TEST OUT TP3 GND C4 4.7μF C μf NOTES: UNLESS OTHERWISE SPECIFIED, [] DO NOT STUFF. RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT993- MHz Differential Amplifier/ADC Driver A V = V/V, NF =.3dB, OIP3 = 3dBm at 7MHz LT MHz Differential Amplifi er/adc Driver A V = 4V/V, NF = 4.dB, OIP3 = 4dBm at 7MHz LT993-7MHz Differential Amplifi er/adc Driver A V = V/V, NF =.7dB, OIP3 = 4dBm at 7MHz LT4 Ultralow Distortion IF Amplifi er/adc Driver Digitally Controlled Gain Output IP3 47dBm at MHz LT64-3MHz Differential Amplifi er/adc Driver A V = db, e n =.6nV/ Hz at MHz, mw LT64-3MHz Differential Amplifi er/adc Driver A V = db, e n =.9nV/ Hz at MHz, mw LT64 6MHz Differential ADC Driver/Dual Selectable Gain Amplifi er 33V/μs Slew Rate, 6mA Current Consumption, Selectable Gain: A V =, +, + LT66- Very Low Noise Differential Amplifi er and MHz Lowpass Filter db S/N with 3V Supply, SO- Package LT66- Very Low Noise Differential Amplifi er and MHz Lowpass Filter db S/N with 3V Supply, SO- Package LT66- Very Low Noise Differential Amplifi er and MHz Lowpass Filter 76dB S/N with 3V Supply, SO- Package 6 Linear Technology Corporation 63 McCarthy Blvd., Milpitas, CA (4) 43-9 FAX: (4) fa LT 7 REV A PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 6