LT6604-2.5 Dual Very Low Noise, Differential Amplifier and 2.5MHz Lowpass Filter FEATURES DESCRIPTION n The LT®6604-2.5 consists of two matched, fully differential amplifiers, each with a 4th order, 2.5MHz lowpass filter. The fixed frequency lowpass filter approximates a Chebyshev response. By integrating a filter and a differential amplifier, distortion and noise are made exceptionally low. At unity gain, the measured in-band signal-to-noise ratio is an impressive 86dB. At higher gains, the input referred noise decreases, allowing the part to process smaller input differential signals without significantly degrading the signal-to-noise ratio. n n n n n n Dual Differential Amplifier with 2.5MHz Lowpass Filters 4th Order Filters Approximates Chebyshev Response Guaranteed Phase and Gain Matching Resistor-Programmable Differential Gain >86dB Signal-to-Noise (3V Supply, 1VRMS Output) Low Distortion (1MHz, 1VRMS Output, 800Ω Load) HD2: 92dBc HD3: 88dBc Specified for Operation with 3V, 5V and ±5V Supplies Fully Differential Inputs and Outputs Adjustable Output Common Mode Voltage Small 4mm × 7mm × 0.75mm QFN Package Gain and phase are well matched between the two channels. Gain for each channel is independently programmed using two external resistors. The LT6604-2.5 enables level shifting by providing an adjustable output common mode voltage, making it ideal for directly interfacing to ADCs. The LT6604-2.5 is fully specified for 3V operation. The differential design enables outstanding performance up to a 4VP-P signal level for a single 3V supply. APPLICATIONS n n n n n n n Dual Differential ADC Driver and Filter Single-Ended to Differential Converter Matched, Dual, Differential Gain or Filter Stage Common Mode Translation of Differential Signals High Speed ADC Antialiasing and DAC Smoothing in Wireless Infrastructure or Networking Applications High Speed Test and Measurement Equipment Medical Imaging See the back page of this datasheet for a complete list of related single and dual differential amplifiers with integrated 2.5MHz to 20MHz lowpass filters. L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.v TYPICAL APPLICATION Channel to Channel Gain Matching 3V 18 + 1580Ω +INA VMIDA 0.01μF – + V+A – VOCMA –INA – + 1580Ω + 1580Ω 0.01μF 1580Ω +OUTA V+B +INB VMIDB – –OUTA + – VOCMB –INB – + LTC22xx 3V 50Ω 50Ω + 18pF AIN DOUT – 3V –OUTB 50Ω +OUTB 50Ω + 18pF AIN 50 TYPICAL UNITS 16 TA = 25°C GAIN = 1 14 fIN = 2.5MHz DUAL ADC NUMBER OF UNITS LT6604-2.5 12 10 8 6 4 DOUT 2 – 0 V– –0.25 660425 TA01 –0.15 –0.05 0 0.05 0.15 GAIN MATCH (dB) 0.25 660425 TA01b 660425f 1 LT6604-2.5 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) 31 V– 32 V– 33 NC TOP VIEW 34 VMIDA Total Supply Voltage .................................................11V Operating Temperature Range (Note 6).... –40°C to 85°C Specified Temperature Range (Note 7) .... –40°C to 85°C Junction Temperature ........................................... 150°C Storage Temperature Range................... –65°C to 150°C Input Voltage +IN, –IN, VOCM, VMID (Note 8) ..............................±VS Input Current +IN, –IN, VOCM, VMID (Note 8) ........................±10mA NC 1 30 NC +INA 2 29 –OUTA NC 3 28 NC –INA 4 27 +OUTA NC 5 26 NC 25 V+A VOCMA 6 V– 7 24 V– 35 VMIDB 8 23 NC NC 9 22 NC +INB 10 21 –OUTB NC 11 20 NC –INB 12 19 +OUTB V+B 17 NC 16 NC 15 18 NC VOCMB 14 NC 13 UFF PACKAGE 34-LEAD (4mm s 7mm) PLASTIC QFN TJMAX = 150°C, θJA = 43°C/W, θJC = 4°C/W EXPOSED PAD (PIN 35) IS V–, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION SPECIFIED TEMPERATURE RANGE LT6604CUFF-2.5#PBF LT6604CUFF-2.5#TRPBF 60425 34-Lead (4mm × 7mm) Plastic QFN 0°C to 70°C LT6604IUFF-2.5#PBF LT6604IUFF-2.5#TRPBF 60425 34-Lead (4mm × 7mm) Plastic QFN –40°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. Unless otherwise specified VS = 5V (V+ = 5V, V– = 0V), RIN = 1580Ω, and RLOAD = 1k. PARAMETER CONDITIONS MIN TYP MAX UNITS Filter Gain Either Channel, VS = 3V VIN = 2VP-P, fIN = DC to 260kHz VIN = 2VP-P, fIN = 700kHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 1.9MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 2.2MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 2.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 7.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 12.5MHz (Gain Relative to 260kHz) –0.5 –0.15 –0.2 –0.6 –2.1 0.1 0 0.2 0.1 –0.9 –34 –51 0.4 0.1 0.6 0.5 0 – 31 dB dB dB dB dB dB dB l l l l l l 660425f 2 LT6604-2.5 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. Unless otherwise specified VS = 5V (V+ = 5V, V– = 0V), RIN = 1580Ω, and RLOAD = 1k. PARAMETER CONDITIONS Matching of Filter Gain, VS = 3V VIN = 2VP-P, fIN = DC to 260kHz VIN = 2VP-P, fIN = 700kHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 1.9MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 2.2MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 2.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 7.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 12.5MHz (Gain Relative to 260kHz) Matching of Filter Phase, VS = 3V MIN TYP MAX l l l l l l 0.04 0.005 0.02 0.03 0.05 0.15 0.05 0.4 0.1 0.3 0.4 0.6 1.1 2.8 dB dB dB dB dB dB dB VIN = 2VP-P, fIN = 700kHz VIN = 2VP-P, fIN = 1.9MHz VIN = 2VP-P, fIN = 2.2MHz l l l 0.2 0.6 0.8 1.5 3.5 4.5 deg deg deg Filter Gain Either Channel, VS = 5V VIN = 2VP-P, fIN = DC to 260kHz VIN = 2VP-P, fIN = 700kHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 1.9MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 2.2MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 2.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 7.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 12.5MHz (Gain Relative to 260kHz) l l l l l l –0.1 0 0.2 0.1 –0.9 –34 –51 0.4 0.1 0.6 0.5 0 – 31 dB dB dB dB dB dB dB Matching of Filter Gain, VS = 5V VIN = 2VP-P, fIN = DC to 260kHz VIN = 2VP-P, fIN = 700kHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 1.9MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 2.2MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 2.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 7.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 12.5MHz (Gain Relative to 260kHz) l l l l l l 0.04 0.005 0.02 0.03 0.05 0.15 0.05 0.4 0.1 0.3 0.4 0.6 1.1 2.8 dB dB dB dB dB dB dB Matching of Filter Phase, VS = 5V VIN = 2VP-P, fIN = 700kHz VIN = 2VP-P, fIN = 1.9MHz VIN = 2VP-P, fIN = 2.2MHz l l l 0.2 0.6 0.8 1.5 3.5 4.5 deg deg deg dB –0.5 –0.15 –0.2 –0.6 –2.1 UNITS Filter Gain Either Channel, VS = ±5V VIN = 2VP-P, fIN = DC to 260kHz –0.6 –0.1 0.4 Filter Gain, RIN = 402Ω VOUT = 2VP-P, fIN = DC to 260kHz VS = 3V VS = 5V VS = ±5V 11.3 11.3 11.2 11.8 11.8 11.7 12.3 12.3 12.2 Filter Gain Temperature Coefficient (Note 2) fIN = 260kHz, VIN = 2VP-P 780 ppm/°C Noise Noise BW = 10kHz to 2.5MHz, RIN = 1580Ω 51 μVRMS Distortion (Note 4) VIN = 1VRMS, fIN = 1MHz, RL = 800Ω 2nd Harmonic 3rd Harmonic 92 88 dBc dBc –119 dB Channel Separation (Note 9) VIN = 2VP-P, fIN = 1MHz Differential Output Swing Measured Between OUT+ and OUT–, VOCM Shorted to VMID VS = 5V VS = 3V l l 8.8 5.1 9.3 5.5 dB dB dB VP-P_DIFF VP-P_DIFF 660425f 3 LT6604-2.5 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. Unless otherwise specified VS = 5V (V+ = 5V, V– = 0V), RIN = 1580Ω, and RLOAD = 1k. PARAMETER CONDITIONS Input Bias Current Average of IN+ and IN– l Input Referred Differential Offset RIN = 1580Ω, Differential Gain = 1V/V VS = 3V VS = 5V VS = ±5V l l l 5 5 5 25 30 35 mV mV mV RIN = 402Ω, Differential Gain = 4V/V VS = 3V VS = 5V VS = ±5V l l l 3 3 3 13 16 20 mV mV mV MIN TYP –35 –15 Differential Offset Drift Input Common Mode Voltage (Note 3) Output Common Mode Voltage (Note 5) Output Common Mode Offset (with Respect to VOCM) Power Supply Current (Per Channel) μV/°C Differential Input = 500mVP-P, RIN ≥402Ω VS = 3V VS = 5V VS = ±5V l l l 0 0 –2.5 1.5 3 1 V V V Differential Output = 2VP-P, VMID at Mid Supply VS = 3V VS = 5V VS = ±5V l l l 1 1.5 –1 1.5 3 2 V V V VS = 3V VS = 5V VS = ±5V l l l –25 –30 –55 45 45 35 mV mV mV 10 5 –10 63 VS = 5V VS = 3V 2.45 2.51 1.5 2.56 V V 7.7 kΩ l 4.3 5.7 l l –15 –10 –3 –3 VS = 3V, VS = 5V VS = 3V, VS = 5V VS = ±5V l l 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 coefficient of the internal feedback resistors assuming a temperature independent external resistor (RIN). Note 3: The input common mode voltage is the average of the voltages applied to the external resistors (RIN). Note 4: Distortion is measured differentially using a differential stimulus. The input common mode voltage, the voltage at VOCM, and the voltage at VMID are equal to one half of the total power supply voltage. Note 5: Output common mode voltage is the average of the +OUT and –OUT voltages. The output common mode voltage is equal to VOCM. Note 6: The LT6604C-2.5 is guaranteed functional over the operating temperature range –40°C to 85°C. dB l VOCM = VMID = VS/2 VS = 5V VS = 3V VMID Input Resistance VOCM Bias Current UNITS μA 10 Common Mode Rejection Ratio Voltage at VMID MAX 26 28 μA μA 30 33 36 mA mA mA Note 7: The LT6604C-2.5 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 –40°C to 85°C. The LT6604I-2.5 is guaranteed to meet specified performance from –40°C to 85°C. Note 8: Input pins (+IN, –IN, VOCM and VMID) are protected by steering diodes to either supply. If the inputs should exceed either supply voltage, the input current should be limited to less than 10mA. In addition, the inputs +IN, –IN are protected by a pair of back-to-back diodes. If the differential input voltage exceeds 1.4V, the input current should be limited to less than 10mA Note 9: Channel separation (the inverse of crosstalk) is measured by driving a signal into one input while terminating the other input. Channel separation is the ratio of the resulting output signal at the driven channel to the output at the channel that is not driven. 660425f 4 LT6604-2.5 TYPICAL PERFORMANCE CHARACTERISTICS Frequency Response 12 VS = p2.5V RIN = 1580Ω GAIN = 1 0 0 GAIN Passband Gain and Group Delay 320 12 300 11 320 GAIN 300 10 280 –24 260 9 260 –3 240 8 240 –4 220 –5 200 –36 –48 –60 GROUP DELAY –6 –72 –84 –96 100k 1M 10M FREQUENCY (Hz) 50M 180 VS = 5V –7 RIN = 1580Ω 160 GAIN = 1 –8 140 TA = 25oC –9 120 0.5 0.75 1.0 1.25 1.5 1.75 2.0 2.25 2.5 2.75 3.0 FREQUENCY (MHz) 660425 G01 7 220 GROUP DELAY 6 200 180 VS = 5V RIN = 402Ω 160 GAIN = 4 140 3 TA = 25oC 120 2 0.5 0.75 1.0 1.25 1.5 1.75 2.0 2.25 2.5 2.75 3.0 FREQUENCY (MHz) 5 4 660425 G03 660425 G02 Output Impedance vs Frequency (OUT+ or OUT–) Common Mode Rejection Ratio 100 Power Supply Rejection Ratio 110 90 VIN = 1VP-P VS = 5V 100 R = 1580Ω IN GAIN = 1 90 CMRR (dB) 10 1 V+ TO DIFFERENTIAL OUT VS = 3V 80 70 60 PSRR (dB) OUTPUT IMPEDANCE (Ω) GAIN (dB) 280 –2 GAIN (dB) –1 GROUP DELAY (ns) –12 GROUP DELAY (ns) GAIN (dB) Passband Gain and Group Delay 1 80 70 50 40 30 60 20 50 0.1 100k 10 40 1M 10M FREQUENCY (Hz) 0 1k 100M 10k 100k 1M FREQUENCY (Hz) 10M 100M 1k 10k 100k 1M FREQUENCY (Hz) 10M 100M 660425 G05 660425 G04 660425 G06 DIFFERENTIAL INPUT, 2ND HARMONIC DIFFERENTIAL INPUT, 3RD HARMONIC SINGLE-ENDED INPUT, 2ND HARMONIC SINGLE-ENDED INPUT, 3RD HARMONIC –80 –90 VIN = 2VP-P VS = 3V RL = 800Ω AT EACH OUTPUT GAIN = 1 –100 –110 0.1 1 FREQUENCY (MHz) 10 660425 G07 Distortion vs Frequency –60 DIFFERENTIAL INPUT, 2ND HARMONIC DIFFERENTIAL INPUT, 3RD HARMONIC SINGLE-ENDED INPUT, 2ND HARMONIC SINGLE-ENDED INPUT, 3RD HARMONIC –70 DISTORTION (dBc) –70 DISTORTION (dBc) Distortion vs Frequency –60 –80 –90 VIN = 2VP-P VS = 5V RL = 800Ω AT EACH OUTPUT GAIN = 1 –100 –110 0.1 DIFFERENTIAL INPUT, 2ND HARMONIC DIFFERENTIAL INPUT, 3RD HARMONIC SINGLE-ENDED INPUT, 2ND HARMONIC SINGLE-ENDED INPUT, 3RD HARMONIC –70 DISTORTION (dBc) Distortion vs Frequency –60 1 FREQUENCY (MHz) 10 –80 –90 VIN = 2VP-P VS = p5V RL = 800Ω AT EACH OUTPUT GAIN = 1 –100 –110 0.1 1 FREQUENCY (MHz) 10 660425 G08 660425 G09 660425f 5 LT6604-2.5 TYPICAL PERFORMANCE CHARACTERISTICS 2ND HARMONIC, DIFFERENTIAL INPUT 3RD HARMONIC, DIFFERENTIAL INPUT 2ND HARMONIC, SINGLE-ENDED INPUT 3RD HARMONIC, SINGLE-ENDED INPUT –60 –70 –80 –90 VS = 3V F = 1MHz RL = 800Ω AT EACH OUTPUT GAIN = 1 –100 –110 0 1 2 3 4 INPUT LEVEL (VP-P) 5 Distortion vs Signal Level –40 2ND HARMONIC, DIFFERENTIAL INPUT 3RD HARMONIC, DIFFERENTIAL INPUT 2ND HARMONIC, SINGLE-ENDED INPUT 3RD HARMONIC, SINGLE-ENDED INPUT –50 DISTORTION (dBc) –50 DISTORTION (dBc) Distortion vs Signal Level –40 –60 –70 –80 –90 VS = 5V F = 1MHz RL = 800Ω AT EACH OUTPUT GAIN = 1 –100 –110 0 6 1 660425 G10 2 3 4 5 6 7 INPUT LEVEL (VP-P) 8 –60 –70 –70 –80 –90 VS = p5V F = 1MHz RL = 800Ω AT EACH OUTPUT GAIN = 1 –110 0 1 2 660425 G11 –40 2VP-P 1MHz INPUT RIN = 1580Ω GAIN = 1 –80 –90 –100 8 9 660425 G12 2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V –50 –60 –70 –80 –90 –100 –110 2VP-P 1MHz INPUT, RIN = 402Ω, GAIN = 4 –110 –2 –1 0 1 2 –3 3 INPUT COMMON MODE VOLTAGE RELATIVE TO VMID (V) –2 –1 0 1 2 –3 3 INPUT COMMON MODE VOLTAGE RELATIVE TO VMID (V) 660425 G13 660425 G14 Single Channel Supply Current vs Total Supply Voltage Distortion vs Output Common Mode Level 2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V 2ND HARMONIC, VS = p5V 3RD HARMONIC, VS = p5V –50 –60 –70 –80 –90 –100 32 30 SUPPLY CURRENT (mA) –40 DISTORTION COMPONENT (dBc) 3 4 5 6 7 INPUT LEVEL (VP-P) Distortion vs Input Common Mode Level DISTORTION COMPONENT (dBc) DISTORTION COMPONENT (dBc) –50 2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V –60 –100 9 Distortion vs Input Common Mode Level –40 2ND HARMONIC, DIFFERENTIAL INPUT 3RD HARMONIC, DIFFERENTIAL INPUT 2ND HARMONIC, SINGLE-ENDED INPUT 3RD HARMONIC, SINGLE-ENDED INPUT –50 DISTORTION (dBc) Distortion vs Signal Level –40 TA = 85°C 28 TA = 25°C 26 24 22 TA = –40°C 20 18 2VP-P 1MHz INPUT, RIN = 1580Ω, GAIN = 1 –110 –1.5 –1.0 –0.5 0 1.0 1.5 2.0 VOLTAGE VOCM TO VMID (V) 0.5 2.5 16 2 3 4 6 8 5 7 9 TOTAL SUPPLY VOLTAGE (V) 10 660425 G15 660425 G16 660425f 6 LT6604-2.5 TYPICAL PERFORMANCE CHARACTERISTICS Channel Separation vs Frequency (Note 9) Transient Response Gain = 1 –10 VOUT+ 50mV/DIV CHANNEL SEPARATION (dB) –30 DIFFERENTIAL INPUT 200mV/DIV 500ns/DIV 660425 G17 –50 –70 –90 –110 –130 100k VIN = 2VP-P VS = 5V RL = 800Ω AT EACH OUTPUT GAIN = 1 1M 10M FREQUENCY (Hz) 100M 660425 G18 PIN FUNCTIONS +INA, –INA (Pins 2, 4): Channel A Input Pins. Signals can be applied to either or both input pins through identical external resistors, RIN. The DC gain from the differential inputs to the differential outputs is 1580Ω/RIN. +INB, –INB (Pins 10, 12): Channel B Input Pins. Signals can be applied to either or both input pins through identical external resistors, RIN. The DC gain from differential inputs to the differential outputs is 1580Ω/RIN. VOCMA (Pin 6): DC Common Mode Reference Voltage for the 2nd Filter Stage in channel A. Its value programs the common mode voltage of the differential output of the filter. Pin 6 is a high impedance input, which can be driven from an external voltage reference, or Pin 6 can be tied to Pin 34 on the PC board. Pin 6 should be bypassed with a 0.01μF ceramic capacitor unless it is connected to a ground plane. VOCMB (Pin 14): DC Common Mode Reference Voltage for the 2nd Filter Stage in Channel B. Its value programs the common mode voltage of the differential output of the filter. Pin 14 is a high impedance input, which can be driven from an external voltage reference, or Pin 14 can be tied to Pin 8 on the PC board. Pin 14 should be bypassed with a 0.01μF ceramic capacitor unless it is connected to a ground plane. V– (Pins 7, 24, 31, 32, 35): Negative Power Supply Pin (can be ground). V+A, V+B (Pins 25, 17): Positive Power Supply Pins for Channels A and B. For a single 3.3V or 5V supply (V– grounded) a quality 0.1μF ceramic bypass capacitor is required from each positive supply pin (V+A, V+B) to the negative supply pin (V–). The bypass should be as close as possible to the IC. For dual supply applications, bypass the negative supply pins to ground and each of the positive supply pins (V+A, V+B) to ground with a quality 0.1μF ceramic capacitor. VMIDB (Pin 8): The VMIDB pin is internally biased at midsupply, see Block Diagram. For single supply operation the VMIDB pin should be bypassed with a quality 0.01μF ceramic capacitor to V–. For dual supply operation, Pin 8 can be bypassed or connected to a high quality DC ground. A ground plane should be used. A poor ground will increase noise and distortion. Pin 8 sets the output common mode voltage of the 1st stage of the filter in channel B. It has a 5.5kΩ impedance, and it can be overridden with an external low impedance voltage source. +OUTB, –OUTB (Pins 19, 21): Output Pins. Pins 19 and 21 are the filter differential outputs for channel B. With a typical short-circuit current limit greater than ±40mA, each pin can drive a 100Ω and/or 50pF load to AC ground. 660425f 7 LT6604-2.5 PIN FUNCTIONS +OUTA, – OUTA (Pins 27, 29): Output Pins. Pins 27 and 29 are the filter differential outputs for channel A. With a typical short-circuit current greater than ±40mA, each pin can drive a 100Ω and/or 50pF load to AC ground. VMIDA (Pin 34): The VMIDA pin is internally biased at midsupply, see Block Diagram. For single supply operation the VMIDA pin should be bypassed with a quality 0.01μF ceramic capacitor to V–. For dual supply operation, Pin 34 can be bypassed or connected to a high quality DC ground. A ground plane should be used. A poor ground will increase noise and distortion. Pin 34 sets the output common mode voltage of the 1st stage of the filter in channel A. It has a 5.5kΩ impedance, and it can be overridden with an external low impedance voltage source. Exposed Pad (Pin 35): V–. The Exposed Pad must be soldered to the PCB. If V– is separate from ground, tie the Exposed Pad to V–. BLOCK DIAGRAM VMIDA V– NC V+A NC RIN NC 11k +INA VIN+A V– LOWPASS FILTER STAGE 1580Ω –OUTA 11k 800Ω V– NC OP AMP + –INA VIN–A + – – VOCM VOCM + – RIN NC 800Ω – + 800Ω NC +OUTA NC 800Ω 1580Ω VOCMA V– VMIDB V+A V+B V– 11k LOWPASS FILTER STAGE 1580Ω NC 11k 800Ω V– NC OP AMP +INB + VIN+B RIN NC 800Ω + – – VOCM NC – –OUTB VOCM + – + 800Ω NC –INB VIN–B +OUTB RIN 800Ω 1580Ω NC NC 660025 BD VOCMB NC NC V+B 660425f 8 LT6604-2.5 APPLICATIONS INFORMATION Interfacing to the LT6604-2.5 operating with a single 3.3V supply and unity passband gain; the input signal is DC-coupled. The common mode input voltage is 0.5V, and the differential input voltage is 2VP-P. The common mode output voltage is 1.65V, and the differential output voltage is 2VP-P for frequencies below 2.5MHz. The common mode output voltage is determined by the voltage at VOCM. Since VOCM is shorted to VMID, 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 VMID. Note: The LT6604-2.5 contains two identical lowpass filters. The following applications information only refers to one filter. The two filters are independent except that they share the same negative supply voltage V–. The two filters can be used simultaneously by replicating the example circuits. The referenced pin numbers correspond to the A channel filter. Each LT6604-2.5 channel requires two equal external resistors, RIN, to set the differential gain to 1580Ω/RIN. The inputs to the filter are the voltages VIN+ and VIN– presented to these external components, Figure 1. The difference between VIN+ and VIN– is the differential input voltage. The average of VIN+ and VIN– is the common mode input voltage. Similarly, the voltages VOUT+ and VOUT– appearing at pins 27 and 29 of the LT6604-2.5 are the filter outputs. The difference between VOUT+ and VOUT– is the differential output voltage. The average of VOUT+ and VOUT– is the common mode output voltage. Figure 1 illustrates the LT6604-2.5 Figure 2 shows how to AC couple signals into the LT66042.5. In this instance, the input is a single-ended signal. AC-coupling allows the processing of single-ended or differential signals with arbitrary common mode levels. The 0.1μF coupling capacitor and the 1580Ω gain setting resistor form a high pass filter, attenuating signals below 1kHz. Larger values of coupling capacitors will proportionally reduce this highpass 3dB frequency. In Figure 3 the LT6604-2.5 channel is providing 12dB of gain. The common mode output voltage is set to 2V. 3.3V 0.1μF V 3 – VIN 2 VIN+ 1 0.01μF + VIN 0 t VIN– V 25 1580Ω 3 4 – 27 34 1/2 + LT6604-2.5 6 VOUT+ – 29 VOUT– 2 + 1580Ω 7 2 VOUT+ 1 VOUT– t 0 660425 F01 Figure 1 3.3V 0.1μF V 0.1μF 2 1580Ω 4 – 27 34 1/2 + LT6604-2.5 6 1 VIN 0 + 0.1μF t VIN 0.01μF 2 + –1 V 25 – + 1580Ω 29 3 VOUT+ 2 VOUT– 1 7 VOUT+ VOUT– 0 660425 F02 t Figure 2 5V 0.1μF V 3 VIN – 4 – 27 34 1/2 + LT6604-2.5 6 2 1 0 0.01μF 500mVP-P (DIFF) VIN+ VIN– VIN t V 25 402Ω 2 + 402Ω + – – + 7 2V 29 3 VOUT+ VOUT+ 2 VOUT– 1 0 VOUT– 660425 F03 t Figure 3 660425f 9 LT6604-2.5 APPLICATIONS INFORMATION Use Figure 4 to determine the interface between the LT6604-2.5 and a current output DAC. The gain, or “transimpedance,” is defined as A = VOUT/IIN. To compute the transimpedance, use the following equation: A= CURRENT OUTPUT DAC 34 0.01μF 6 660425 F04 R1 2 R2 LT6604-2.5 – + 7 29 29 402Ω 0.1μF 660425 F05 –2.5V Figure 5 Differential and Common Mode Voltage Ranges 25 27 7 50Ω VOUT– 0.1μF – 1/2 + + NETWORK ANALYZER INPUT VOUT+ 3.3V 4 – COILCRAFT TTWB-16A 4:1 402Ω 53.6Ω and 392Ω resistors satisfy the two constraints above. The transformer converts the single-ended source into a differential stimulus. Similarly, the output of the LT6604-2.5 will have lower distortion with larger load resistance yet the analyzer input is typically 50Ω. The 4:1 turns (16:1 impedance) transformer and the two 402Ω resistors of Figure 5, present the output of the LT6604-2.5 with a 1600Ω differential load, or the equivalent of 800Ω to ground at each output. The impedance seen by the network analyzer input is still 50Ω, reducing reflections in the cabling between the transformer and analyzer input. R1 R1• R2 + IIN • R1+ R2 + 1580 R1+ R2 = 26mV + IIN • 48.3Ω IIN+ 53.6Ω 25 – 27 34 1/2 + LT6604-2.5 6 392Ω VDAC = VMID • R1 NETWORK ANALYZER SOURCE COILCRAFT TTWB-1010 1:1 392Ω 4 2 By setting R1 + R2 = 1580Ω, the gain equation reduces to A = R1(Ω). The voltage at the pins of the DAC is determined by R1, R2, the voltage on VMID and the DAC output current. Consider Figure 4 with R1 = 49.9Ω and R2 = 1540Ω. The voltage at VMID, for VS = 3.3V, is 1.65V. The voltage at the DAC pins is given by: R2 0.1μF 50Ω 1580 • R1 (Ω) (R1+ R2) IIN– 2.5V VOUT+ – VOUT– IIN+ – IIN– = 1580 • R1 R1 + R2 Figure 4 Evaluating the LT6604-2.5 The low impedance levels and high frequency operation of the LT6604-2.5 require some attention to the impedance matching networks between the LT6604-2.5 and other devices. The previous examples assume an ideal (0Ω) source impedance and a large (1k) load resistance. Among practical examples where impedance must be considered is the evaluation of the LT6604-2.5 with a network analyzer. Figure 5 is a laboratory setup that can be used to characterize the LT6604-2.5 using single-ended instruments with 50Ω source impedance and 50Ω input impedance. For a 12dB gain configuration the LT6604-2.5 requires a 402Ω source resistance yet the network analyzer output is calibrated for a 50Ω load resistance. The 1:1 transformer, The rail-to-rail output stage of the LT6604-2.5 can process large differential signal levels. On a 3V supply, the output signal can be 5.1VP-P. Similarly, a 5V supply can support signals as large as 8.8VP-P. To prevent excessive power dissipation in the internal circuitry, the user must limit differential signal levels to 9VP-P. The two amplifiers inside the LT6604-2.5 channel have independent control of their output common mode voltage (see the “Block Diagram” section). The following guidelines will optimize the performance of the filter. VMID can be allowed to float, but it must be bypassed to an AC ground with a 0.01μF capacitor or instability may be observed. VMID can be driven from a low impedance source, provided it remains at least 1.5V above V– and at least 1.5V below V+. An internal resistor divider sets the voltage of VMID. 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 VMID. 660425f 10 LT6604-2.5 APPLICATIONS INFORMATION VOCM can be shorted to VMID for simplicity. If a different common mode output voltage is required, connect VOCM to a voltage source or resistor network. For 3V and 3.3V supplies the voltage at VOCM must be less than or equal to the mid supply level. For example, voltage (VOCM) ≤ 1.65V on a single 3.3V supply. For power supply voltages higher than 3.3V the voltage at VOCM can be set above mid supply, as shown in Table 1. The voltage on VOCM should not be more than 1V below the voltage on VMID. VOCM is a high impedance input. Table 1. Output Common Mode Range for Various Supplies SUPPLY VOLTAGE DIFFERENTIAL OUT VOLTAGE SWING OUTPUT COMMON MODE RANGE FOR LOW DISTORTION 3V 4VP-P 2VP-P 1VP-P 1.4V ≤ VOCM ≤ 1.6V 1V ≤ VOCM ≤ 1.6V 0.75V ≤ VOCM ≤ 1.6V 5V 8VP-P 4VP-P 2VP-P 1VP-P 2.4V ≤ VOCM ≤ 2.6V 1.5V ≤ VOCM ≤ 3.5V 1V ≤ VOCM ≤ 3.75V 0.75V ≤ VOCM ≤ 3.75V ±5V 9VP-P 4VP-P 2VP-P 1VP-P –2V ≤ VOCM ≤ 2V –3.5V ≤ VOCM ≤ 3.5V –3.75V ≤ VOCM ≤ 3.75V –4.25V ≤ VOCM ≤ 3.75V NOTE: The voltage at VOCM should not be more than 1V below the voltage at VMID. To achieve some of the output common mode ranges shown in the table, the voltage at VMID must be set externally to a value below mid supply. The LT6604-2.5 was designed to process a variety of input signals including signals centered on the mid-supply voltage and signals that swing between ground and a positive voltage in a single supply system (Figure 1). The allowable range of the input common mode voltage (the average of VIN+ and VIN– in Figure 1) is determined by the power supply level and gain setting (see “Electrical Characteristics”). 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 1580Ω feedback resistor and the external 402Ω input resistor. The resulting 1.25mA common mode DC current in each input path, must be absorbed by the sources VIN+ and VIN–. VOCM sets the common mode output voltage of the 2nd differential amplifier inside the LT6604-2.5 channel, and therefore sets the common mode output voltage of the filter. Since, in the example of Figure 3, VOCM differs from VMID by 0.5V, an additional 625μA (312μA per side) of DC current will flow in the resistors coupling the 1st differential amplifier output stage to the filter output. Thus, a total of 3.125mA is used to translate the common mode voltages. A simple modification to Figure 3 will reduce the DC common mode currents by 36%. If VMID is shorted to VOCM 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 625μA. Noise The noise performance of the LT6604-2.5 channel can be evaluated with the circuit of Figure 6. Given the low noise output of the LT6604-2.5 and the 6dB attenuation of the transformer coupling network, it is necessary to measure the noise floor of the spectrum analyzer and subtract the instrument noise from the filter noise measurement. Example: With the IC removed and the 25Ω resistors grounded, Figure 6, measure the total integrated noise (eS) of the spectrum analyzer from 10kHz to 2.5MHz. With the IC inserted, the signal source (VIN) disconnected, and the Common Mode DC Currents In applications like Figure 1 and Figure 3 where the LT66042.5 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 between input and output terminals. Minimize these currents to decrease power dissipation and distortion. 2.5V 0.1μF VIN RIN 25 4 – 27 34 1/2 + LT6604-2.5 6 2 RIN – + 29 7 COILCRAFT TTWB-1010 25Ω 1:1 SPECTRUM ANALYZER INPUT 50Ω 25Ω 660425 F06 Consider the application in Figure 3. VMID sets the output common mode voltage of the 1st differential amplifier inside the LT6604-2.5 channel (see the “Block Diagram” section) 0.1μF –2.5V Figure 6 660425f 11 LT6604-2.5 APPLICATIONS INFORMATION input resistors grounded, measure the total integrated noise out of the filter (eO). With the signal source connected, set the frequency to 100kHz and adjust the amplitude until VIN measures 100mVP-P. Measure the output amplitude, VOUT, and compute the passband gain A = VOUT/VIN. Now compute the input referred integrated noise (eIN) as: (eO )2 – (eS )2 eIN = A Table 2 lists the typical input referred integrated noise for various values of RIN. Table 2. Noise Performance RIN INPUT REFERRED INTEGRATED NOISE 10kHz TO 2.5MHz INPUT REFERRED INTEGRATED NOISE 10kHz TO 5MHz 4 402Ω 18μVRMS 23μVRMS 2 806Ω 29μVRMS 39μVRMS 1 1580Ω 51μVRMS 73μVRMS PASSBAND GAIN Figure 7 is plot of the noise spectral density as a function of frequency for an LT6604-2.5 channel with RIN = 1580Ω using the fixture 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. 100 80 40 SPECTRAL DENSITY 30 60 20 40 10 20 INTEGRATED 0 0.01 0 0.1 1 10 FREQUENCY (MHz) 660425 F07 INTEGRATED NOISE (μVRMS) NOISE SPECTRAL DENSITY (nVRMS/√Hz) 50 Conversely, if each output is measured individually and the noise power added together, the resulting calculated noise level will be higher than the true differential noise. Power Dissipation The LT6604-2.5 amplifiers combine high speed with large signal currents in a small package. There is a need to ensure that the die’s junction temperature does not exceed 150°C. The LT6604-2.5 has an exposed pad (pin 35) which is connected to the negative supply (V–). Connecting the pad to a ground plane helps to dissipate the heat generated by the chip. Metal trace and plated through-holes can be used to spread the heat generated by the device to the backside of the PC board. Junction temperature, TJ, is calculated from the ambient temperature, TA, and power dissipation, PD. The power dissipation is the product of supply voltage, VS, and total supply current, IS. Therefore, the junction temperature is given by: TJ = TA + (PD • θJA) = TA + (VS • IS • θJA) where the supply current, IS, is a function of signal level, load impedance, temperature and common mode voltages. For a given supply voltage, the worst-case power dissipation occurs when the differential input signal is maximum, the common mode currents are maximum (see Applications Information regarding Common Mode DC Currents), the load impedance is small and the ambient temperature is maximum. To compute the junction temperature, measure the supply current under these worst-case conditions, use 43°C/W as the package thermal resistance, then apply the equation for TJ. 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 85°C, the supply current (current into V+) measures 37.6mA per channel. The resulting junction temperature is: TJ = TA + (PD • θJA) = 85 + (5 • 2 • 0.0376 • 43) = 101°C. The thermal resistance can be affected by the amount of copper on the PCB that is connected to V–. The thermal resistance of the circuit can increase if the Exposed Pad is not connected to a large ground plane with a number of vias. Figure 7. Input Referred Noise, Gain = 1 660425f 12 LT6604-2.5 TYPICAL APPLICATIONS IQ DAC Output Filter 5V 0.1μF 5V 1580Ω 52.3Ω 16 BIT 4kHz to 2.5MHz DISCRETE MULTI-TONE SIGNAL @ 50MSPS LADCOM IOUT A 52.3Ω LTC1668 56pF IOUT B 25 4 – 27 34 1/2 + LT6604-2.5 6 2 CLK + 1580Ω QOUT – 29 7 0.1μF –5V 50MHz –5V 5V 0.1μF 5V 1580Ω 52.3Ω LADCOM IOUT A 52.3Ω LTC1668 56pF IOUT B 17 – 19 8 1/2 + LT6604-2.5 14 12 10 CLK 1580Ω + IOUT – 21 24 0.1μF –5V 50MHz –5V DAC Output Spectrum LT6604-2.5 Output Spectrum 0 0 –10 –10 –20 –20 BASEBAND SIGNAL –30 –30 –40 DAC OUTPUT IMAGE –50 (dBm) (dBm) 660425 TA02a –40 –50 –60 –60 –70 –70 –80 –80 –90 0 12 24 36 48 60 72 84 96 108 120 FREQUENCY (MHz) –90 0 12 24 36 48 60 72 84 96 108 120 FREQUENCY (MHz) 660425 TA02b 660025 TA02c 660425f 13 LT6604-2.5 TYPICAL APPLICATIONS Dual, Matched 5th Order, 2.5MHz Lowpass Filter, Gain = 1 V+ 0.1μF VIN– R 787Ω R 787Ω C 82pF VIN+ 25 – 27 1/2 34 + LT6604-2.5 6 4 2 R 787Ω + R 787Ω QOUT – 29 7 0.1μF 1 C= 2π • R • 2.5MHz GAIN = VIN– V– 1580Ω 2R R 787Ω V+ 0.1μF R 787Ω C 82pF VIN+ 17 – 19 8 1/2 + LT6604-2.5 14 12 10 R 787Ω R 787Ω + QOUT – 21 24 0.1μF V– 660425 TA03a Frequency Response 10 Transient Response Gain = 1 VS = ±2.5V GAIN = 1 R = 787Ω TA = 25°C 0 –10 VOUT+ 50mV/DIV GAIN (dB) –20 –30 DIFFERENTIAL INPUT 200mV/DIV –40 –50 –60 –70 500ns/DIV –80 –90 100k 1M FREQUENCY (Hz) 660425 TA03c 10M 20M 660425 TA03b 660425f 14 LT6604-2.5 PACKAGE DESCRIPTION UFF Package 34-Lead Plastic QFN (4mm × 7mm) (Reference LTC DWG # 05-08-1758 Rev Ø) 0.70 ± 0.05 1.90 ± 0.05 4.50 ± 0.05 PACKAGE OUTLINE 1.83 ± 0.05 3.10 ± 0.05 1.50 REF 1.90 ± 0.05 1.47 ± 0.05 2.64 ± 0.05 1.29 ± 0.05 0.25 ± 0.05 0.50 BSC 6.00 REF 6.10 ± 0.05 7.50 ± 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ± 0.10 PIN 1 NOTCH R = 0.30 OR 0.25 × 45° CHAMFER R = 0.10 TYP 0.75 ± 0.05 1.50 REF 33 34 0.40 ± 0.10 1 PIN 1 TOP MARK (NOTE 6) 1.90 ± 0.10 2 1.47 ± 0.10 7.00 ± 0.10 6.00 REF 1.83 ± 0.10 1.90 ± 0.10 2.64 ± 0.10 (UFF34) QFN 0807 REV Ø 0.200 REF R = 0.125 TYP 0.00 – 0.05 0.25 ± 0.05 0.50 BSC 0.99 ± 0.10 BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 660425f Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 15 LT6604-2.5 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS Integrated Filters LTC1562-2 Very Low Noise, 8th Order Filter Building Block Lowpass and Bandpass Filters up to 300kHz LTC1565-31 650kHz Linear Phase Lowpass Filter Continuous Time, 7th Order, Differential LTC1566-1 Low Noise, 2.3MHz Lowpass Filter Continuous Time, 7th Order, Differential LT1568 Very Low Noise, 4th Order Filter Building Block Lowpass and Bandpass Filters up to 10MHz LTC1569-7 Linear Phase, Tunable 10th Order Lowpass Filter Single-Resistor Programmable Cut-Off to 300kHz LT6600-2.5 Very Low Noise Differential 2.5MHz Lowpass Filter SNR = 86dB at 3V Supply, 4th Order Filter LT6600-5 Very Low Noise Differential 5MHz Lowpass Filter SNR = 82dB at 3V Supply, 4th Order Filter LT6600-10 Very Low Noise Differential 10MHz Lowpass Filter SNR = 82dB at 3V Supply, 4th Order Filter LT6600-15 Very Low Noise Differential 15MHz Lowpass Filter SNR = 76dB at 3V Supply, 4th Order Filter LT6600-20 Very Low Noise Differential 20MHz Lowpass Filter SNR = 76dB at 3V Supply, 4th Order Filter LTC6601 Low Noise, Fully Differential, Pin Configurable Amplifier/Driver/2nd Order Filter Building Block LTC6602 Dual Adjustable Lowpass Filter for RFID LTC6603 Dual Adjustable Lowpass Filter for Communications LT6604-5 Dual Very Low Noise, Differential Amplifier and 5MHz Lowpass Filter SNR = 82dB at 3V Supply, 4th Order Filter LT6604-10 Dual Very Low Noise, Differential Amplifier and 10MHz Lowpass Filter SNR = 82dB at 3V Supply, 4th Order Filter LT6604-15 Dual Very Low Noise, Differential Amplifier and 15MHz Lowpass Filter SNR = 76dB at 3V Supply, 4th Order Filter 660425f 16 Linear Technology Corporation LT 0708 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2008