LT6600-20

LT6600-20 Very Low Noise, Differential Amplifier and 20MHz Lowpass Filter FEATURES s s s s DESCRIPTIO s s s s s Programmable Differential Gain via Two External Resistors Adjustable Output Common Mode Voltage Operates and Specified with 3V, 5V, ±5V Supplies 0.5dB Ripple 4th Order Lowpass Filter with 20MHz Cutoff 76dB S/N with 3V Supply and 2VP-P Output Low Distortion, 2VP-P, 800Ω Load 2.5MHz: 83dBc 2nd, 88dBc 3rd 20MHz: 63dBc 2nd, 64dBc 3rd Fully Differential Inputs and Outputs SO-8 Package Compatible with Popular Differential Amplifier Pinouts The LT®6600-20 combines a fully differential amplifier with a 4th order 20MHz lowpass filter approximating a Chebyshev frequency response. Most differential amplifiers require many precision external components to tailor gain and bandwidth. In contrast, with the LT6600-20, two external resistors program differential gain, and the filter’s 20MHz cutoff frequency and passband ripple are internally set. The LT6600-20 also provides the necessary level shifting to set its output common mode voltage to accommodate the reference voltage requirements of A/Ds. Using a proprietary internal architecture, the LT6600-20 integrates an antialiasing filter and a differential amplifier/ driver without compromising distortion or low noise performance. At unity gain the measured in band signal-to-noise ratio is an impressive 76dB. At higher gains the input referred noise decreases so the part can process smaller input differential signals without significantly degrading the output signal-to-noise ratio. The LT6600-20 also features low voltage operation. The differential design provides outstanding performance for a 2VP-P signal level while the part operates with a single 3V supply. The LT6600-20 is packaged in an SO-8 and is pin compatible with stand alone differential amplifiers. APPLICATIO S s s s s High Speed ADC Antialiasing and DAC Smoothing in Networking or Cellular Base Station Applications High Speed Test and Measurement Equipment Medical Imaging Drop-in Replacement for Differential Amplifiers , LTC and LT are registered trademarks of Linear Technology Corporation. TYPICAL APPLICATIO LT6600-20 5V 0.1µF RIN 402Ω 1 7 0.01µF VIN RIN 402Ω 2 8 3 An 8192 Point FFT Spectrum A/D LTC1748 5V AMPLITUDE (dB) 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 –120 66002 TA01a – VMID VOCM + – 4 49.9Ω 49.9Ω 18pF + AIN V+ DOUT VCM V– 1µF + 6 5 – GAIN = 402Ω/RIN 0 U INPUT 5.9MHz 2VP-P fSAMPLE = 80MHz 10 20 FREQUENCY (MHz) 66002 TA01b U U 30 40 66002f 1 LT6600-20 ABSOLUTE (Note 1) AXI U RATI GS PACKAGE/ORDER I FOR ATIO TOP VIEW IN – 1 VOCM 2 V+ 3 OUT + 4 8 7 6 5 IN + VMID V– OUT – 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 Lead Temperature (Soldering, 10 sec).................. 300°C ORDER PART NUMBER LT6600CS8-20 LT6600IS8-20 S8 PART MARKING 660020 600I20 S8 PACKAGE 8-LEAD PLASTIC SO TJMAX = 150°C, θJA = 100°C/W Consult LTC Marketing for parts specified with wider operating temperature ranges. The q denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25°C. Unless otherwise specified VS = 5V (V+ = 5V, V – = 0V), RIN = 402Ω, and RLOAD = 1k. PARAMETER Filter Gain, VS = 3V CONDITIONS VIN = 2VP-P, fIN = DC to 260kHz VIN = 2VP-P, fIN = 2MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 10MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 16MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 20MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 60MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 100MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = DC to 260kHz VIN = 2VP-P, fIN = 2MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 10MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 16MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 20MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 60MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 100MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = DC to 260kHz VIN = 2VP-P, fIN = DC to 260kHz, VS = 3V VIN = 2VP-P, fIN = DC to 260kHz, VS = 5V VIN = 2VP-P, fIN = DC to 260kHz, VS = ±5V fIN = 250kHz, VIN = 2VP-P Noise BW = 10kHz to 20MHz 2.5MHz, 2VP-P, RL = 800Ω 2nd Harmonic 3rd Harmonic 20MHz, 2VP-P, RL = 800Ω 2nd Harmonic 3rd Harmonic Measured Between Pins 4 and 5 VS = 5V VS = 3V Average of Pin 1 and Pin 8 MIN – 0.4 – 0.1 – 0.2 – 0.1 – 0.8 TYP 0.1 0 0.1 0.5 0 – 33 – 50 0 0 0.1 0.4 –0.4 – 33 – 50 – 0.1 12.1 12.0 11.9 780 118 83 88 63 64 4.75 4.50 – 50 MAX 0.5 0.1 0.5 1.9 1 – 28 0.5 0.1 0.4 1.6 0.6 –28 0.4 12.6 12.5 12.4 UNITS dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB ppm/C µVRMS dBc dBc dBc dBc VP-P DIFF VP-P DIFF µA ELECTRICAL CHARACTERISTICS q q q q q q q q q q q q Filter Gain, VS = 5V – 0.5 – 0.1 – 0.2 – 0.3 – 1.3 Filter Gain, VS = ± 5V Filter Gain, RIN = 100Ω – 0.6 11.6 11.5 11.4 Filter Gain Temperature Coefficient (Note 2) Noise Distortion (Note 4) Differential Output Swing Input Bias Current q q q 3.80 3.75 – 95 66002f 2 U W U U WW W LT6600-20 The q denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25°C. Unless otherwise specified VS = 5V (V+ = 5V, V – = 0V), RIN = 402Ω, and RLOAD = 1k. PARAMETER Input Referred Differential Offset CONDITIONS RIN = 402Ω RIN = 100Ω MIN V S = 3V V S = 5V VS = ±5V V S = 3V V S = 5V VS = ±5V VS = 3V VS = 5V VS = ±5V VS = 3V VS = 5V VS = ±5V VS = 3V VS = 5V VS = ±5V VS = 5 VS = 3 VOCM = VMID= VS/2 V S = 5V V S = 3V VS = 3V, VS = 5 VS = 3V, VS = 5 VS = ±5V q q q q q q q q q q q q q q q q q q q q q ELECTRICAL CHARACTERISTICS Differential Offset Drift Input Common Mode Voltage (Note 3) TYP 5 10 10 5 5 5 10 MAX 25 30 35 15 17 20 1.5 3.0 1.0 1.5 3.0 2.0 40 40 35 2.55 7.65 Differential Input = 500mVP-P, RIN = 100Ω Differential Input = 2VP-P, Pin 7 at Mid-Supply Common Mode Voltage at Pin 2 Output Common Mode Voltage (Note 5) Output Common Mode Offset (with Respect to Pin 2) Common Mode Rejection Ratio Voltage at VMID (Pin 7) VMID Input Resistance VOCM Bias Current Power Supply Current 0.0 0.0 –2.5 1.0 1.5 –1.0 –35 –40 –55 2.46 4.35 –15 –10 5 0 –5 66 2.51 1.5 5.7 –3 –3 42 46 46 53 56 UNITS mV mV mV mV mV mV µV/°C V V V V V V mV mV mV dB V V kΩ µA µA mA mA mA Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: This is the temperature coefficient of the internal feedback resistors assuming a temperature independent external resistor (RIN). Note 3: The input common mode voltage is the average of the voltages applied to the external resistors (RIN). Specification guaranteed for RIN ≥ 100Ω. Note 4: Distortion is measured differentially using a differential stimulus, The input common mode voltage, the voltage at Pin 2, and the voltage at Pin 7 are equal to one half of the total power supply voltage. Note 5: Output common mode voltage is the average of the voltages at Pins 4 and 5. The output common mode voltage is equal to the voltage applied to Pin 2. Note 6: The LT6600C-20 is guaranteed functional over the operating temperature range –40°C to 85°C. Note 7: The LT6600C-20 is guaranteed to meet 0°C to 70°C specifications and is designed, characterized and expected to meet the extended temperature limits, but is not tested at –40°C and 85°C. The LT6600I-20 is guaranteed to meet specified performance from –40°C to 85°C. 66002f 3 LT6600-20 TYPICAL PERFOR A CE CHARACTERISTICS Amplitude Response 10 0 –10 –20 GAIN (dB) –40 –50 –60 –70 –80 VS = 5V GAIN = 1 TA = 25°C 1 10 FREQUENCY (MHz) 100 66002 G01 GAIN (dB) GAIN (dB) –30 –90 0.1 Passband Gain and Group Delay 14 12 10 8 GAIN V S = 5V GAIN = 4 TA = 25°C 50 45 OUTPUT IMPEDANCE (Ω) GAIN (dB) 6 4 2 0 –2 –4 –6 0.5 30 25 20 15 10 5 CMRR (dB) GROUP DELAY 6.5 18.5 24.5 12.5 FREQUENCY (MHz) Power Supply Rejection Ratio 100 90 80 70 V + TO DIFFOUT VS = 3V TA = 25°C DISTORTION (dB) –60 –70 –80 –90 DISTORTION (dB) PSRR (dB) 60 50 40 30 20 10 0 0.001 0.01 0.1 1 FREQUENCY (MHz) 4 UW 66002 G04 Passband Gain and Phase 2 0 –2 –4 –6 –8 –10 –12 –14 –16 –18 0.5 6.5 18.5 24.5 12.5 FREQUENCY (MHz) PHASE GAIN VS = 5V GAIN = 1 TA = 25°C 95 50 5 –40 2 0 –2 –4 Passband Gain and Group Delay GAIN VS = 5V GAIN = 1 TA = 25°C 50 45 40 35 GROUP DELAY 30 25 20 15 10 5 6.5 18.5 24.5 12.5 FREQUENCY (MHz) 0 30.5 66002 G03 GROUP DELAY (ns) PHASE (DEG) –85 –130 –175 –6 –8 –10 –12 –14 –16 –18 0.5 –220 –265 –310 –355 30.5 66002 G02 Output Impedance 100 VS = 5V GAIN = 1 TA = 25°C 80 75 70 65 60 55 50 45 40 35 Common Mode Rejection Ratio INPUT = 1VP-P VS = 5V GAIN = 1 TA = 25°C 40 35 GROUP DELAY (ns) 10 1 0 30.5 0.1 0.1 30 1 10 FREQUENCY (MHz) 100 66002 G05 0.1 1 10 FREQUENCY (MHz) 100 66002 G06 Distortion vs Frequency –40 –50 DIFFERENTIAL INPUT, 2ND HARMONIC DIFFERENTIAL INPUT, 3RD HARMONIC SINGLE-ENDED INPUT, 2ND HARMONIC SINGLE-ENDED INPUT, 3RD HARMONIC –40 –50 –60 –70 –80 –90 –100 Distortion vs Signal Level, VS = 3V 3RD HARMONIC VS = 3V 10MHz INPUT RL = 800Ω AT EACH OUTPUT GAIN = 1 2ND TA = 25°C HARMONIC 10MHz INPUT 3RD HARMONIC 1MHz INPUT 2ND HARMONIC 1MHz INPUT –100 10 100 66002 G07 VIN = 2VP-P VS = 3V RL = 800Ω AT EACH OUTPUT GAIN = 1 TA = 25°C 0.1 1 10 FREQUENCY (MHz) 100 66002 G08 0 1 2 3 INPUT LEVEL (VP-P) 4 5 66002 G09 66002f LT6600-20 TYPICAL PERFOR A CE CHARACTERISTICS Distortion vs Signal Level, VS = ±5V –40 –50 DISTORTION (dB) –60 –70 –80 –90 –100 0 1 2ND HARMONIC, 10MHz INPUT 3RD HARMONIC, 10MHz INPUT 2ND HARMONIC, 1MHz INPUT 3RD HARMONIC, 1MHz INPUT –40 DISTORTION COMPONENT (dB) –60 –70 –80 –90 DISTORTION COMPONENT (dB) VS = ± 5V RL = 800Ω AT EACH OUTPUT GAIN = 1 TA = 25°C 2 3 INPUT LEVEL (VP-P) 4 5 66002 G10 DISTORTION COMPONENT (dB) Total Supply Current vs Total Supply Voltage 60 TA = 85°C TA = 25°C TA = –40°C TOTAL SUPPLY CURRENT (mA) 50 40 30 20 10 0 0 1 2345678 TOTAL SUPPLY VOLTAGE (V) UW Distortion vs Input Common Mode Level 2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V 2VP-P 1MHz INPUT RL = 800Ω AT EACH OUTPUT GAIN = 1 TA = 25°C –40 –50 –60 –70 –80 –90 Distortion vs Input Common Mode Level 2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V –50 –100 –3 –2 –1 0 1 2 3 INPUT COMMON MODE VOTLAGE RELATIVE TO PIN 7 (V) 66002 G11 –100 500mVP-P 1MHz INPUT, GAIN = 4, RL = 800Ω AT EACH OUTPUT –3 –2 –1 0 1 2 3 INPUT COMMON MODE VOTLAGE RELATIVE TO PIN 7 (V) 66002 G12 Distortion vs Output Common Mode –40 –50 –60 –70 –80 –90 –100 –110 2VP-P 1MHz INPUT, GAIN = 1, RL = 800Ω AT EACH OUTPUT –2 –1.5 –1 –0.5 0 0.5 1 1.5 VOLTAGE PIN 2 TO PIN 7 (V) 2 2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V 2ND HARMONIC, VS = ± 5V 3RD HARMONIC, VS = ± 5V 66002 G13 Transient Response, Gain = 1 VOUT+ 50mV/DIV DIFFERENTIAL INPUT 200mV/DIV 100ns/DIV 66002 G15 9 10 66002 G14 66002f 5 LT6600-20 PI FU CTIO S IN – and IN + (Pins 1, 8): 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 402Ω/RIN. VOCM (Pin 2): Is the DC Common Mode Reference Voltage for the 2nd Filter Stage. Its value programs the common mode voltage of the differential output of the filter. Pin 2 is a high impedance input, which can be driven from an external voltage reference, or Pin 2 can be tied to Pin 7 on the PC board. Pin 2 should be bypassed with a 0.01µF ceramic capacitor unless it is connected to a ground plane. V+ and V – (Pins 3, 6): Power Supply Pins. For a single 3.3V or 5V supply (Pin 6 grounded) a quality 0.1µF ceramic bypass capacitor is required from the positive supply pin (Pin 3) to the negative supply pin (Pin 6). The bypass should be as close as possible to the IC. For dual supply applications, bypass Pin 3 to ground and Pin 6 to ground with a quality 0.1µF ceramic capacitor. OUT+ and OUT – (Pins 4, 5): Output Pins. Pins 4 and 5 are the filter differential outputs. Each pin can drive a 100Ω and/or 50pF load. VMID (Pin 7): The VMID pin is internally biased at midsupply, see block diagram. For single supply operation, the VMID pin should be bypassed with a quality 0.01µF ceramic capacitor to Pin 6. For dual supply operation, Pin 7 can be bypassed or connected to a high quality DC ground. A ground plane should be used. A poor ground will increase noise and distortion. Pin 7 sets the output common mode voltage of the 1st stage of the filter. It has a 5.5kΩ impedance, and it can be overridden with an external low impedance voltage source. BLOCK DIAGRA VIN+ VIN– 6 W U U U RIN 8 IN + VMID 7 V+ 11k 402Ω 11k V– 6 OUT – 5 PROPRIETARY LOWPASS FILTER STAGE 200Ω V– OP AMP + VOCM – + 200Ω +– VOCM – 200Ω –+ 200Ω 402Ω 1 RIN IN – 2 VOCM 3 V+ 4 66002 BD OUT + 66002f LT6600-20 APPLICATIO S I FOR ATIO Interfacing to the LT6600-20 The LT6600-20 requires two equal external resistors, RIN, to set the differential gain to 402Ω/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 4 and 5 of the LT6600-20 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 LT6600-20 operating with a single 3.3V supply and unity passband gain; the input signal is DC coupled. The common mode input voltage is 0.5V, and the differential input voltage is 2VP-P. The common mode V 3 2 1 0 VIN+ + VIN VIN – 402Ω VIN– t 402Ω V 0.1µF 2 1 0 –1 VIN+ 0.1µF t VIN + V 3 2 1 0 500mVP-P (DIFF) VIN+ VIN– t + VIN VIN – U output voltage is 1.65V, and the differential output voltage is 2VP-P for frequencies below 20MHz. The common mode output voltage is determined by the voltage at pin 2. Since pin 2 is shorted to pin 7, the output common mode is the mid-supply voltage. In addition, the common mode input voltage can be equal to the mid-supply voltage of Pin 7 (see the Distortion vs Input Common Mode Level graphs in the Typical Performance Characteristics). Figure 2 shows how to AC couple signals into the LT6600-20. In this instance, the input is a single-ended signal. AC coupling allows the processing of single-ended or differential signals with arbitrary common mode levels. The 0.1µF coupling capacitor and the 402Ω gain setting resistor form a high pass filter, attenuating signals below 4kHz. Larger values of coupling capacitors will proportionally reduce this highpass 3dB frequency. 3.3V 0.1µF 1 7 0.01µF 2 8 3 V 3 W UU – LT6600-20 + 4 VOUT+ VOUT– 2 1 0 VOUT+ VOUT– t 66002 F01 + 6 –5 Figure 1 3.3V 0.1µF 402Ω 1 7 0.01µF 402Ω 2 8 3 V 3 VOUT+ VOUT– 2 1 0 VOUT+ VOUT– – LT6600-20 + 4 + 6 – 5 66002 F02 Figure 2 62pF 5V 3 0.1µF 3 VOUT+ 2 VOUT– 1 0 VOUT– VOUT+ V 100Ω 1 7 2 8 – LT6600-20 + 4 0.01µF 100Ω + 6 – 5 + – 2V t 66002 F03 62pF Figure 3 66002f 7 LT6600-20 APPLICATIO S I FOR ATIO In Figure 3 the LT6600-20 is providing 12dB of gain. The gain resistor has an optional 62pF in parallel to improve the passband flatness near 20MHz. The common mode output voltage is set to 2V. Use Figure 4 to determine the interface between the LT6600-20 and a current output DAC. The gain, or “transimpedance,” is defined as A = VOUT/IIN. To compute the transimpedance, use the following equation: A= 402 • R1 (Ω ) (R1+ R2) By setting R1 + R2 = 402Ω, the gain equation reduces to A = R1(Ω). The voltage at the pins of the DAC is determined by R1, R2, the voltage on Pin 7 and the DAC output current. Consider Figure 4 with R1 = 49.9Ω and R2 = 348Ω. The voltage at Pin 7 is 1.65V. The voltage at the DAC pins is given by: R1 R1 • R2 VDAC = VPIN7 • + IIN • R1 + R2 + 402 R1 + R2 = 26mV + IIN • 48.3Ω IIN is IIN+ or IIN–. The transimpedance in this example is 50.4Ω. Evaluating the LT6600-20 The low impedance levels and high frequency operation of the LT6600-20 require some attention to the matching networks between the LT6600-20 and other devices. The previous examples assume an ideal (0Ω) source impedance and a large (1kΩ) load resistance. Among practical CURRENT OUTPUT DAC IIN– R1 IIN+ R1 R2 0.01µF R2 1 7 8 3.3V 0.1µF 3 –+ + 6 4 VOUT+ VOUT– 2 LT6600-20 – 5 66002 F04 Figure 4 8 U examples where impedance must be considered is the evaluation of the LT6600-20 with a network analyzer. Figure 5 is a laboratory setup that can be used to characterize the LT6600-20 using single-ended instruments with 50Ω source impedance and 50Ω input impedance. For a unity gain configuration the LT6600-20 requires a 402Ω source resistance yet the network analyzer output is calibrated for a 50Ω load resistance. The 1:1 transformer, 53.6Ω and 388Ω resistors satisfy the two constraints above. The transformer converts the single-ended source into a differential stimulus. Similarly, the output of the LT6600-20 will have lower distortion with larger load resistance yet the analyzer input is typically 50Ω. The 4:1 turns (16:1 impedance) transformer and the two 402Ω resistors of Figure 5, present the output of the LT6600-20 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. Differential and Common Mode Voltage Ranges The differential amplifiers inside the LT6600-20 contain circuitry to limit the maximum peak-to-peak differential voltage through the filter. This limiting function prevents excessive power dissipation in the internal circuitry and provides output short-circuit protection. The limiting function begins to take effect at output signal levels above 2VP-P and it becomes noticeable above 3.5VP-P. This is illustrated in Figure 6; the LT6600-20 was configured with unity passband gain and the input of the filter was driven with a 1MHz signal. Because this voltage limiting takes place well before the output stage of the filter reaches the 2.5V 0.1µF NETWORK ANALYZER SOURCE 50Ω COILCRAFT TTWB-1010 1:1 388Ω 1 7 53.6Ω 388Ω 2 8 COILCRAFT TTWB-16A 4:1 402Ω NETWORK ANALYZER INPUT 50Ω 402Ω 66002 F05 W U U 3 – + – 6 4 LT6600-20 + 5 0.1µF – 2.5V Figure 5 66002f LT6600-20 APPLICATIO S I FOR ATIO 20 0 OUTPUT LEVEL (dBV) 1dB PASSBAND GAIN COMPRESSION POINTS 1MHz 25°C 1MHz 85°C –20 –40 –60 –80 –100 3RD HARMONIC 85°C 3RD HARMONIC 25°C 2ND HARMONIC 25°C 2ND HARMONIC 85°C 0 1 4 3 5 2 1MHz INPUT LEVEL (VP-P) 6 7 –120 66002 F06 Figure 6. Output Level vs Input Level, Differential 1MHz Input, Gain = 1 supply rails, the input/output behavior of the IC shown in Figure 6 is relatively independent of the power supply voltage. The two amplifiers inside the LT6600-20 have independent control of their output common mode voltage (see the “block diagram” section). The following guidelines will optimize the performance of the filter. Pin 7 must be bypassed to an AC ground with a 0.01µF or larger capacitor. Pin 7 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 Pin 7. While the internal 11k resistors are well matched, their absolute value can vary by ±20%. This should be taken into consideration when connecting an external resistor network to alter the voltage of Pin 7. Pin 2 can be shorted to Pin 7 for simplicity. If a different common mode output voltage is required, connect Pin 2 to a voltage source or resistor network. For 3V and 3.3V supplies the voltage at Pin 2 must be less than or equal to the mid supply level. For example, voltage (Pin 2) ≤ 1.65V on a single 3.3V supply. For power supply voltages higher than 3.3V the voltage at Pin 2 should be within the voltage of Pin 7 – 1V to the voltage of Pin 7 + 2V. Pin 2 is a high impedance input. U The LT6600-20 was designed to process a variety of input signals including signals centered around the mid-supply voltage and signals that swing between ground and a positive voltage in a single supply system (Figure 1). The range of allowable input common mode voltage (the average of VIN+ and VIN– in Figure 1) is determined by the power supply level and gain setting (see Distortion vs Input Common Mode Level in the Typical Performance Characteristics). Common Mode DC Currents In applications like Figure 1 and Figure 3 where the LT6600-20 not only provides lowpass filtering but also level shifts the common mode voltage of the input signal, DC currents will be generated through the DC path between input and output terminals. Minimize these currents to decrease power dissipation and distortion. Consider the application in Figure 3. Pin 7 sets the output common mode voltage of the 1st differential amplifier inside the LT6600-20 (see the “Block Diagram” section) at 2.5V. Since the input common mode voltage is near 0V, there will be approximately a total of 2.5V drop across the series combination of the internal 402Ω feedback resistor and the external 100Ω input resistor. The resulting 5mA common mode DC current in each input path, must be absorbed by the sources VIN+ and VIN–. Pin 2 sets the common mode output voltage of the 2nd differential amplifier inside the LT6600-20, and therefore sets the common mode output voltage of the filter. Since, in the example of Figure 3, Pin 2 differs from Pin 7 by 0.5V, an additional 2.5mA (1.25mA per side) of DC current will flow in the resistors coupling the 1st differential amplifier output stage to filter output. Thus, a total of 12.5mA is used to translate the common mode voltages. A simple modification to Figure 3 will reduce the DC common mode currents by 36%. If Pin 7 is shorted to Pin 2 the common mode output voltage of both op amp stages will be 2V and the resulting DC current will be 8mA. Of course, by AC coupling the inputs of Figure 3, the common mode DC current can be reduced to 2.5mA. 66002f W U U 9 LT6600-20 APPLICATIO S I FOR ATIO Noise The noise performance of the LT6600-20 can be evaluated with the circuit of Figure 7. Given the low noise output of the LT6600-20 and the 6dB attenuation of the transformer coupling network, it is necessary to measure the noise floor of the spectrum analyzer and subtract the instrument noise from the filter noise measurement. Example: With the IC removed and the 25Ω resistors grounded, Figure 7, measure the total integrated noise (eS) of the spectrum analyzer from 10kHz to 20MHz. With the IC inserted, the signal source (VIN) disconnected, and the input resistors grounded, measure the total integrated noise out of the filter (eO). With the signal source connected, set the frequency to 1MHz 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: NOISE SPECTRAL DENSITY (nVRMS/√Hz) eIN = (eO )2 – (eS )2 A Table 1 lists the typical input referred integrated noise for various values of RIN. Figure 8 is plot of the noise spectral density as a function of frequency for an LT6600-20 with RIN = 402Ω using the fixture of Figure 7 (the instrument noise has been subtracted from the results). Table 1. Noise Performance PASSBAND GAIN (V/V) 4 2 1 INPUT REFERRED INTEGRATED NOISE 10kHz TO 20MHz 42µVRMS 67µVRMS 118µVRMS INPUT REFERRED NOISE dBm/Hz –148 –143 –139 RIN 100Ω 200Ω 402Ω The noise at each output is comprised of a differential component and a common mode component. Using a transformer or combiner to convert the differential outputs to single-ended signal rejects the common mode noise and gives a true measure of the S/N achievable in the system. Conversely, if each output is measured individually and the 10 U 2.5V 0.1µF VIN RIN 3 COILCRAFT TTWB-1010 25Ω 1:1 SPECTRUM ANALYZER INPUT 50Ω 25Ω 66002 F07 W U U 1 7 2 8 –+ + 6 4 LT6600-20 – RIN 5 0.1µF – 2.5V Figure 7 50 VS = 5V 250 40 200 INTEGRATED NOISE (µVRMS) 30 SPECTRAL DENSITY 20 150 100 10 INTEGRATED 50 0 0.1 1 10 FREQUENCY (MHz) 0 100 66002 F08 Figure 8. Input Referred Noise, Gain = 1 noise power added together, the resulting calculated noise level will be higher than the true differential noise. Power Dissipation The LT6600-20 amplifiers combine high speed with largesignal currents in a small package. There is a need to ensure that the die junction temperature does not exceed 150°C. The LT6600-20 package has Pin 6 fused to the lead frame to enhance thermal conduction when connecting to a ground plane or a large metal trace. Metal trace and plated through-holes can be used to spread the heat generated by the device to the backside of the PC board. For example, on a 3/32" FR-4 board with 2oz copper, a total of 660 square millimeters connected to Pin 6 of the LT6600-20 (330 square millimeters on each side of the PC board) will result in a thermal resistance, θJA, of about 85°C/W. Without the extra metal trace connected to the V – pin to provide a heat sink, the thermal resistance will be around 105°C/W. Table 2 can be used as a guide when considering thermal resistance. 66002f LT6600-20 APPLICATIO S I FOR ATIO COPPER AREA TOPSIDE (mm2) 1100 330 35 35 0 BACKSIDE (mm2) 1100 330 35 0 0 BOARD AREA (mm2) 2500 2500 2500 2500 2500 Table 2. LT6600-20 SO-8 Package Thermal Resistance THERMAL RESISTANCE (JUNCTION-TO-AMBIENT) 65°C/W 85°C/W 95°C/W 100°C/W 105°C/W 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 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. PACKAGE DESCRIPTIO S8 Package 8-Lead Plastic Small Outline (Narrow .150 Inch) (Reference LTC DWG # 05-08-1610) .045 ±.005 .050 BSC 8 .189 – .197 (4.801 – 5.004) NOTE 3 7 6 5 .245 MIN .030 ±.005 TYP RECOMMENDED SOLDER PAD LAYOUT .010 – .020 × 45° (0.254 – 0.508) .008 – .010 (0.203 – 0.254) 0°– 8° TYP .016 – .050 (0.406 – 1.270) NOTE: 1. DIMENSIONS IN INCHES (MILLIMETERS) 2. DRAWING NOT TO SCALE 3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm) Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. U 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 worstcase conditions, estimate the thermal resistance from Table 2, then apply the equation for TJ. For example, using the circuit in Figure 3 with a DC differential input voltage of 250mV, a differential output voltage of 1V, no load resistance and an ambient temperature of 85°C, the supply current (current into Pin 3) measures 55.5mA. Assuming a PC board layout with a 35mm2 copper trace, the θJA is 100°C/W. The resulting junction temperature is: TJ = TA + (PD • θJA) = 85 + (5 • 0.0555 • 100) = 113°C When using higher supply voltages or when driving small impedances, more copper may be necessary to keep TJ below 150°C. .160 ±.005 .228 – .244 (5.791 – 6.197) .150 – .157 (3.810 – 3.988) NOTE 3 1 2 3 4 .053 – .069 (1.346 – 1.752) .004 – .010 (0.101 – 0.254) .014 – .019 (0.355 – 0.483) TYP .050 (1.270) BSC SO8 0303 W U U U 66002f 11 LT6600-20 TYPICAL APPLICATIO Amplitude Response 10 0 –10 –20 GAIN (dB) –30 –40 –50 –60 –70 –80 VS = ± 2.5V GAIN = 1 C = 39pF R = 200Ω TA = 25°C 10 1 FREQUENCY (MHz) 100 66002 TA04 –90 0.1 RELATED PARTS PART NUMBER LTC 1565-31 LTC1566-1 LT1567 LT1568 LT6600-2.5 LT6600-10 ® DESCRIPTION 650kHz Linear Phase Lowpass Filter Low Noise, 2.3MHz Lowpass Filter Very Low Noise, High Frequency Filter Building Block Very Low Noise, 4th Order Building Block Very Low Noise Differential Amplifier and 2.5MHz Lowpass Filter Very Low Noise Differential Amplifier and 10MHz Lowpass Filter 12 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 q FAX: (408) 434-0507 q www.linear.com U A 5th Order, 20MHz Lowpass Filter V+ 3 0.1µF VIN– R R C VIN+ C= R R 1 7 2 8 – + – 6 4 VOUT+ VOUT– LT6600-20 + 5 0.1µF 1 2π • R • 20MHz GAIN = 402Ω , MAXIMUM GAIN = 4 V – 2R 66002 TA02a Transient Response, Gain = 1 VOUT+ 50mV/DIV DIFFERENTIAL INPUT 200mV/DIV 100ns/DIV 66002 TA03 COMMENTS Continuous Time, SO8 Package, Fully Differential Continuous Time, SO8 Package 1.4nV/√Hz Op Amp, MSOP Package, Fully Differential Lowpass and Bandpass Filter Designs Up to 10MHz, Differential Outputs 86dB S/N with 3V Supply, SO-8 82dB S/N with 3V Supply, SO-8 66002f LT/TP 0503 1K • PRINTED IN USA © LINEAR TECHNOLOGY CORPORATION 2003