LT6600-5

LT6600-5 Very Low Noise, Differential Amplifier and 5MHz Lowpass Filter FEATURES n n n n DESCRIPTION The LT®6600-5 combines a fully differential amplifier with a 4th order 5MHz lowpass filter approximating a Chebyshev frequency response. Most differential amplifiers require many precision external components to tailor gain and bandwidth. In contrast, with the LT6600-5, two external resistors program differential gain, and the filter’s 5MHz cutoff frequency and passband ripple are internally set. The LT6600-5 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 LT6600-5 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 82dB. 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 LT6600-5 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. For similar devices with other cutoff frequencies, refer to the LT6600-20, LT6600-10 and LT6600-2.5. n n n n n 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 5MHz Cutoff 82dB S/N with 3V Supply and 2VP-P Output Low Distortion, 2VP-P, 800Ω Load 1MHz: 93dBc 2nd, 96dBc 3rd Fully Differential Inputs and Outputs Compatible with Popular Differential Amplifier Pinouts Available in an SO-8 Package APPLICATIONS n n n n 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 L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATION Dual, Matched, 5MHz Lowpass Filter 3V 0.1μF RIN 0.01μF IIN 30 1 7 2 8 RIN VOCM (1V-1.5V) RIN 0.01μF QIN 1 7 2 8 RIN 3 5MHz Phase Distribution (50 Units) –+ +– 6 4 QOUT PERCENTAGE OF UNITS (%) 25 20 15 10 5 IOUT 0 5 –135 –134.5 –134 –133.5 –133 –132.5 –132 –131.5 5MHz PHASE (DEG) 66005 TA01 LT6600-5 5 GAIN = 3V 0.1μF 3 806Ω RIN –+ +– 6 4 LT6600-5 66005fa 1 LT6600-5 ABSOLUTE MAXIMUM RATINGS (Note 1) PIN CONFIGURATION TOP VIEW IN– 1 VOCM 2 V+ 3 OUT+ 4 8 7 6 5 IN+ VMID V– OUT– Total Supply Voltage .................................................11V Input Voltage (Note 8)...............................................±VS Input Current (Note 8)..........................................±10mA 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 S8 PACKAGE 8-LEAD PLASTIC SO TJMAX = 150°C, θJA = 100°C/W ORDER INFORMATION LEAD FREE FINISH LT6600CS8-5#PBF LT6600IS8-5#PBF LEAD BASED FINISH LT6600CS8-5 LT6600IS8-5 TAPE AND REEL LT6600CS8-5#TRPBF LT6600IS8-5#TRPBF TAPE AND REEL LT6600CS8-5#TR LT6600IS8-5#TR PART MARKING 66005 6600I5 PART MARKING 66005 6600I5 PACKAGE DESCRIPTION 8-Lead Plastic SO 8-Lead Plastic SO PACKAGE DESCRIPTION 8-Lead Plastic SO 8-Lead Plastic SO SPECIFIED TEMPERATURE RANGE –40°C to 85°C –40°C to 85°C SPECIFIED TEMPERATURE RANGE –40°C to 85°C –40°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. 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/ 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 = 806Ω, and RLOAD = 1k. PARAMETER Filter Gain, VS = 3V CONDITIONS VIN = 2VP-P, fIN = DC to 260kHz VIN = 2VP-P, fIN = 500k (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 2.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 4MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 15MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 25MHz (Gain Relative to 260kHz) Filter Gain, VS = 5V VIN = 2VP-P, fIN = DC to 260kHz VIN = 2VP-P, fIN = 500k (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 2.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 4MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 15MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 25MHz (Gain Relative to 260kHz) Filter Gain, VS = ± 5V VIN = 2VP-P, fIN = DC to 260kHz l l l l l l l l l l l l ELECTRICAL CHARACTERISTICS MIN – 0.5 –0.15 –0.4 – 0.7 –1.1 TYP 0 0 – 0.1 – 0.1 –0.2 – 28 –44 MAX 0.5 0.1 0.3 0.6 0.8 –25 0.5 0.1 0.3 0.6 0.8 –25 0.4 UNITS dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB 66005fa – 0.5 – 0.15 –0.4 – 0.7 – 1.1 0 0 – 0.1 –0.1 –0.2 – 28 – 44 – 0.6 –0.1 2 LT6600-5 ELECTRICAL CHARACTERISTICS PARAMETER Filter Gain, RIN = 229Ω CONDITIONS VIN = 2VP-P, fIN = DC to 260kHz VS = 3V VS = 5V VS = ±5V 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 = 806Ω, and RLOAD = 1k. MIN 10.4 10.3 10.1 TYP 10.9 10.8 10.7 780 45 2nd Harmonic 3rd Harmonic 2nd Harmonic 3rd Harmonic VS = 5V VS = 3V VS = 3V VS = 5V VS = ±5V VS = 3V VS = 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 VS = 5 VS = 3 VS = 3V, VS = 5 VS = 3V, VS = 5 VS = ±5V l l l l l l l l l l l l l l l l l l l l l l l l MAX 11.5 11.4 11.3 UNITS dB dB dB ppm/C μVRMS dBc dBc dBc dBc VP-P DIFF VP-P DIFF μA Filter Gain Temperature Coefficient (Note 2) fIN = 260kHz, VIN = 2VP-P Noise Distortion (Note 4) Noise BW = 10kHz to 5MHz, RIN = 806Ω 1MHz, 2VP-P, RL = 800Ω 5MHz, 2VP-P, RL = 800Ω Differential Output Swing Input Bias Current Input Referred Differential Offset Measured Between Pins 4 and 5 Pin 7 Shorted to Pin 2 Average of Pin 1 and Pin 8 RIN = 806Ω 93 96 66 73 3.85 3.85 –70 4.8 4.8 –30 5 10 8 5 5 5 10 0.0 0.0 –2.5 1.0 1.5 –2.5 –25 –30 –55 2.46 4.3 –15 –10 5 0 –5 61 2.51 1.5 5.5 –3 –3 28 30 31 34 38 2.55 7.7 25 30 35 13 16 20 1.5 3.0 1.0 1.5 3.0 2.0 50 45 35 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 RIN = 229Ω Differential Offset Drift Input Common Mode Voltage (Note 3) Differential Input = 500mVP-P, RIN = 229Ω Differential Output = 2VP-P, Pin 7 at Midsupply 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 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). Specification guaranteed for RIN ≥ 229Ω. 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 is guaranteed functional over the operating temperature range –40°C to 85°C. Note 7: The LT6600C 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 is guaranteed to meet specified performance from –40°C to 85°C. Note 8: The inputs are protected by back-to-back diodes. If the differential input voltage exceeds 1.4V, the input current should be limited to less than 10mA. 66005fa 3 LT6600-5 TYPICAL PERFORMANCE CHARACTERISTICS Amplitude Response 10 0 –10 –20 GAIN (dB) GAIN (dB) –30 –40 –50 –60 –70 –80 0.1 1 10 FREQUENCY (MHz) 100 66005 G01 Passband Gain and Delay 1 VS = 5V GAIN = 1 TA = 25°C 0 –1 –2 –3 –4 –5 –6 –7 –8 GAIN = 1 TA = 25°C –9 012 DELAY GAIN 120 110 100 90 DELAY (ns) GAIN (dB) 80 70 60 50 40 30 34567 FREQUENCY (MHz) 8 9 20 10 13 12 11 10 9 8 7 6 5 Passband Gain and Delay 120 GAIN 110 100 90 DELAY 80 70 60 50 40 30 20 8 9 10 DELAY (ns) 4 GAIN = 4 TA = 25°C 3 01234567 FREQUENCY (MHz) 66005 G02 66005 G03 Output Impedance vs Frequency 100 VS = 5V GAIN = 1 TA = 25°C 90 80 70 CMRR (dB) Common Mode Rejection Ratio VS = 5V GAIN = 1 VIN = 1VP-P TA = 25°C PSRR (dB) 80 70 60 50 40 30 20 40 10 0.1 1 10 FREQUENCY (MHz) 100 66005 G05 Power Supply Rejection Ratio OUTPUT IMPEDANCE (Ω) 10 60 50 1 0.1 0.1 1 10 FREQUENCY (MHz) 100 66005 G04 30 0.01 0 0.01 VS = 3V VIN = 200mVP-P TA = 25°C V+ TO DIFFOUT 0.1 1 10 FREQUENCY (MHz) 100 66005 G06 Distortion vs Frequency –50 –60 DISTORTION (dB) –70 –80 –90 DIFFERENTIAL INPUT, 2ND HARMONIC DIFFERENTIAL INPUT, 3RD HARMONIC SINGLE-ENDED INPUT, 2ND HARMONIC SINGLE-ENDED INPUT, 3RD HARMONIC –50 –60 DISTORTION (dB) –70 –80 –90 Distortion vs Frequency DIFFERENTIAL INPUT, 2ND HARMONIC DIFFERENTIAL INPUT, 3RD HARMONIC SINGLE-ENDED INPUT, 2ND HARMONIC SINGLE-ENDED INPUT, 3RD HARMONIC –40 Distortion vs Signal Level VS = 3V RL = 800Ω –50 T = 25°C A –60 –70 –80 –90 3RD HARMONIC, 5MHz INPUT 2ND HARMONIC, 5MHz INPUT 3RD HARMONIC, 1MHz INPUT –100 –110 VS = 3V, VIN = 2VP-P RL = 800Ω, TA = 25°C 0.1 1 FREQUENCY (MHz) 10 66005 G07 –100 –110 VS = ±5V, VIN = 2VP-P RL = 800Ω, TA = 25°C 0.1 1 FREQUENCY (MHz) 10 66005 G08 DISTORTION (dB) –100 –110 0 1 2ND HARMONIC, 1MHz INPUT 2 3 INPUT LEVEL (VP-P) 4 5 66005 G09 66005fa 4 LT6600-5 TYPICAL PERFORMANCE CHARACTERISTICS Distortion vs Signal Level –40 DISTORTION COMPONENT (dB) –50 DISTORTION (dB) –60 –70 –80 –90 –100 –110 0 3RD HARMONIC 1MHz INPUT 2ND HARMONIC 1MHz INPUT VS = ±5V RL = 800Ω, TA = 25°C 1 2 3 4 5 66005 G10 Distortion vs Input Common Mode –40 –50 –60 –70 –80 –90 2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V –40 DISTORTION COMPONENT (dB) –50 –60 –70 –80 –90 Distortion vs Input Common Mode 2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V 3RD HARMONIC 5MHz INPUT 2ND HARMONIC 5MHz INPUT INPUT LEVEL (VP-P) –100 GAIN = 1, PIN 7 = VS/2 2VP-P 1MHz INPUT RL = 800Ω, TA = 25°C –110 –1 0 1 2 3 –3 –2 INPUT COMMON MODE VOLTAGE RELATIVE TO PIN 7 (V) 66005 G11 –100 GAIN = 4, PIN 7 = VS/2 2VP-P 1MHz INPUT RL = 800Ω, TA = 25°C –110 2 3 –3 –1 0 1 –2 INPUT COMMON MODE VOLTAGE RELATIVE TO PIN 7 (V) 66005 G12 Power Supply Current vs Power Supply Voltage 36 POWER SUPPLY CURRENT (mA) 34 32 30 28 26 24 22 20 2 6 10 4 8 TOTAL SUPPLY VOLTAGE (V) 12 66005 G13 Transient Response, Differential Gain = 1, Single-Ended Input, Differential Output 20 OUT– 200mV/DIV 0 OUTPUT LEVEL (dBV) –20 –40 –60 –80 Distortion vs Temperature 1dB PASSBAND GAIN COMPRESSION POINTS 1MHz TA = 25°C 1MHz TA = 85°C 3RD HARMONIC TA = 85°C 3RD HARMONIC TA = 25°C TA = 85°C OUT+ 200mV/DIV TA = 25°C TA = –40°C IN– 500mV/DIV IN+ 100ns/DIV 66005 G14 –100 –120 0 1 2ND HARMONIC TA = 85°C 2ND HARMONIC TA = 25°C 4 3 5 2 1MHz INPUT LEVEL (VP-P) 6 7 66005 G15 Distortion vs Output Common Mode –40 DISTORTION COMPONENT (dB) GAIN = 4 PIN 7 = VS/2 –50 TA = 25°C 0.5VP-P 1MHz INPUT –60 RL = 800Ω –70 –80 –90 2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V 2ND HARMONIC, VS = ±5V 3RD HARMONIC, VS = ±5V 45 40 NOISE DENSITY (nV/√Hz) 35 30 25 20 15 10 5 2.5 Input Referred Noise INTEGRATED NOISE, GAIN = 1X INTEGRATED NOISE, GAIN = 4X NOISE DENSITY, GAIN = 1X NOISE DENSITY, GAIN = 4X 90 80 70 60 50 40 30 20 10 0.1 FREQUENCY (MHz) 66005 G17 INTEGRATED NOISE (μV) –100 –110 –1.5 –1.0 –0.5 0 0.5 1.0 1.5 2.0 VOLTAGE PIN 2 TO PIN 7 (V) 0 0.01 10 0 100 66005 G16 66005fa 5 LT6600-5 PIN FUNCTIONS 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 806Ω/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 to AC ground. 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 DIAGRAM VIN+ RIN 8 IN+ VMID 7 V+ 11k 806Ω 11k 400Ω V– OP AMP V– 6 OUT– 5 PROPRIETARY LOWPASS FILTER STAGE + VOCM 400Ω – + 400Ω +– VOCM – –+ 400Ω 806Ω 1 VIN– RIN IN– 2 VOCM 3 V+ 4 66005 BD OUT+ 66005fa 6 LT6600-5 APPLICATIONS INFORMATION Interfacing to the LT6600-5 The LT6600-5 requires 2 equal external resistors, RIN, to set the differential gain to 806Ω/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 LT6600-5 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 LT6600-5 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 V 3 2 1 0 VIN+ VIN VIN– t + is 2VP-P for frequencies below 5MHz. 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 (refer to the Distortion vs Input Common Mode Level graphs in the Typical Performance Characteristics). Figure 2 shows how to AC couple signals into the LT6600-5. 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 806Ω gain setting resistor form a high pass filter, attenuating signals below 2kHz. Larger values of coupling capacitors will proportionally reduce this highpass 3dB frequency. In Figure 3 the LT6600-5 is providing 12dB of gain. The gain resistor has an optional 62pF in parallel to improve 3.3V 0.1μF V 3 3 VIN – 806Ω 1 7 0.01μF 2 8 – LT6600-5 + 4 VOUT+ VOUT– 2 1 0 VOUT+ VOUT– t 66005 F01 + 6 –5 806Ω Figure 1 3.3V V 0.1μF 2 1 0 –1 VIN+ 0.1μF t VIN + 0.1μF 806Ω 1 7 0.01μF 806Ω 2 8 3 V 3 VOUT+ VOUT– 2 1 0 VOUT+ VOUT– – LT6600-5 + 4 + 6 – 5 66005 F02 Figure 2 62pF V 3 2 1 0 500mVP-P (DIFF) VIN+ VIN– t 62pF 0.01μF VIN + 5V 0.1μF V 3 VOUT+ 2 VOUT– 1 0 VOUT– 3 VIN – 200Ω 1 7 2 8 – LT6600-5 + 4 VOUT+ + 6 – 5 200Ω + – 2V 0.01μF 66005 F03 t Figure 3 66005fa 7 LT6600-5 APPLICATIONS INFORMATION the passband flatness near 5MHz. The common mode output voltage is set to 2V. Use Figure 4 to determine the interface between the LT6600-5 and a current output DAC. The gain, or “transimpedance,” is defined as A = VOUT/IIN Ω. To compute the transimpedance, use the following equation: A= 806 • R1 Ω R1+ R2 Figure 5 is a laboratory setup that can be used to characterize the LT6600-5 using single-ended instruments with 50Ω source impedance and 50Ω input impedance. For a unity gain configuration the LT6600-5 requires a 806Ω source resistance yet the network analyzer output is calibrated for a 50Ω load resistance. The 1:1 transformer, 51.1Ω and 787Ω resistors satisfy the two constraints above. The transformer converts the single-ended source into a differential stimulus. Similarly, the output the LT6600-5 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 LT6600-5 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. 2.5V 0.1μF NETWORK ANALYZER SOURCE 50Ω COILCRAFT TTWB-1010 1:1 787Ω 1 7 51.1Ω 2 8 787Ω CURRENT OUTPUT DAC IIN– R1 IIN+ R1 R2 1 7 0.01μF R2 2 8 3.3V 0.1μF –2.5V 3 COILCRAFT TTWB-16A 4:1 402Ω NETWORK ANALYZER INPUT By setting R1 + R2 = 806Ω, the gain equation reduces to A = R1Ω. The voltage at the pins of the DAC is determined by R1, R2, the voltage on Pin 7 and the DAC output current (IIN+ or IIN–). Consider Figure 4 with R1 = 49.9Ω and R2 = 750Ω. The voltage at Pin 7 is 1.65V. The voltage at the DAC pins is given by: VDAC = VPIN7 • R1 R1• R2 + IIN R1+ R2 + 806 R1+ R2 = 51mV + IIN 46.8Ω 3 IIN is IIN– or IIN+. The transimpedance in this example is 50.3Ω. – + – 6 4 LT6600-5 402Ω 5 50Ω + 0.1μF 66005 F05 – + – 6 4 VOUT+ VOUT– Figure 5 LT6600-5 Differential and Common Mode Voltage Ranges The differential amplifiers inside the LT6600-5 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 LTC6600-5 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 supply rails, the input/output behavior of the IC shown in Figure 6 is relatively independent of the power supply voltage. 66005fa + 5 66005 F04 Figure 4 Evaluating the LT6600-5 The low impedance levels and high frequency operation of the LT6600-5 require some attention to the matching networks between the LT6600-5 and other devices. The previous examples assume an ideal (0Ω) source impedance and a large (1kΩ) load resistance. Among practical examples where impedance must be considered is the evaluation of the LT6600-5 with a network analyzer. 8 LT6600-5 APPLICATIONS INFORMATION 20 0 OUTPUT LEVEL (dBV) –20 –40 –60 –80 2ND HARMONIC TA = 85°C 2ND HARMONIC TA = 25°C 0 1 4 3 5 2 1MHz INPUT LEVEL (VP-P) 6 7 66005 F06 1dB PASSBAND GAIN COMPRESSION POINTS 1MHz TA = 25°C 1MHz TA = 85°C the power supply level and gain setting (see “Electrical Characteristics”). Common Mode DC Currents In applications like Figure 1 and Figure 3 where the LT6600-5 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. Consider the application in Figure 3. Pin 7 sets the output common mode voltage of the 1st differential amplifier inside the LT6600-5 (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 806Ω feedback resistor and the external 200Ω input resistor. The resulting 2.5mA 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 LT6600-5, and therefore sets the common mode output voltage of the filter. Since in the example, Figure 3, Pin 2 differs from Pin 7 by 0.5V, an additional 1.25mA (0.625mA per side) of DC current will flow in the resistors coupling the 1st differential amplifier output stage to filter output. Thus, a total of 6.25mA 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 4mA. Of course, by AC coupling the inputs of Figure 3 and shorting Pin 7 to Pin 2, the common mode DC current is eliminated. Noise The noise performance of the LT6600-5 can be evaluated with the circuit of Figure 7. Given the low noise output of the LT6600-5 and the 6dB attenuation of the transformer coupling network, it will be necessary to measure the noise floor of the spectrum analyzer and subtract the instrument noise from the filter noise measurement. 66005fa 3RD HARMONIC TA = 85°C 3RD HARMONIC TA = 25°C –100 –120 Figure 6 The two amplifiers inside the LT6600-5 have independent control of their output common mode voltage (see the “block diagram” section). The following guidelines will optimize the performance of the filter for single supply operation. Pin 7 must be bypassed to an AC ground with a 0.01μF or higher 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 can be set above mid-supply. The voltage on Pin 2 should not be more than 1V below the voltage on Pin 7. The voltage on Pin 2 should not be more than 2V above the voltage on Pin 7. Pin 2 is a high impedance input. The LT6600-5 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 9 LT6600-5 APPLICATIONS INFORMATION 2.5V 0.1μF VIN RIN 3 NOISE DENSITY (nV/√Hz) COILCRAFT TTWB-1010 25Ω 1:1 25Ω SPECTRUM ANALYZER INPUT 45 40 35 30 25 20 15 10 5 0 0.01 0.1 FREQUENCY (MHz) 66005 G08 INTEGRATED NOISE, GAIN = 1X INTEGRATED NOISE, GAIN = 4X NOISE DENSITY, GAIN = 1X NOISE DENSITY, GAIN = 4X 90 80 70 60 50 40 30 20 10 INTEGRATED NOISE (μV) 1 7 2 8 –+ + 6 4 LT6600-5 50Ω – RIN 5 0.1μF 66005 F07 –2.5V Figure 7 10 0 100 Example: With the IC removed and the 25Ω resistors grounded, measure the total integrated noise (eS) of the spectrum analyzer from 10kHz to 5MHz. 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: Figure 8 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 LT6600-5 amplifiers combine high speed with largesignal currents in a small package. There is a need to ensure that the dies’s junction temperature does not exceed 150°C. The LT6600-5 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 LT6600-5 (330 square millimeters on each side of the PC board) will result in a thermal resistance, θJA, of about 85°C/W. Without 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. Table 2. LT6600-5 SO-8 Package Thermal Resistance COPPER AREA TOPSIDE (mm2) 1100 330 35 35 0 BACKSIDE (mm2) 1100 330 35 0 0 BOARD AREA (mm2) 2500 2500 2500 2500 2500 THERMAL RESISTANCE (JUNCTION-TO-AMBIENT) 65°C/W 85°C/W 95°C/W 100°C/W 105°C/W 66005fa 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 LT6600-5 with RIN = 806Ω and 200Ω 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 10MHz 24μVRMS 38μVRMS 69μVRMS INPUT REFERRED NOISE dBm/Hz –149 –145 –140 RIN 200Ω 402Ω 806Ω 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. 10 LT6600-5 APPLICATIONS INFORMATION 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. 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, estimate the thermal resistance from Table 2, then apply the equation for TJ. For example, using the circuit in Figure 3 with DC differential input voltage of 250mV, a differential output voltage of 1V, 1kΩ load resistance and an ambient temperature of 85°C, the supply current (current into Pin 3) measures 32.2mA. 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 • 0.0322 • 100) = 101°C When using higher supply voltages or when driving small impedances, more copper may be necessary to keep TJ below 150°C. PACKAGE DESCRIPTION S8 Package 8-Lead Plastic Small Outline (Narrow .150 Inch) (Reference LTC DWG # 05-08-1610) .189 – .197 (4.801 – 5.004) NOTE 3 8 7 6 5 .050 BSC .045 ±.005 .245 MIN .160 ±.005 .228 – .244 (5.791 – 6.197) .150 – .157 (3.810 – 3.988) NOTE 3 .030 ±.005 TYP RECOMMENDED SOLDER PAD LAYOUT .010 – .020 × 45° (0.254 – 0.508) .008 – .010 (0.203 – 0.254) .016 – .050 (0.406 – 1.270) NOTE: 1. DIMENSIONS IN 0°– 8° TYP 1 2 3 4 .053 – .069 (1.346 – 1.752) .004 – .010 (0.101 – 0.254) 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) .014 – .019 (0.355 – 0.483) TYP .050 (1.270) BSC SO8 0303 66005fa 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. 11 LT6600-5 TYPICAL APPLICATION Dual, Matched, 6th Order, 5MHz Lowpass Filter Single-Ended Input (IIN and QIN) and Differential Output (IOUT and QOUT) IIN 0.1μF V+ 0.1μF V+ 806Ω 1 249Ω QIN 249Ω 249Ω 2 3 4 5 6 7 8 0.1μF V– V + 1 7 LT1568 15 INVA INVB 14 SA SB 13 OUTA OUTB 12 OUTA OUTB 11 GNDA GNDB 10 NC EN – –9 V V V + 16 –+ +– 6 V– 3 4 IOUT 249Ω 249Ω 2 8 806Ω LT6600-5 5 0.1μF 249Ω Q I GAIN = OUT OR OUT = 1 IIN QIN 806Ω 1 7 2 8 806Ω V+ 0.1μF –+ +– 6 V– 3 4 QOUT LT6600-5 5 0.1μF 66005 TA02 Amplitude Response 12 0 GAIN (dB) 20 LOG (IOUT/IIN) OR 20 LOG (QOUT/QIN) –12 –24 –36 –48 –60 –72 –84 –96 –108 100 1 10 FREQUENCY (Hz) 40 66005 TA02b Transient Response OUTPUT (IOUT OR QOUT) 200mV/DIV INPUT (IIN OR QIN) 500mV/DIV 100ns/DIV 66005 TA02c RELATED PARTS PART NUMBER LTC 1565-31 LTC1566-1 LT1567 LT1568 LTC1569-7 LT6600-2.5 LT6600-10 LT6600-20 ® 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 Linear Phase, DC Accurate, Tunable 10th Order Lowpass Filter Very Low Noise, Differential Amplifier and 2.5MHz Lowpass Filter Very Low Noise, Differential Amplifier and 10MHz Lowpass Filter Very Low Noise, Differential Amplifier and 20MHz Lowpass Filter COMMENTS Continuous Time, SO8 Package, Fully Differential Continuous Time, SO8 Package, Fully Differential 1.4nV/√Hz Op Amp, MSOP Package, Differential Output Lowpass and Bandpass Filter Designs Up to 10MHz, Differential Outputs One External Resistor Sets Filter Cutoff Frequency, Differential Inputs Adjustable Output Common Mode Voltage Adjustable Output Common Mode Output Voltage Adjustable Output Common Mode Voltage 66005fa 12 Linear Technology Corporation (408) 432-1900 ● FAX: (408) 434-0507 ● LT 0408 REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 www.linear.com © LINEAR TECHNOLOGY CORPORATION 2004