LT6600-2.5 LINER
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LT6600-2.5 660025i , LTC and LT are registered trademarks of Linear Technology Corporation. I Programmable Differential Gain via Two External Resistors I Adjustable Output Common Mode Voltage I Operates and Specified with 3V, 5V, ±5V Supplies I 0.5dB Ripple 4th Order Lowpass Filter with 2.5MHz Cutoff I 86dB S/N with 3V Supply and 1VRMS Output I Low Distortion, 1VRMS, 800Ω Load 1MHz: 95dBc 2nd, 88dBc 3rd I Fully Differential Inputs and Outputs I Compatible with Popular Differential Amplifier Pinouts Very Low Noise, Differential Amplifier and 2.5MHz Lowpass Filter June 2003 I High Speed ADC Antialiasing and DAC Smoothing in Networking or Cellular Base Station Applications I High Speed Test and Measurement Equipment I Medical Imaging I Drop-in Replacement for Differential Amplifiers The LT ®6600-2.5 combines a fully differential amplifier with a 4th order 2.5MHz lowpass filter approximating a Chebyshev frequency response. Most differential amplifi- ers require many precision external components to tailor gain and bandwidth. In contrast, with the LT6600-2.5, two external resistors program differential gain, and the filter’s 2.5MHz cutoff frequency and passband ripple are inter- nally set. The LT6600-2.5 also provides the necessary level shifting to set its output common mode voltage to accom- modate the reference voltage requirements of A/Ds. Using a proprietary internal architecture, the LT6600-2.5 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 86dB. At higher gains the input referred noise decreases so the part can process smaller input differential signals without signifi- cantly degrading the output signal-to-noise ratio. The LT6600-2.5 also features low voltage operation. The differential design provides outstanding performance for a 4VP-P signal level while the part operates with a single 3V supply. The LT6600-2.5 is available in an SO-8 package. For similar devices with higher cutoff frequency, refer to the LT6600-10 and LT6600-20 data sheets. Amplitude Response DESCRIPTIO U
FEATURES
U TYPICAL APPLICATIO U Final Electrical Specifications 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 represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 1580Ω 1580Ω 0.01µF 0.1µF LT6600-2.5 VIN+ VIN– VOUT–
660025 TA01a
FREQUENCY (Hz) 100k –36 GAIN (dB) –24 –12 10M 50M
660025 TA01b
–48 –60 –84 –96 –72 VS = ±2.5V
LT6600-2.5 660025i PARAMETER CONDITIONS MIN TYP MAX UNITS Filter Gain, VS = 3V VIN = 2VP-P, fIN = DC to 260kHz – 0.5 0.1 0.4 dB RIN = 1580Ω VIN = 2VP-P, fIN = 700kHz (Gain Relative to 260kHz) G – 0.15 0.1 dB VIN = 2VP-P, fIN = 1.9MHz (Gain Relative to 260kHz) G – 0.2 0.2 0.6 dB VIN = 2VP-P, fIN = 2.2MHz (Gain Relative to 260kHz) G – 0.6 0.1 0.5 dB VIN = 2VP-P, fIN = 2.5MHz (Gain Relative to 260kHz) G –2.1 0.9 dB VIN = 2VP-P, fIN = 7.5MHz (Gain Relative to 260kHz) G – 34 – 31 dB VIN = 2VP-P, fIN = 12.5MHz (Gain Relative to 260kHz) G – 51 dB Filter Gain, VS = 5V VIN = 2VP-P, fIN = DC to 260kHz –0.5 –0.1 0.4 dB RIN = 1580Ω VIN = 2VP-P, fIN = 700kHz (Gain Relative to 260kHz) G – 0.15 0.1 dB VIN = 2VP-P, fIN = 1.9MHz (Gain Relative to 260kHz) G – 0.2 0.2 0.6 dB VIN = 2VP-P, fIN = 2.2MHz (Gain Relative to 260kHz) G – 0.6 0.1 0.5 dB VIN = 2VP-P, fIN = 2.5MHz (Gain Relative to 260kHz) G – 2.1 –0.9 dB VIN = 2VP-P, fIN = 7.5MHz (Gain Relative to 260kHz) G –34 –31 dB VIN = 2VP-P, fIN = 12.5MHz (Gain Relative to 260kHz) G –51 dB Filter Gain, VS = ±5V VIN = 2VP-P, fIN = DC to 260kHz –0.6 –0.1 0.4 dB Filter Gain, RIN = 402Ω VIN = 2VP-P, fIN = DC to 260kHz, VS = 3V 11.3 11.8 12.3 dB VIN = 2VP-P, fIN = DC to 260kHz, VS = 5V 11.3 11.8 12.3 dB VIN = 2VP-P, fIN = DC to 260kHz, VS = ±5V 11.2 11.7 12.2 dB Filter Gain Temperature Coefficient (Note 2) fIN = 260kHz, VIN = 2VP-P 780 ppm/C Noise Noise BW = 10kHz to 2.5MHz µVRMS Distortion (Note 4) 1MHz, 1VRMS, RL = 800Ω 2nd Harmonic dBc 3rd Harmonic dBc Differential Output Swing Measured Between Pins 4 and 5 VS = 5V G 8.8 9.3 VP-P DIFF VS = 3V G 5.1 5.5 VP-P DIFF Input Bias Current Average of Pin 1 and Pin 8 G – 35 –15 µA Operating Temperature Range (Note 6) ...–40°C to 85°C Specified Temperature Range (Note 7)....–40°C to 85°C ORDER PART NUMBER S8 PART MARKING TJMAX = 150°C, θJA = 100°C/W 660025 600I25 ABSOLUTE AXI U RATI GS W W W U PACKAGE/ORDER I FOR ATIO U U W (Note 1)
ELECTRICAL CHARACTERISTICS
Consult LTC Marketing for parts specified with wider operating temperature ranges. The G 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 = 1580Ω, and RLOAD = 1k. LT6600CS8-2.5 LT6600IS8-2.5 TOP VIEW IN+ VMID OUT– IN– VOCM OUT+ S8 PACKAGE 8-LEAD PLASTIC SO
LT6600-2.5 660025i 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 ≥ 402Ω. Note 4: Distortion is measured differentially using a single-ended 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. PARAMETER CONDITIONS MIN TYP MAX UNITS Input Referred Differential Offset RIN = 1580Ω VS = 3V G mV VS = 5V G mV VS = ±5V G mV RIN = 402Ω VS = 3V G mV VS = 5V G mV VS = ±5V G mV Differential Offset Drift µV/°C Input Common Mode Voltage (Note 3) Differential Input = 500mVP-P, VS = 3V G 0.0 1.5 V RIN = 402Ω VS = 5V G 0.0 3.0 V VS = ±5V G –2.5 1.0 V Output Common Mode Voltage (Note 5) Differential Input = 2VP-P, VS = 3V G 1.0 1.5 V Pin 7 at Mid-Supply VS = 5V G 1.5 3.0 V VS = ±5V G –1.0 2.0 V Output Common Mode Offset VS = 3V G –25 mV (with Respect to Pin 2) VS = 5V G –30 mV VS = ±5V G –55 –10 mV Common Mode Rejection Ratio dB Voltage at VMID (Pin 7) VS = 5V G 2.46 2.51 2.55 V VS = 3V 1.5 V VMID Input Resistance G 4.3 5.7 7.7 kΩ VOCM Bias Current VOCM = VMID= VS/2 VS = 5V G –15 µA VS = 3V G –10 µA Power Supply Current VS = 3V, VS = 5V mA VS = 3V, VS = 5V G mA VS = ±5V G mA 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: Both the LT6600CS8-2.5 and LT6600IS8-2.5 are guaranteed functional over the operating temperature range of –40°C to 85°C. Note 7: The LT6600CS8-2.5 is guaranteed to meet specified performance from 0°C to 70°C and is designed, characterized and expected to meet specified performance from –40°C and 85°C, but is not tested or QA sampled at these temperatures. The LT6600IS8-2.5 is guaranteed to meet specified performance from –40°C to 85°C. The G 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 = 1580Ω, and RLOAD = 1k.
LT6600-2.5 660025i U U U 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 1580Ω/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 mid- supply, 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.
LT6600-2.5 660025i BLOCK DIAGRA W VOCM VOCM 1580Ω 1580Ω 800Ω 800Ω 800Ω 800Ω 11k 11k OP AMP PROPRIETARY LOWPASS FILTER STAGE VIN– VIN RIN RIN 660025 BD IN+ VOCM OUT+ OUT– VMID IN–
LT6600-2.5 660025i APPLICATIO S I FOR ATIO W U U U Figure 1 Figure 2 Figure 3 Interfacing to the LT6600-2.5 The LT6600-2.5 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 4 and 5 of the LT6600-2.5 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 LT6600-2.5 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 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. Figure 2 shows how to AC couple signals into the LT6600-2.5. 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 propor- tionally reduce this highpass 3dB frequency. 1580Ω 1580Ω 0.01µF 0.1µF 3.3V VIN VIN
660025 F01
VOUT– V t VIN+ VIN– V t VOUT+ LT6600-2.5 VOUT– 1580Ω 1580Ω 0.01µF 0.1µF 0.1µF 0.1µF 3.3V VIN
660025 F02
VOUT– V V t VIN+ LT6600-2.5 VOUT+ VOUT– 402Ω 402Ω 0.1µF 0.01µF VIN VIN
660025 F03
VOUT– V t VIN+ VIN– V t VOUT+ VOUT– LT6600-2.5 500mVP-P (DIFF)
LT6600-2.5 660025i APPLICATIO S I FOR ATIO W U U U In Figure 3 the LT6600-2.5 is providing 12dB of gain. The common mode output voltage is set to 2V. Use Figure 4 to determine the interface between the LT6600-2.5 and a current output DAC. The gain, or “trans- impedance,” is defined as A = VOUT/IIN. To compute the transimpedance, use the following equation: A R R R Ω ( ) 1580 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 Pin 7 and the DAC output current. Consider Figure 4 with R1 = 49.9Ω and R2 = 1540Ω. The voltage at Pin 7 is 1.65V. The voltage at the DAC pins is given by: V V R R R I R R R R mV I DAC PIN IN IN Ω 1580 48 3 IIN is IIN+ or IIN–. The transimpedance in this example is 49.6Ω. Evaluating the LT6600-2.5 The low impedance levels and high frequency operation of the LT6600-2.5 require some attention to the matching networks between the LT6600-2.5 and other devices. The previous examples assume an ideal (0Ω) source imped- ance and a large (1kΩ) load resistance. Among practical examples where impedance must be considered is the evaluation of the LT6600-2.5 with a network analyzer. Figure 5 Figure 5 is a laboratory setup that can be used to charac- terize the LT6600-2.5 using single-ended instruments with 50Ω source impedance and 50Ω input impedance. For a 12dB gain configuration the LT6600-2.5 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 LT6600-2.5 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 LT6600-2.5 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. Differential and Common Mode Voltage Ranges The rail-to-rail output stage of the LT6600-2.5 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 LT6600-2.5 have indepen- dent control of their output common mode voltage (see the “Block Diagram” section). The following guidelines will optimize the performance of the filter. Pin 7 can be allowed to float; Pin 7 must be bypassed to an AC ground with a 0.01µF capacitor or some instability may be observed. Pin 7 can be driven from a low impedance Figure 4 0.1µF 3.3V LT6600-2.5 0.01µF CURRENT OUTPUT DAC VOUT+ VOUT–
660025 F04
IIN– IIN+ 0.1µF 0.1µF 2.5V –2.5V LT6600-2.5
660025 F05
402Ω 402Ω NETWORK ANALYZER INPUT 50Ω COILCRAFT TTWB-16A 4:1 NETWORK ANALYZER SOURCE COILCRAFT TTWB-1010 1:1 50Ω 53.6Ω 388Ω 388Ω
LT6600-2.5 660025i APPLICATIO S I FOR ATIO W U U U 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 exceed 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 LT6600-2.5 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 the power supply level and gain setting (see “Electrical Characteristics”). Common Mode DC Currents In applications like Figure 1 and Figure 3 where the LT6600-2.5 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 be- tween 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 LT6600-2.5 (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 1580Ω feedback resis- tor 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–. Pin 2 sets the common mode output voltage of the 2nd differential amplifier inside the LT6600-2.5, 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 625µA (312µA per side) of DC current will flow in the resistors coupling the 1st differential amplifier output stage to 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 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 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 LT6600-2.5 can be evalu- ated with the circuit of Figure 6. Given the low noise output of the LT6600-2.5 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. 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 input resistors grounded, measure the total integrated noise out of the filter (eO). With the signal source con- nected, set the frequency to 100kHz and adjust the ampli- tude until VIN measures 100mVP-P. Measure the output amplitude, VOUT, and compute the passband gain 0.1µF 0.1µF 2.5V –2.5V LT6600-2.5 RIN RIN 25Ω 25Ω
66002 F06
50Ω VIN COILCRAFT TTWB-1010 1:1 Figure 6
subtracted from the results). Table 1. Noise Performance gives a true measure of the S/N achievable in the system. level will be higher than the true differential noise. considering thermal resistance. Figure 7. Input Referred Noise, Gain = 1
66002 F07
Table 2. LT6600-2.5 SO-8 Package Thermal Resistance
LT6600-2.5 660025i Currents), the load impedance is small and the ambient temperature is maximum. To compute the junction tem- perature, 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 1V, a differential output voltage of 4V, no load resistance and an ambient temperature of 85°C, the supply current APPLICATIO S I FOR ATIO W U U U (current into Pin 3) measures 37.6mA. Assuming a PC board layout with a 35mm2 copper trace, the θJA is 100°C/W. The resulting junction temperature is: When using higher supply voltages or when driving small impedances, more copper may be necessary to keep TJ below 150°C.
LT6600-2.5 660025i 8-Lead Plastic Small Outline (Narrow .150 Inch) (Reference LTC DWG # 05-08-1610) U PACKAGE DESCRIPTIO .016 – .050 (0.406 – 1.270) .010 – .020 0°– 8° TYP .008 – .010 (0.203 – 0.254) SO8 0303 .053 – .069 (1.346 – 1.752) .014 – .019 (0.355 – 0.483) TYP .004 – .010 (0.101 – 0.254) .050 (1.270) BSC .150 – .157 (3.810 – 3.988) NOTE 3 .189 – .197 (4.801 – 5.004) NOTE 3 .228 – .244 (5.791 – 6.197) .245 MIN .160 ±.005 RECOMMENDED SOLDER PAD LAYOUT .045 ±.005 .050 BSC .030 ±.005 TYP INCHES (MILLIMETERS) NOTE: 1. DIMENSIONS IN 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)
LT6600-2.5 660025i RELATED PARTS PART NUMBER
DESCRIPTION
®1565-31 650kHz Linear Phase Lowpass Filter Continuous Time, SO8 Package, Fully Differential LTC1566-1 Low Noise, 2.3MHz Lowpass Filter Continuous Time, SO8 Package LT1567 Very Low Noise, High Frequency Filter Building Block 1.4nV/√Hz Op Amp, MSOP Package, Fully Differential LT1568 Very Low Noise, 4th Order Building Block Lowpass and Bandpass Filter Designs Up to 10MHz, Differential Outputs LT6600-10 Very Low Noise Differential Amplifier and 10MHz 82dB S/N with 3V Supply, SO-8 Package Lowpass Filter LT6600-20 Very Low Noise Differential Amplifier and 20MHz 76dB S/N with 3V Supply, SO-8 Package Lowpass Filter Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 G FAX: (408) 434-0507 G www.linear.com LINEAR TECHNOLOGY CORPORATION 2003 LT/TP 0603 1K • PRINTED IN USA