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LTC6401-8 Datasheet(PDF) 9 Page - Linear Technology |
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LTC6401-8 Datasheet(HTML) 9 Page - Linear Technology |
9 / 16 page LTC6401-8 9 64018f APPLICATIONS INFORMATION Circuit Operation The LTC6401-8 is a low noise and low distortion fully differential op amp/ADC driver with: • Operation from DC to 2.2GHz –3dB bandwidth • Fixed gain of 2.5V/V (8dB) • Differential input impedance 400Ω • Differential output impedance 25Ω • Differential impedance of output filter 100Ω The LTC6401-8 is composed of a fully differential amplifier with on chip feedback and output common mode voltage control circuitry. Differential gain and input impedance are set by 200Ω/500Ω resistors in the feedback network. Small output resistors of 12.5Ω improve the circuit stability over various load conditions. They also provide a possible external filtering option, which is often desirable when the load is an ADC. Filter resistors of 50Ω are available for additional filtering. Lowpass/bandpass filters are easily implemented with just a couple of external components. Moreover, they offer single-ended 50Ω matching in wideband applications and no external resistor is needed. The LTC6401-8 is very flexible in terms of I/O coupling. It can be AC- or DC-coupled at the inputs, the outputs or both. Due to the internal connection between input and output, users are advised to keep input common mode voltage between 1V and 1.6V for proper operation. If the inputs are AC-coupled, the input common mode voltage is automatically biased approximately 250mV above VOCM and thus no external circuitry is needed for bias. The LTC6401-8 provides an output common mode voltage set by VOCM, which allows driving ADC directly without external components such as transformer or AC coupling capacitors. The input signal can be either single-ended or differential with only minor difference in distortion performance. Input Impedance and Matching The differential input impedance of the LTC6401-8 is 400Ω. Usually the differential inputs need to be terminated to a lower value impedance, e.g. 50Ω, in order to provide an impedance match for the source. Several choices are avail- able. One approach is to use a differential shunt resistor (Figure 1). Another approach is to employ a wideband transformer and shunt resistor (Figure 2). Both methods provide a wideband match. The termination resistor or the transformer must be placed close to the input pins in order to minimize the reflection due to input mismatch. Alternatively, one could apply a narrowband impedance match at the inputs of the LTC6401-8 for frequency selec- tion and/or noise reduction. Figure 1. Input Termination for Differential 50Ω Input Impedance Using Shunt Resistor Figure 2. Input Termination for Differential 50Ω Input Impedance Using a Balun 64018 F01 +OUT +OUTF –OUTF –OUT +IN IN+ OUT– IN– OUT+ +IN –IN –IN 200 Ω 57.6 Ω 12.5 Ω 500 Ω LTC6401-8 200 Ω 25 Ω 25 Ω VIN 500 Ω 12.5 Ω 50 Ω 50 Ω 2.7pF 13 14 15 16 7 5 6 8 + – 64018 F02 +OUT +OUTF –OUTF –OUT +IN IN+ OUT– IN– OUT+ +IN –IN MINI CIRCUITS TCM4-19 –IN 200 Ω 12.5 Ω 500 Ω LTC6401-8 200 Ω 25 Ω 402 Ω 25 Ω VIN 500 Ω 12.5 Ω 50 Ω 50 Ω 2.7pF 13 14 15 16 7 5 6 8 + – 1:4 • |
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