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LT6600-10 Datasheet(PDF) 11 Page - Linear Technology |
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LT6600-10 Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page LTC6400-14 11 640014fb APPLICATIONS INFORMATION Circuit Operation The LTC6400-14 is a low noise and low distortion fully differential op amp/ADC driver with: • Operation from DC to 2.4GHz (–3dB bandwidth) • Fixed gain of 5V/V (14dB) • Differential input impedance 200 Ω • Differential output impedance 25 Ω • On-Chip 590MHz output filter The LTC6400 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 100 Ω/1000Ω resistors in the feedback network. Small output resistors of 12.5 Ωimprovethecircuitstability 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 50 Ω are available for additional filtering. Lowpass/bandpass filters are easily implemented with just a couple of external components. Moreover, they of- fer single-ended 50 Ω matching in wideband applications and no external resistor is needed. The LTC6400-14 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 1.8V for proper operation. If the inputs are AC-coupled, the input common mode voltage is automatically biased approximately 450mV above VOCM and thus no external circuitry is needed for bias. The LTC6400-14 provides an output common mode voltage set by VOCM, which allows driving an ADC directly without external components such as a transformer or AC coupling capacitors. The input signal can be either single-ended or differential with only minor differences in distortion performance. Input Impedance and Matching The differential input impedance of the LTC6400-14 is 200 Ω. If a 200Ω source impedance is unavailable, then the differential inputs may need to be terminated to a lower value impedance, e.g. 50 Ω, in order to provide an imped- ance match for the source. Several choices are available. One approach is to use a differential shunt resistor (Figure 1). Another approach is to employ a wide band transformer (Figure 2). Both methods provide a wide band impedance 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 LTC6400-14 for frequency selection and/or noise reduction. Referring to Figure 3, LTC6400-14 can be easily configured for single-ended input and differential output without a balun. The signal is fed to one of the inputs through a matching network while the other input is connected to the same matching network and a source resistor. Because the return ratios of the two feedback paths are equal, the two outputs have the same gain and thus symmetrical swing. Figure 1. Input Termination for Differential 50 Ω Input Impedance Using Shunt Resistor Figure 2. Input Termination for Differential 50 Ω Input Impedance Using a 1:4 Balun 640014 F01 +OUT +OUTF –OUTF –OUT +IN IN+ OUT– IN– OUT+ +IN –IN –IN 100Ω 66.5Ω 12.5Ω 500Ω LTC6400-14 100Ω 25Ω 25Ω VIN 500Ω 12.5Ω 50Ω 50Ω 2.7pF 13 14 15 16 7 5 6 8 + – 640014 F02 +OUT +OUTF –OUTF –OUT +IN IN+ OUT– IN– OUT+ +IN –IN –IN 100Ω 12.5Ω 500Ω LTC6400-14 100Ω 25Ω 25Ω VIN 500Ω 12.5Ω 50Ω 50Ω 2.7pF 13 14 15 16 7 5 6 8 + – 1:4 • • MINI-CIRCUITS TCM4-19 |
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