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LTC6420IUDC-20-PBF Datasheet(PDF) 9 Page - Linear Technology |
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LTC6420IUDC-20-PBF Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page LTC6420-20 9 642020fa 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. In general, the single-ended input impedance and termination resistor RT are determined by the combination of RS, RG and RF. For example, when RS is 50Ω, it is found that the single-ended input impedance is 202Ω and RT is 66.5Ω in order to match to a 50Ω source impedance. The LTC6420-20 is unconditionally stable. However, the overall differential gain is affected by both source impedance and load impedance as follows: A V = V OUT V IN = 2000 R S + 200 • R L 25 +R L Output Impedance Match The LTC6420-20 can drive an ADC directly without external output impedance matching. Alternatively, the differential output impedance of 25Ω can be matched to a higher value impedance, e.g. 50Ω, by series resistors or an LC network. Output Common Mode Adjustment The output common mode voltage is set by the VOCM pin, which is a high impedance input. The output common mode voltage is capable of tracking VOCM in a range from 1.1V to 1.6V. The bandwidth of VOCM control is typically 15MHz, which is dominated by a low pass filter connected to the VOCM pin and is aimed to reduce common mode noise generation at the outputs. The internal common mode feedback loop has a –3dB bandwidth of 300MHz, allowing fast rejection of any common mode output voltage disturbance. The VOCM pin should be tied to a DC bias voltage with a 0.1μF bypass capacitor. When interfacing with A/D converters such as the LTC22xx families, the VOCM pin can be connected to the VCM pin of the ADC. Driving A/D Converters The LTC6420-20 has been specifically designed to interface directly with high speed A/D converters. The back page of this data sheet shows the LTC6420-20 driving an LTC2285, which is a dual 14-bit, 125Msps ADC. The VOCM pins of the LTC6420-20 are connected to the VCM pins of the LTC2285, which provide a DC voltage level of 1.5V. Both ICs are powered from the same 3V supply voltage. The inputs to the LTC6420-20 can be configured in various ways, as described in the Input Impedance and Matching section of this data sheet. The outputs of the LTC6420-20 may be connected directly to the analog inputs of an ADC, or a simple lowpass or bandpass filter network may be inserted to reduce out-of-band noise. Test Circuits Due to the fully-differential design of the LTC6420 and its usefulness in applications with differing characteristic specifications, two test circuits are used to generate the information in this data sheet. Test Circuit A is DC1299, a two-port demonstration circuit for the LTC6420/LTC6421 family. The schematic and silkscreen are shown in Figure 4. This circuit includes input and output transformers (baluns) for single-ended-to-differential conversion and impedance transformation, allowing direct hook-up to a 2-port network analyzer. There are also series resistors at the output to avoid loading the amplifier directly with a 50Ω load. Due to the input and output transformers, the –3dB bandwidth is reduced from 1.8GHz to approximately 1.3GHz. Test Circuit B uses a 4-port network analyzer to measure S-parameters and gain/phase response. This removes the effects of the wideband baluns and associated circuitry, for a true picture of the >1GHz S-parameters and AC characteristics. APPLICATIONS INFORMATION |
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