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Michael Steffes

Designing higher frequency active filters to drive differential input high-speed ADCs

Michael Steffes
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This part 3 will progress through the different VFA approaches with Part 4 moving on to CFA and FDA DC precision considerations
Modern Rail-to-rail (R-R) output stage op amps, or fully differential amplifiers (FDAs), can sometimes get into a low phase margin condition for relatively simple circuits. Using the phase margin simulation test of Reference 1, a few simple external phase margin improvement methods will be shown. These tests and fixes are best done in simulation prior to board layout. Where the device model for a R-R output device is perhaps not including the higher frequency reactance, places for these tuning elements might be put into the board layout as a hedge against future stability problems.
Modern high-speed op amps and Fully Differential Amplifiers (FDAs) push the limits in every direction to deliver the best harmonic distortion at the lowest quiescent power. This often leads to relatively low phase margin designs in what would normally appear to be benign application circuits.
The differential outputs provided by fully differential amplifiers (FDA) can be used in a positive feedback scheme to generate a partially active output impedance
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