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Posted by John Wetherell on November 1, 2008, 9:02 pm
I have two questions:
1. Do you think it's possible to use switched CMFB (for continuous-time circuits? The most obvious continuous time circuit is filter. Would the following stages of filter out any common-mode bounce? How much common-mode bounce would get converted to differential? Would it just appear as a offset or gain error or distortion?
2. How do you use switched capacitor CMFB for a two-stage amplifier? You can just apply the CMFB to the second stage, but the first stage would need an additional CMFB. Would settling suffer? You can modify the swit-cap CMFB to be inverting, but then it's parasitics might affect the common-mode offset.

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Reply by ryan on November 5, 2008, 6:18 pm
1) The CMFB circuit wouldnt care, your circuit may or may not care depending on the common mode gain and bandwidth. The Differential transfer function would be different for the 2 states of the SW-CMFB circuits giving you mixing effect which would cause distortion (time variant).2)you would use 2 loops, an inner fast CT loop and an outer loop that nested the inner one.In general if you have schematics it is easier to answer questions.Best Regards

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