vivkr
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Hi CherryQ,
I just meant that you ought to ensure that the common-mode sensing and feedback circuitry is linear enough that large differential signals at the output of your amplifier do not lead the CMFB going out of its linear range, or cause some differential mode signal from leaking into the common mode path.
To put it on a concrete footing, if you have outputs Vp and Vn, then your common-mode sensing circuitry should always detect Vcm = 0.5*(Vp + Vn).
Due to nonidealities or poor choice of architecture, you might pass the signal Vcm = 0.5*(Vp+Vn) + Admcm(Vp - Vn) into your common-mode path. However, your problem could be more due to the common-mode sensing circuitry having a limited range of input swing (smaller than swing actually seen in amp).
Vivek
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