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Do u mean do transient simulation with different common mode voltage?
And why THD is -3dB worse beyong common mode range?
I think THD is mostly related to the amplitude of the signal.
Could we just change the bias voltage of the differential amplifier and check its operating point?
Renwl's way is very good but difficult and time consuming.
there is another way,
Plot output dc voltage vs. input dc voltage.
(sweep input dc voltage)
note that you must plot single ended output .
input range that cause linear curve, is ICMR.
(it is if you don't have swing in input)