Is it possible to very roughly estimate the CMRR degredation of a differential amplifier due to source imbalance given its datasheet
Your model is inaccurate. 

Because CM noise has a much higher impedance than the wire, this can be modeled as a weak transformer coupling by mutual inductance ( B field) or stray capacitance (E field). 

It must be given a value then also the cable impedance and mismatch which tends to dominate the results for CMRR.  Also RLC values of every wire and interference source must be estimated.

But if you were to direct connect a CM voltage of 0 Ohms then yes, the induced differential voltage from CM voltage is reduced by 60dB. But that does not apply to 230V.







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