# Does input impedance of an isolation amplifier or a transformer have impact on common mode to differential mode conversion?

I have a question where I can only explain it clearly by an example. It is about conversion of common-mode interference to differential mode due to overall system imbalance.

I will use below two models where a common mode voltage is applied through a cable to amplifiers in both cases. In Figure 1 differential amplifier is the receiver; and in Figure 2 an isolation amplifier or any system which has isolation like a transformer is the receiver. Let's first look at Figure 1. Rs1, Rs2 can be source output resistances and C1, C2 can be the cable's capacitance to the ground. Rin1 and Rin2 represents the input impedance of the differential amplifier. Here it is apparent that if we want to estimate how much Vcm is converted to differential mode then we need to know Rs1, Rs2, C1, C2 and also Rin1 and Rin2. We can also see that even Rin1=Rin2, still value of input impedance have an impact on the value of common mode to differential mode conversion.

Now let's examine the case for Figure 2 where the isolation amplifier is used ans same Vcm is present. For simplicity I tried to model as it has input impedance Rin and parasitic isolation capacitance as Cp1 and Cp2 as shown. In this case it is to me apparent that Rs1, Rs2, C1 and C2 again determines the conversion from CM to DM. But it is not very obvious whether the value of Rin has impact on the value of the common mode to differential mode conversion.

Regarding Figure 2 my questions are:

It seems the common mode currents will not flow through Rin but they will flow through Cp1 and Cp2. Is that correct? If so does that mean the input impedance value of the isolation amplifier has no effect on CM to DM conversion and act almost as an open circuit to the common mode voltages?