Below circuit shows the issue when the output impedance of a DC-like source V is not balanced when coupled to a Diff Input(differential input of a data-acquisition board). As you see as an example for the source V, I defined the positive lead output impedance Rs = 1k and the negative lead as 2 Ohm. So when for example a capacitively coupled common mode interference Vcm occurs, due to those source output impedances the common mode interference will be differential hence noise will be observed. Here below is the problematic unbalanced source lines when a capacitively coupled common-mode interference hits.


simulate this circuit – Schematic created using CircuitLab

As a remedy to this a single-ended to diff-ended or a kind of unbalanced to balanced interface is recommended. I called such interface an impedance balancer. It could be a transformer if it were AC but in this case the source is DC. So this interface could be a driver for differential line or a sort of interface where carefully buffers the source output and equalizes the impedances. I called such interface Impedance Balancer in the below diagrams:


simulate this circuit

My question is:

(Here I'm also assuming the transmission line used is balanced(STP cable), and the operating frequency of the signal have a wavelength that is much greater than the STP cable length)

1-) In Figure1 the impedance is made balanced at the source side but in Figure2 it is made balanced right before the differential input side. Would there be any difference between using such an Impedance Balancer as in Figure1 and Figure2?

2-) Would an isolation amplifier as in Figure3 be used as such a balancer interface?


It seems to me that if EMI hits as shown below, it will create noise for Figure2 but not for Figure1.

enter image description here

But my question is about whether there is any difference for the Vcm capacitive coupling case(as shown in my question) not for the EMI hitting on the cable.


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