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This is a conceptual question about long analog signal transmission exposed to capacitive coupling and EMI noise as shown in below figures:

schematic

simulate this circuit – Schematic created using CircuitLab

In Figure1 the input impedance of the differential amplifier/instrumentation amplifier is 100Meg Ohm. And in Figure2 the input impedance of the differential amplifier/instrumentation amplifier is reduced to 100k Ohm by adding it as shunt.

I can say that since the source resistance is 1k, in Figure2 the DC error is larger which can be calibrated out as well.

But I'm wondering the what happens to the effect of the capacitance coupling(like 50Hz power supply leakage) and common mode EMI(coupling to the cable wires) when we insert that 100k shunt resistor as in Figure2. If we decrease the input resistance by adding that 100k the amount of current looping is increasing. Does that affect the SNR or provide better noise immunity for EMI or capacitive coupling?

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  • \$\begingroup\$ Your model 10% complete of distributed noise current B field, and stray voltage E field with distributed inductance and capacitance on the cable and input resistance and capacitance, Common Mode impedance and CM noise sources. . Fix it \$\endgroup\$
    – D.A.S.
    Commented Jul 15, 2018 at 1:25
  • \$\begingroup\$ How balanced are the two wires? This sets the injected imbalance of Electric Fields. And is TwistedPair used? This affects both Electric Field imbalance and Magnetic Field injection. How tightly and perfectly symmetric is the TwistedPair twisted? \$\endgroup\$
    – analogsystemsrf
    Commented Jul 15, 2018 at 3:23

1 Answer 1

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Does that affect the SNR or provide better noise immunity for EMI or capacitive coupling?

If you are going to consider the effects of driving an unbalanced signal (only one Rs) down a balanced line to a balanced receiver, the biggest problem is the unbalanced signal in the first place. It is unbalanced because there is only one Rs. To vastly improve the situation you apply another impedance of equal value in the 0 volt/return wire.

Now you have an impedance balanced transmission: -

enter image description here

Both wires are susceptible to EMI but, with a balanced impedance driver (due to having 2 x Rs), EMI affects both balanced wires the same and the receiver cancels those effects out.

There is no point considering the effects of EMI on a balanced cable unless you drive it with equal impedances (irrespective of whether you drive differential voltages or not).

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