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Below shows a perfectly balanced system assuming cables are balanced and twisted: enter image description here Im wondering if the transmitter is not impedance balanced, would a balanced receiver reject CM interference still much better than single ended receiver:

For example below the total imbalance seems 100 Ohm: enter image description here

But if one uses the receiver as single ended the total impedance seems like 101kOhm. enter image description here

Would that make a huge difference also in terms of common mode interference rejection? And is there a way to quantify it?

Are there transducers which are not differential signalling but impedance balanced?

user1999
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  • As drawn, the yellow resistors provide no termination in either case. They are shorted by the signal source. – The Photon Oct 20 '18 at 22:37
  • Your first circuit doesn't represent what I would call a balanced signal transmission system. It then follows that the rest of your circuits would be flawed as they seek to modify the original circuit. This means your question doesn't work. – Andy aka Oct 21 '18 at 16:34

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If you do the maths you will find that the impact depends on the ratio of the imbalance to the RX common mode impedance, 100R:100k for example will give a common mode to differential mode conversion of -60dB, 100R:10k will be -40dB, 100R:1k will be -20dB and so on, it is just a potential divider.

It is for this reason that one figure of merit for a differential receiver is a very high common mode impedance, and why things like the THAT Corp bootstrapped line receivers are such a brilliant idea.

There are plenty of microphones that are impedance balanced, and they work perfectly well.

Dan Mills
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