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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?

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  • \$\begingroup\$ As drawn, the yellow resistors provide no termination in either case. They are shorted by the signal source. \$\endgroup\$ – The Photon Oct 20 '18 at 22:37
  • \$\begingroup\$ 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. \$\endgroup\$ – 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.

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This is how you screw CMRR. Common mode becomes differential mode, once anything is not symmetrical. In digital line you can survive it, but in analog you see very clearly. For example using 1% resistors vs 0.1% resistors in an amplifier reduces CMRR spectacularly. Bottom line, keep everything symmetrical for differential signals.

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  • \$\begingroup\$ I know but Im comparing two cases and need to see how different they are. Are you saying 100 Ohm imbalance and 101kOhm imbalance have same effects ? Can we quantify the difference which becomes diff mode from common mode? \$\endgroup\$ – user1999 Oct 20 '18 at 21:19
  • \$\begingroup\$ I don't have enough data for that. If you can specify your common mode, try simulation. Like I said, digital signals survive quite serious conditions, analog- don't. \$\endgroup\$ – Gregory Kornblum Oct 20 '18 at 21:25

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