In the figure below, the common mode impedances of the line driver and receiver are represented by RCM+ and RC-:

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(What I understand by the phrase CM impedance is that it is the resistance or impedance seen by the common mode voltage.)

This CM impedance parameters sometimes are not given in a datasheet directly.

If the CMRR of an amplifier or a transformer ect. is given as 100dB at a particular frequency can we estimate the common mode impedance?

Since -100dB is 10^-5, can we than say the common mode impedance is 100k Ohm?

For instance in this module data-sheet in the second page at specifications, the CMR, the NMR and the input impedance/resistance are explicitly given. Can we also estimate the common mode impedance as well?


1 Answer 1


CMRR is a ratio of Ad (differential mode gain) to Acm (common mode gain) which is 20log(Ad/Acm). With a high CMRR you may imply that input impedance is high but I don't think you can calculate it directly from CMRR. CMRR has more to do with the balance of the amplifier and its ability to reject, or not amplify, noise which is present at both (inverting and non inverting) inputs.


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