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Common mode chokes for signal lines.

treez

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Hi,
For a comms system going from one board to another via a cable, (eg CAN, USB, RS232, RS484 etc etc) one can place common mode chokes in the signal lines. Should one place the common mode chokes in the signal lines at the transceiver on both PCBs?

Is there any instance when such common mode chokes should not be used? (I mean, I would have thought one could get unfortunate ringing between the stray diff mode inductance of the common mode choke and the stray capacitance at the transceivers?)

Common mode chokes for signal lines.
https://www.murata.com/-/media/webrenewal/products/emc/emifil/cmcc/pdf/consumer-signal.ashx?la=en-gb
 

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Common mode chokes make little sense for single ended signals, e.g. RS232., except for isolated interfaces with the signal ground included.

They are rather usual for differential signals, e.g. RS-485, CAN. To effectively improve the noise immunity (increase the receiver AC common mode range) you may need a common mode termination impedance.
--- Updated ---

See e.g. this example of a RS-422 DM and CM termination:

1595926914787.png
 

treez

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Thanks, and i imagine the termination impedance depends on the characteristic impedance of the particular cable?
I am also under the impression that if you are sending say a CAN bus (cable) from one PCB to another...then its sometimes best to have isolators at each board, so that the ground bit in the cable is isolated from the grounds at each PCB...then you provide an isolated PSU to power the "cable side" of the interface ICs. Doing this, prevents ground loops, and ensures that system ground volt drops dont mess up the communication?
 

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