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Differential signal Ground - CAN bus
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mattko:
Hello,
I have some doubts about differential signals and their grounding, mostly CAN bus.
If I understand correctly, you do not need a GND wire to go with differential pair wires IF the "Absolute maximum: CANH DC voltage versus GND" is high enough. In other words, to keep common mode voltage on the differential pair in a small range, you need a GND wire. Is that correct? How to ensure that common mode voltage (CANH+CANL/2) will remain inside the specified tolerance?
And if you don't have a GND wire, then common mode EMI is not a problem?
I read some articles, but my doubt remains.
Thanks!
capt bullshot:
CAN is designed to work with all transceivers tied to a common GND potential. As RS485 is.
This stuff simply won't work reliably without the common GND.
If used correctly (including the common GND) the internal circuitry of the transceiver chips "magically" take care of the common mode voltage at the transmitting side. For the receiving side, it's up to you to provide the receiver with the correct GND potential.
In some (rare and special) cases CAN (and RS485) will work without a solid GND connection, the common mode level could be restored by some kind of resistive divider or clamping diodes then. But the result will be way more sensitive to all kind of EMC disturbances.
Inserting a transformer (to isolate the GND potentials) makes 2-wire (without GND) differential signal transmission work in the real work (like Ethernet).
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