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Electronics => Beginners => Topic started by: e100 on December 04, 2020, 02:36:37 pm

Title: Is it important to avoid ground loops in outdoor CAN bus systems?
Post by: e100 on December 04, 2020, 02:36:37 pm

Some CAN interface boards designed for use with microcontrollers have isolation barriers and some don't. Those with isolation barriers have more components, are more expensive and physically larger. I believe the NMEA 2000 standard used on boats, which uses CAN as the transport layer, requires all nodes to be electrically isolated.

In audio circuits ground loops produce annoying mains 50/60 Hz frequency signals. Is there a dummies guide that explains the pitfalls of ground loops in the context of data cables where the data rate is far higher than 50/60 Hz power supply?

Title: Re: Is it important to avoid ground loops in outdoor CAN bus systems?
Post by: Doctorandus_P on December 05, 2020, 11:00:42 am

CAN and RS485 are quite good at filtering out noise, but there are limits.

As far as I know RS485 goes from a common mode voltage of -7V to +12V and the limits for CAN are a bit less.
As long as the noise is not so big that the signals go out of this common mode range it should work. It is the magic of differential signalling.

https://html.duckduckgo.com/html?q=differential+bus+noise+immunity (https://html.duckduckgo.com/html?q=differential+bus+noise+immunity)

If you have big ground loops (for example 2 different power cables going to a PCB via different routes) and there is a strong magnetic field present, then you may have trouble with currents going round in the ground loop, and a power resistor or extra filtering may help to reduce that current.

Title: Re: Is it important to avoid ground loops in outdoor CAN bus systems?
Post by: Siwastaja on December 05, 2020, 06:46:25 pm

Differential audio (aka balanced audio) doesn't hum in presence of ground loops, and Just Works as long as ground differences are within the common mode range of audio opamps (say, around +/-12V for example).

CAN is differential like differential audio is, so the same applies.

If you have high-current devices that have separate power and communication connectors, isolation of the comms may be a good idea because if non-isolated, disconnecting the power wire but leaving the communication connector provides a power path through the communication connector which may not be rated for such high current. At least use fuses to prevent damage.

Title: Re: Is it important to avoid ground loops in outdoor CAN bus systems?
Post by: e100 on December 06, 2020, 02:40:11 am

Thanks for both replies. It makes sense now.