CAN bus termination - split with a capacitor and common mode chokes

[Postal2]

... CAN transceivers might use voltage dividers on their inputs in order to extend the common mode input range beyond supply rails.

This is true.

[Simon]

Yes the CAN bus voltage ground will shift. But to rely on that too much as part of design is a bad idea. What happens when there is a transient in the separate power circuit whose ground is also your can bus ground, ah, that common mode input range helps it cope, but if you are using that range by design then you can't use it to deal with the unexpected.

So I have a 30A motor driver that can do 60A briefly, what sort of shit will that throw around my ground as the designers have chosen to not isolate the CAN bus, because of an amount of common mode range that won't be a problem, but now if I put my other can bus nodes very far away and carry power on the same cable that uses the can bus ground as the negative supply and use up that common mode range in voltage drop now what happens when there are motor driver transients?

You can use the common mode range for whatever you like, just don't use it twice....

[Postal2]

Yes the CAN bus voltage ground will shift. But to rely on that too much as part of design is a bad idea. ....

Well, the CAN bus was invented by fools. A new generation grew up and came up with the idea of ​​putting a capacitor there.

... that common mode input range helps it cope, ....

With a capacitor present, the input signal is out of phase because the load cannot conduct current between the drains while being pulled up to a different level.

... what happens when there are motor driver transients? ...

Without a capacitor, the exchange on the bus will proceed normally and nothing will happen.

I also assume that a capacitor with a capacity less than 1/4 of the line capacity will not create a negative effect, but then it turns out that it is not needed at all, since the introduced imbalance of the CMC coil will negate the removal of interference to the near ground.

[Niklas]

Common mode chokes were introduced for CAN to handle the mismatch in timing on CANH and CANL due to difference between N and P channel transistors in the transceiver. As transceiver got better, but still maximum 1 Mbits/s, it was possible in some cases to remove the choke based on the EMC test results, but it also depends on different OEM's requirements. Some specify the CMC as a must, some as optional if it passes EMC without it. With higher bit rate, like CAN FD, the CMC might be needed depending on the application.
Termination has also changed over time. 10 years ago it was most common to leave it out and the OEM handled it on the vehicle harness. One German OEM had a specific requirement for a 120 ohm termination with an open end to be bridged in their harness if the unit was located at the end of the bus. A few years ago some OEM's started to ask for high impedance split termination with 2*1.3k.
Being able to handle the full supply voltage on CANH and CANL for more than 1 minute was also mandatory. This was also part of the miswiring test in ISO 16750-2 were all combinations were tested in a sequence. This test has implications on both transceiver selection (CM voltage range), TVS breakdown voltage and termination resistor power rating.
TVS diodes were not optional for the product I worked with as most transceiver handled up to 8 kV, but the OEM requirements were 15 kV for ESD.
One thing to look out for is connector pinout and how the CAN bus wires are located with respect to other wires with transients. On an older product our pinout on an internal harness was Vbat, CANH, CANL, 0V. During the ISO 7637-2 Pulse 1 immunity test (-600V, 50 ohm on Vbat) we got error frames on CAN. The product had a CMC and 120 ohm termination. Problem solved by twisting the internal harness to minimize the loop sizes and hence the mutual inductance coupling between the supply voltage pair and the CAN bus.
Also the wire insulation can have an impact as the harness impedance will change with material and thickness. Not sure how much it will affect immunity, but it will impact emissions.