Question:
How to isolate an off-the-shelf CAN device?
Answer:
A digital isolator can be placed on the non-bus side of the CAN transceivers to achieve isolation, i.e. between the CAN controller and the CAN transceiver.
The isolators, however, cannot be placed on the CAN bus side as it requires resistor connections to achieve the recessive state, which is typically VCC/2, and the combination of CANH and CANL to achieve the dominant state. Digital isolators do not support this signal standard. Therefore, it is not possible to insert a digital isolator between the CAN bus transceivers and the cable.
Therefore, CAN bus signal path isolation is accomplished by designing isolators into the digital signal path between the transceivers and the local CAN bus controller. The system side of the CAN bus transceivers use digital logic level signals of 0 V to 5 V or 0 V to 3 V, and typically connect to a CAN controller or processor. Our digital isolators contain input and output circuits that are isolated from each other. Placing a digital isolator in this location electrically isolates the CAN bus cable signals from each system connected to it.
The customer could also use an isolated power supply (isolated dc to dc converter) which will supply power to the local CAN bus transceiver and CAN bus side of the isolator.
There block diagram below is a good reference on how to isolate CAN: