I have designed an alternator regulator which uses an ATtiny84 and includes I2C to talk to an Arduino. The 5v power and the ground for the ATtiny85 is supplied from the same rails the main Arduino (a Mega) uses. This is powered from a switchmode power supply and filtered to provide very clean power. The problem I have is how to isolate the noisy alternator ground from the ATtiny but still allow such things as the voltage divider, chopper MOSFET and shunt resistors to work. For the MOSFET I could use an optoisolator, but due to their nonlinearity I am unable to use one for the analog shunt nor voltage divider. I also can’t use it for I2C due to it needing to be bidirectional. I don’t want to connect the alternator and switchmode PSU grounds due to that creating a ground loop. Would adding two RS422 differential chips (one attached with the ATtiny to the noisy alternator supply/ground, and one to the clean switchmode power supply/ground) do the trick?
Probably quite an unusual question but if anyone has any help I’d appreciate it.
Schematic is below…