I'm using a MOSFET for single direction motor speed control via PWM from a PIC. I'm thinking about isolating the power supply that is used for motor control (via the FET) from the supply used for the control electronics (PIC). What I'm unclear about is if the FETs common needs to be tied to the control electronics common. I know with a standard transistor you have to tie commons to one another; however I wasn't sure if that was the case with a MOSFET.

So tie commons, no isolation?
Don't tie commons, isolation (expect for PWM from PIC)?