Neale - I think by asking you know the answer already - it' rarely a good idea to mix analogue and digital grounds. It might work, but it's not designed to.

Of course, an alternative is a RS422 interface between the controlling PC and the inverter - I'm not saying this is the best solution but it can (in my experience of dodgy earths) avoid some analogue noise, and requires only a 2-wire interface and a £3 USB device to provide digital speed and direction control.