>> The problem is that I am just laying out the board now. I need to
>> determine how much inductance to expect. I guess if I had to I could
>> hack something up to make some measurements, though.
>
>I'd like to see a schematic and layout on this one.
>It could be very nasty.

Sounds to me more like a case of making sure there is a single point ground 
attachment between the two systems, and keeping the power rails in separate 
areas. Proper earthing rules need to be stringently applied to stop the high 
current ground bounce getting back into the logic circuitry - and dealing 
with the gate drive will be another exciting part of this, meaning there are 
three items that all need to meet at the grounding point. 

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