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The problem has been addressed with isolation amplifiers. My initial search turned up the Avagotech HCPL 7510, stocked by Mouser under $5. The application note is our exact application, and the description looks good. Need a +5 supply on each side of the isolation barrier. Three other possibilities I looked at, AD 215, AD204, and TI Iso 122 were all more expensive, though they allow different solutions to the isolated power supply problem. I have ordered a couple of the Avagotech 7510 pieces from Mouser, and lash up a test circuit.
Nearly 50 years ago, I was a test tech at Dynamics Instrumentation, which built isolated strain gauge amplifiers in a 3 by 5 by 12 inch package, with Russian doll shielding, and able to see a 10 mV signal in hundreds of volts of common mode. Now a fi'dolla chip does it if we tend to the power supplies.
A further search may turn up a better solution, but this should work. It will give 2 to 3.5 volts for 0 to 200 mV in on the shunt side.
As I read the sideways roadmap in the wiki, there are supplies on each side of U11, and the comparator U8A is what the current sensing feeds, so we can do level shifting there. What was the output of the Hall device? It may be best to have a little add on board that plugs in where the Hall device did.
Hey, if Avagotech did the job they claim, this IS easy! we will see.
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