I asked this question a few days ago, but I don't think I was clear enough, so I'm going to try again.
Here is a rough diagram of my sputtering system:
Basically, a DC voltage supply holds the substrate at a large negative voltage (~-1000V) relative to the chamber chassis (Vs) and a DC current supply pushes a large current (~120A) through a plasma into the chassis and substrate (It). Only a small fraction of the total plasma current actually goes through the substrate (~2A out of the total 120A). Most of the current passes directly into the chassis. So, to clarify, there are two power supplies. The voltage supply is providing -1000V to the substrate through which ~2A are passing. The current supply is providing 120A through the target/plasma (at about 20V).
The resistor between the chassis and ground indicates that the chassis is poorly grounded. Furthermore, the resistor should be taken to be variable as the current flow through the stainless steel chassis fluctuates, meaning that the voltage of the chassis relative to earth ground fluctuate significantly over time.
Here is the problem: I want to measure the voltage between the substrate and a particular spot on the chassis without measuring any of the voltage due to the large current flowing through the chassis. The measurement doesn't have to be very precise, it's only used as a check (i.e., +/- 5V is fine).
Right now, this is accomplished using a Fluke battery powered DMM. Since it's battery powered, it makes a real floating differential measurement. What I'd like to do is replace this handheld meter with a non-battery powered solution that could also be hooked to a computer for data-logging purposes. I thought maybe using a 110V AC to 9V DC wall adapter that could hook into the Fluke's battery terminals might be an idea, but I guess there is no electrical isolation and all the current in the chassis would get dumped through the Fluke to the mains ground.
Can anyone suggest an approach? I'm pretty ignorant about this stuff. I've tried reading up on it but can't figure out anything that might work. If I can clarify in any way, I'd be happy to. Any suggestions (including "you're stupid. this can never work") would be appreciated!
Thanks a lot in advance, Brian