Analog Opto Isolator?

Hi,
I'm messing with a very basic RC EDM (Jon Eleson's) and want to
measure the voltage drop across a resistor using a micro (coridium
Armite) with an analog 5 volt input to sense the spark gap. The EDM
voltage is adjustable so I can't use a fixed resistor network to keep
the sense voltage within 5 volts. I need some form of isolation to
protect the micro from over voltage on the sense line after adjusting
the EDM voltage. I don't need anything automatic as I can manually
re-adjust a pot on the sense line to get the voltage back into range
after adjusting the EDM voltage. I'm looking for something simple.

Any suggestions? Is there such a thing as an input voltage tolerant
analog opto-isolator? An i2c A/D with isolation and over voltage
protection?

Roger