Atom058, Is the millivolt measurement Isolated? If so I would use a 10x amplifier stage and then an amplifier configured as an active rectifier. With this arrangement you should be able to read the peak current (ac) and then multiply it by .707 to get the RMS value or .637 to get the average value. At 500 mv. x 10 = 5.0 v you will be using the fullscale 10 bit capability of the a/d on board the microprocessor.

Dave Purola,
N8NTA