Hi,

The simplest way to measure a sinewave ac is to rectify the input and do an ADC. Simply multiply the result with 0.707 to get the RMS value. If your input range is too low then some amplification and active rectification through OP-AMPS maybe needed. In fact the gain of the op-amp stage can be calibrated to give you an average dc reading (no need for maths in the PIC side). That is what is done in low cost multimeters. To get best results (depending on the speed needed) do an averaging of the samples collected. Please let me know your exact requirement

1. Range of Input Voltage

2. Maximum Frequency

3. Input waveform

I will try to help.

Regards

Sougata