OK, no problem.
You understand that we have no idea of your project other than what you tell us.
OPA [operational amplifiers], some of the older types are not very suitable for this job.
So if you know whats available in your location a design could be drawn.
The other important question what is the 'ac' line, is it part of the mains wiring.? If its not mains what is the frequency.?
It is possible to amplify a 20mV ac signal to 5Vppk then rectify it to give about +5Vdc suitable for a PIC's adc.