Hi,

For instrumentation purposes it should be kept in mind that the ADC in the PIC is poor in the lower range (Less than 2 Volts). So you need a shifting reference that actually increases the resolution.

If you measure the AC in chunks you need a zero cross sense circuit to kick your measurement routines.Also noise (motor current) would make it worse.

As mentioned by lab310 distortion and PF in the input waveform would give erroneous result. You may use a software based pseudo differential measurement technique.

I believe depending on the accuracy you need you should choose a solution. As mentioned amplifying the AC and introducing a gain of 0.707 do give results but in the lower side the accuracy is not good enough due to the conversion slope.You can handle this in software.

I would suggest that you go for a RMS converter and use a temperature compensated reference for the PIC. Do the AD in sleep via interrupt. Sample 16 times and do an average.This gives best results.

Good Luck.

Regards

Sougata