I'm debating on a method to sample an AC voltage between 100 to 400 Hz. I can either rectify it, smooth it, and read it through the ADC (from which I can only get voltage characteristics, which is ok but not desirable). Or I can take several samples in order to determine the frequency as well as the voltage characteristics.

To do this second method, I would need a very fast ADCIN sample rate. What sort of sample rate could I get using ADCIN with a 20Mhz xtal?

Thanks in advance for any advice offered!
Dave