AC Signal Conditioning for ADC (Design Choices)
Looking at your choice of ADC you have 8 input channels. Why not have 1 input just scaled and a second input inverted and scaled.

Otherwise you need to AC couple but at this (low) frequency you probably need a fairly large quality capacitor, buffer and scaling to your ADC.

I don't think a precision rectifier is what you want; you'll loose all negative signal.

EDIT
Just found this article describing an absolute value circuit. At 50Hz I would expect precision to be good. You are always going to get some additional distortion from any diode based op-amp circuit.