Hi again,
Well detecting the zero crossing close to zero should help. For example, a comparator powered with plus and minus 10 volts can be used by grounding one input. When the sine goes through zero, the output will change state. It will be as close to zero as the input offset spec of the comparator, which is typically 2 or 3mv.
It has to be close to zero because when the sine amplitude varies that means if the threshold is at some offset like 0.5v, that voltage occurs at different points in time for different amplitudes, or to put it another way, that point occurs at a different phase for a different amplitude.
For example, with a 0.5v threshold and a 1v peak sine, the trip point occurs at 30 degrees, but with an amplitude of 2v peak, the trip point occurs at about 14.5 degrees.