Here is an idea for a one bit A to D converter.
Vinput --- [ one bit AtoD ] --- Voutput
If the input is between Vmax and Vmax/2 subtract Vmax/2
Vbit = Vmax * ( Vinput > Vmax/2 )
Voutput = ( Vin - Vbit ) * 2
These one bit AtoD converters can be chained together until tolerences break it.