1. A circuit for converting an analog voltage into a digital signal, comprising: a first A/D converter for converting the analog voltage into a higher-order portion of the digital signal; a neural network for converting the analog voltage into a lower-order portion of the digital signal having a plurality of ordered bits, said neural network including, for each bit of said lower-order portion of said digital signal, a voltage source and a comparator, at least one of said comparators having a voltage reference level provided by the voltage source having an output voltage determined by an output of a comparator associated with a higher-order bit of said lower-order portion of the digital signal.