1. Apparatus for measuring a process variable comprising: a first sensing capacitor having a capacitance, C 1, based on the process variable; a second sensing capacitor having a capacitance, C 2, based on the process variable; a bridge node coupling a first side of the first sensing capacitor to a first side of the second sensing capacitor; an excitation source supplying at least first and second voltage levels; and a switch circuit selectively coupling a second side of each of the first and second sensing capacitors to the excitation source to derive a representation of C 1 -C 2 at the bridge node.