Abstract: A circuit for physical placement within an underground well to amplify low level differential DC voltage signals measured at a well casing is disclosed. The circuit incorporates an instrument amplifier with a first switching device before the amplifier and an inverter after the amplifier in the signal path. A second switching device selectively connects one of the amplifier and inverter outputs to a low-pass filter. The switching devices operate synchronously under control of an operating clock signal so that DC error terms associated with the amplifier are converted into a substantially AC error signal while maintaining the signal being amplified as a DC signal. The duty cycle of the operating clock signal is established to cause the substantially AC error signal to include a DC component of appropriate polarity and magnitude to compensate for DC error terms associated with the inverter and low-pass filter.