Patent ID: 8923956

Claim:
A sensor system comprising: a differential amplifier; a pair of electrodynamic sensors connected to the differential amplifier so as to generate a differential output signal at an output of the differential amplifier; wherein each electrodynamic sensor comprises: an input electrode for detecting small electrical potentials originating from an object under test and for generating detection signals, said input electrode being configured for capacitive coupling with said object without direct electrical contact, and a high input impedance electrometer arranged to receive detection signals from the input electrode and measure said small electrical potentials, the electrometer comprising: an operational amplifier having an input connected to said input electrode and an output connected to an input of the differential amplifier, wherein the output of the operational amplifier is to provide sensor output signals; a dc input bias arrangement for providing a steady input bias current for said amplifier, said dc input bias arrangement including a biasing resistor, and a combination of ancillary circuits providing feedback between the output and the input of the amplifier and configured cumulatively to increase sensitivity of said electrometer to said small electrical potentials whilst not perturbing an electrical field associated therewith, the ancillary circuits comprising: a guard circuit providing a shield surrounding said one input electrode and input wiring for said operational amplifier, said guard circuit being driven by the output of the operational amplifier to maintain the same potential on said input electrode as on said shield, and a feedback arrangement between the output and the input of the operational amplifier, the feedback arrangement comprising: a bootstrapping circuit for bootstrapping the biasing resistor of the dc input biasing arrangement, wherein the bootstrapping circuit includes a capacitor, and wherein the bootstrapping circuit couples the output of the operational amplifier to the input of the operational amplifier through the capacitor and the biasing resistor, and one of a neutralization circuit, a supply modulation circuit and an offset correction circuit for said operational amplifier.