Patent ID: 7885700
Filing Date: 2011-02-08
Classification: A61B,G01R

Abstract:
1. An electrodynamic sensor comprising: an input probe for detecting small electrical potentials originating from an object under test and for generating detection signals, said input probe being configured for capacitive coupling with said object without direct electrical contact and comprising an electrode substrate having thereon a thin film of insulating material, and a high input impedance electrometer arranged to receive the detection signals and measure said electric potentials, the electrometer comprising: an amplifier, a dc input bias arrangement for providing a steady input bias current for said amplifier, said dc input bias arrangement including a biasing resistor, a dc to low frequency negative feedback loop comprising a low-pass filter circuit connected to receive amplified measurement signals and to feed a filtered stabilizing signal into the dc input bias arrangement for maintaining a stable dc operating point for the amplifier; and a combination of ancillary circuits configured cumulatively to increase the sensitivity of said electrometer to said small electrical potentials whilst not perturbing the electrical field associated therewith, the ancillary circuits including a guard circuit providing a shield surrounding said electrode substrate and input wiring for said amplifier, said guard circuit being driven by the output of the amplifier and being arranged to maintain the same potential on said one probe as on said shield, and a feedback arrangement between the output and the input of the amplifier, said feedback arrangement comprising a bootstrapping circuit for bootstrapping the biasing resistor of the dc input bias arrangement, and one of a neutralization circuit, a supply modulation circuit and an offset correction circuit for said amplifier.