Precision Measurement with the Total-Feedback Electrometer

Abstract

The total-feedback electrometer finds frequent ap-plication in biomedical instrumentation. Three methods of using this electrometer: measurement of feedback current, measurement of feedback voltage, and a null method, are described and compared from the viewpoint of convenience and precision. The theory of the total-feedback electrometer, relevant to measurement precision, together with formulae for use with the feedback voltage method, are given. It is emphasized that the precision measurement of feedback voltage on a digital voltmeter provides a rapid and convenient method of reading data from a total-feedback electrometer, and is capable of a precision of about 0.1% for a single reading. Determination of electrometer gain as a function of signal voltage gives the basic information needed to correct for the error inherent in the small input signal voltage.