A low noise isolated amplifier system for electrophysiological measurements: basic considerations and design

An instrumentation amplifier for medical and biological research applications is described. It consists of an isolated preamplifier with a high signal/ noise ratio and a computer or microprocessor controlled main amplifier that incorporates variable highpass and lowpass filtering. The preamplifier includes an input stage, a compensation circuit for electrode offset voltage and an isolation amplifier. Recently introduced ultra low noise operational amplifiers used in the input stage ensure good noise behaviour. Isolation is obtained with an inductive isolation amplifier, which also delivers the supply voltages for the input stage. Accordingly, battery power is not needed, in contrast with optically isolated amplifiers. The main amplifier has computer control for gain, lowpass cut-off frequency and highpass cut-off frequency settings. This features dynamic changes in signal conditioning and data acquisition by the same computer that collects the data. Data acquisition and noise behaviour are discussed. The filter configuration is designed in such a way that several user-defined options can be implemented on the same circuit board. The trade-off between the order of the lowpass filters, their roll-off, and the sampling rate which is used in the subsequent computing system, are of major importance and will be discussed. The amplifier system is at present in sue in laboratories of neurology (BEP, EEG), opthalmology (VEP, EOG, ERG) and cardiology (noninvasive His bundle recordings) from which some results will be presented. The amplifiers form a part of a microprocessor-controlled data acquisition system.