A General Approach to Common-Mode Signal Rejection with Application to Myo-electric Signal Measurements

Abstract

<p>A typical instrumentation problem is the detection of a differential signal in the presence of a common-mode signal. In the case of measuring myo-electric signals, the common-mode signals present can be 10²-10⁴ larger than the differential signal. In such instances, the attenuation of these unwanted common-mode signals require complex circuit designs.</p> <p>In this work an investigation of the common-mode signal path led to the development of a general approach to the problem of unwanted signals arising from common-mode signal sources. It was found possible to write the overall common-mode gain as a product of the differential amplifier common-mode response and the isolation gain. The isolation gain depends on the ratio of the impedance of the common-mode signal path to the differential amplifier input impedance. The impedance of the common-mode signal path may be increased by employing in the amplifier system a self-contained power supply and an output coupler device such as a transformer.</p> <p>The general approach was used to design an amplifier system to measure myo-electric signals. This amplifier system gave a much improved common-mode signal rejection over currently used systems which typically have 40 db of differential gain but only -30 db of common-mode gain at 60 Hz. The improvement in common-mode signal rejection was 25 db.</p>