Noise properties of the 3-electrode skin admittance measuring circuit

Abstract

The 3-electrode skin admittance op.amp measuring circuit has the unique property of measuring the admittance of one single electrode (M) by the use of an additional reference electrode (R) in addition to the second current carrying electrode (CC). It is a monopolar set-up with the R and CC electrodes usually being considered uncritical.

There are two special properties of the 3-electrode system. If an electrode pair or the skin itself generates a DC potential the 3-electrode circuit sets up a substantial and continuous DC current flow. Electrolysis and skin irritation may be the result.

An attractive feature of the 3-electrode circuit is that any external ground referenced noise signal capacitively coupled to the body is cancelled from the current reading channel. But if the CC electrode is small and the skin under the CC electrode is very dry the voltage drive capability of the op.amp.A may be insufficient. The product of the impedance of the CC electrode and the noise current has a critical value above which the op.amp is driven into saturation and the measurement is completely destroyed. Therefore the CC electrode sometimes is critical.