Effects of Stray Capacitance to Ground in Tetrapolar Bioimpedance Measurements

Abstract

Previous works have shown that tetrapolar bioimpedance measurements are susceptible to stray capacitance between the subjects’ body and ground. This paper provides a circuit model to analyze that effect, which includes, among other, the non-negligible impedance of electrodes and the finite input impedance of the voltage amplifier when measuring by applying a voltage and measuring the resulting current and the potential difference between two points on the surface of the volume conductor. When assuming common values for the parameters involved in the circuit, the model predicts gain and nonlinearity errors at any frequency, and resonance. In addition to the stray capacitance to ground, the other relevant parameters are the capacitance of the “low-current” electrode and the input capacitance of the differential voltage amplifier.