Effects of Stray Capacitance to Ground in Bipolar Material Impedance Measurements Based on Direct-Contact Electrodes

Bipolar material impedance measurements are easier to implement than tetrapolar measurements but are more affected by electrode impedance. This effect can be reduced by measuring at a high-enough frequency albeit with the risk of deviations due to stray capacitances. High frequency measurements intended to reduce the influence of material conductivity when its permittivity is of interest also suffer from the effects of parasitic capacitances. In this paper, the effect of stray capacitance to ground in bipolar material impedance measurements implemented with direct-contact electrodes has been analyzed to identify the relevant parameters that determine the best frequency range to measure the conductivity or permittivity of the material when impedance analyzers based on an auto-balancing bridge are used. Stray capacitance to ground yields inductive effects proportional to it and also resistive and capacitive effects. If the ratio between this stray capacitance and the capacitance of the material under test is high enough, the modulus of the measured impedance can display a peak at relatively low frequency (about 100 kHz) and its phase angle can become positive. The consequences are that the best estimate of the conductivity of the material is that obtained from the modulus of the impedance at about midrange between the characteristic frequency of electrodes and that of the material, whereas the permittivity cannot be estimated from the impedance modulus measured at high frequency unless the sample capacitance is much larger than its capacitance to ground and propagation effects are negligible.

I. Introduction

The impedance measured between the two electrodes is defined as the ratio , where is the potential difference (phasor) between the two electrodes, high potential electrode (H) and low potential electrode (L), and is the current (phasor) through them at a given frequency. Two-electrode (bipolar) impedance measurements can be performed by the method (current through H measured when L is connected to circuit common or earth ground) or using an auto-balancing bridge (current through L measured while connected to virtual ground) [1].