Quantification of edge effects in capacitive biopotential sensing*

Knowledge of capacitance and factors contributing to its variation is important in capacitive biopotential sensing where the dimension of the electrode is much less than the dielectric (skin). As such, this study aimed to quantify the actual capacitance exhibited by an electrode, by accounting for fringe fields (edge effects). This study simulated two different dual-electrode configurations to calculate the capacitance of each electrode in the presence and absence of the other electrode. The results were compared with existing expressions from the literature to investigate their reliability in quantifying edge effects. It was found that the capacitance expression for electrode smaller than the body shows the least difference from simulated capacitance than other expressions. However, the difference increased with increasing airgap. Also, the concentric configuration identified that the outer electrode acts as a guard to the inner electrode and can be approximated with minimum error. This will help estimate the actual effective electrode capacitance in single and multi-electrode systems and allow for efficient measurement circuit design and optimal signal processing of the biopotential of interest.