The impact of active electrode guard layer in capacitive measurements of biosignals

This article presents an analysis of active electrode design for capacitively coupled measurement of bioelectrical signals. It deals with constructing an active electrode using a guard layer to decrease its input's parasitic capacitance. Three dielectric materials (cotton, polyester, wool) were examined under different pressure (1.23 kPa, 3.07 kPa) of the active electrodes. The Common Mode Rejection Ratio (CMRR) analysis of the system using guarded and unguarded electrodes is also investigated. The results show that the guard layer increases the active electrode's gain in the whole measured frequency range for all insulation materials and pressures. An unguarded active electrode reaches a gain of -7.45 dB while using cotton as dielectric material under the pressure of 1.23 kPa compared to the guarded active electrode, which reaches the gain value of -0.47 dB at the same conditions. The measurements of the CMRR prove that the guard layer increases the system CMRR.