Input Resistance Boosting for Capacitive Biosignal Acquisition Electrodes

Capacitive electrodes are a promising alternative to conventional wet Ag/AgCl electrodes in the acquisition of biological signals. They consist of a metallic sensing layer covered by an insulating material that contacts the human body. They have the advantage of measuring biopotentials in humans through clothing, hair, and small air gaps. The electrode capacitance creates a high-pass filter with the analog front-end’s (AFE) input resistance. Hence, the bandwidth of the system, especially the low cut-off frequency, depends on the dielectric layers and the characteristics of the body-electrode contact. Moreover, capacitive electrodes suffer from motion artifacts (MAs) that also modify the electrode capacitance. This article proposes an electrode topology with boosted input resistance and compensation for the electrode capacitance changes. To achieve such characteristics, the proposed circuit comprises a negative impedance converter (NIC) that increases the input resistance, which allows the addition of a capacitor in series with the electrode capacitance to reduce the effects of capacitance changes. The proposed electrode’s cut-off frequency was investigated in a controlled test bench. For the worst case scenario of electrode capacitance (1 pF), the proposed topology achieved a cut-off frequency of 1.5 Hz while the reference circuit had a cut-off frequency of 72 Hz. The proposed topology also outperformed the reference electrode in common-mode rejection ratio (CMRR) and through clothing ECG acquisition.