Modeling and Analyzing the Performance of a High-Impedance Analog Front End for Noncontact Bioelectricity Measurement

The sensitivity and baseline of the analog front end (AFE) with the noncontact capacitive probe is influenced by the skin-probe distance, thereby restricting its application of noncontact bioelectricity measurement. In this article, a high-impedance AFE is proposed, which exhibits independence from the skin-probe gap and enables nonattenuation sensing of bioelectrical signals. The equivalent circuit model and transfer function of this AFE integrated with bootstrapped circuit, driven guard, and neutralization circuit have been developed. The lower cutoff frequency, passband gain, stability condition, and nonattenuation condition of the bandpass AFE are analytically expressed. The theoretical derivation and experimental verification of the circuit parameters of AFE for achieving signal nonattenuation detection (0 dB passband gain) have been conducted. The feedback resistor is, moreover, incorporated into the bootstrapped circuit to enhance the stability of the AFE. The experimental capacitive electrocardiography illustrates that the AFE is capable of detecting human bioelectrical signals without attenuation, even when a 0.4 mm cotton T-shirt is used as an insulation layer. This demonstrates the potential of the proposed AFE to accurately and sensitively capture a variety of biopotential signals without being affected by the changes in the relative distance between the skin and the capacitive probe.