Modeling and Analyzing the Performance of a High-impedance Analog Front End for Non-contact Bio-electricity Measurement

The sensitivity and baseline of the analog front end (AFE) with the non-contact capacitive probe is influenced by the skin-probe distance, thereby restricting its application of non-contact bio-electricity measurement. In this paper, a high-impedance AFE is proposed, which exhibits independence from the skin-probe gap and enables non-attenuation sensing of bio-electrical 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 cut-off frequency, passband gain, stability condition, and non-attenuation condition of the band-pass AFE are analytically expressed. The theoretical derivation and experimental verification of the circuit parameters of AFE for achieving signal non-attenuation detection (0 dB passband gain) have been conducted. Moreover, the feedback resistor is 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 bio-electrical signals without attenuation, even when a 0.4mm 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 bio-potential signals without being affected by the changes in the relative distance between the skin and the capacitive probe.