A 136-G \Omega Input-Impedance Active Electrode for Non-Contact Biopotential Signals Monitoring

• Published in: IEEE journal of solid-state circuits Vol. 59; no. 2; pp. 1 - 11
• Main Authors: Qu, Tianxiang, Wang, Peizhuo, Lei, Liangbo, Hong, Zhiliang, Xu, Jiawei
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.02.2024; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Active electrode (AE); Calibration; Capacitance; capacitor scaling; Capacitors; Circuit boards; Electrocardiography; Electrodes; Electroencephalography; Electrostatic discharges; Feedback loops; Impedance; Input impedance; input-impedance boosting; Integrated circuits; non-contact; Parasitic capacitance; Parasitics (electronics); Positive feedback; Printed circuits; Reference signals; Signal monitoring; System-on-chip; ultra-high input impedance
• ISSN: 0018-9200; 1558-173X
• DOI: 10.1109/JSSC.2023.3284524

This article describes an ultra-high input-impedance active electrode (AE) circuit and the system to sense biopotential signals through a capacitively coupled interface. Various techniques from both circuit and system aspects are used to eliminate the input parasitic capacitance of the AE. On-chip parasitic capacitance is compensated by an auto-calibrated positive feedback loop (PFL) without applying any reference signal. A capacitor down-scaling technique combined with SAR-assisted PFL calibration enables femtofarad-level resolution of the capacitor array, alleviating the practical constraints of the conventional PFL to implement a small capacitor below 10 fF. Besides, a dummy input structure ensures that the pad and electrostatic discharge (ESD) capacitances are also canceled by the PFL, while off-chip parasitic capacitance on the printed circuit board (PCB) is nulled by active shielding. Fabricated in a standard 0.18-<inline-formula> <tex-math notation="LaTeX">\mu</tex-math> </inline-formula>m 1P6M CMOS process, the AE achieves an ultra-high input impedance of 136 G<inline-formula> <tex-math notation="LaTeX">\Omega</tex-math> </inline-formula> at 60 Hz (average of 10 samples). This is equivalent to an input capacitance of 19.5 fF and corresponds to a 2.7<inline-formula> <tex-math notation="LaTeX">\times</tex-math> </inline-formula>-68<inline-formula> <tex-math notation="LaTeX">\times</tex-math> </inline-formula> improvement over the state-of-the-art. The AE exhibits an input signal range of 700 mV<inline-formula> <tex-math notation="LaTeX">_{\mathrm{pp}}</tex-math> </inline-formula> and an input-referred noise of 0.72 <inline-formula> <tex-math notation="LaTeX">\mu </tex-math> </inline-formula>V<inline-formula> <tex-math notation="LaTeX">_{\mathrm{rms}}</tex-math> </inline-formula> (0.5-100 Hz) while consuming 10.46 <inline-formula> <tex-math notation="LaTeX">\mu </tex-math> </inline-formula>W from a 1.2-V supply. Each AE integrates an IC and a 3-cm<inline-formula> <tex-math notation="LaTeX">^{2}</tex-math> </inline-formula> copper electrode on the PCB, and the wearable system prototypes successfully measured high-quality electrocardiogram (ECG) and electroencephalogram (EEG) signals from test subjects through capacitively coupled interfaces.