A Front-end Amplifier with Fast Reset Scheme for Capacitive Heart Rate Monitoring

Noncontact capacitive electrodes are a suitable choice for unobtrusive and long-term ECG/heart-rate monitoring applications but require a front-end amplifier with large input impedance, low noise, and motion artifact (MA) reduction methods. This article proposes a front-end amplifier structure with a fast reset scheme. It enables MA recovery prior to saturation and, therefore, avoids a long recovery process, which may take up to several seconds and results in signal loss in typical capacitive sensing systems. A reconstruction algorithm restores the waveform and, thus, extends the dynamic range beyond the original saturation level. The proposed fast-reset amplifier and a conventional pseudo-resistor biased amplifier were fabricated in a 180-nm CMOS technology. The electrical characterization results show that the fast-reset version is superior to the conventional structure, by achieving an input resistance of 1.8 TΩ , a 3-100-Hz, 2.3- μV rms integrated noise level with a 10-pF source impedance, and the ability to deal with MA saturation. Finally, a four-channel system employing the proposed amplifiers was built. It can successfully measure ECG R-peaks through clothes, while the fast-reset scheme enables the continuous detection in the presence of MAs.