Capacitively coupled ECG sensor system with digitally assisted noise cancellation for wearable application

This paper describes a digitally assisted noise cancellation method for a capacitively coupled electrocardiogram (ECG) sensor. This sensor using an insulated electrode can measure ECG through an insulator such as clothing without direct skin contact. In wearable applications, this type of ECG sensor is superior in terms of usability compared with the pasted type ECG sensor. However, noise immunity is an important difficulty related to capacitively coupled ECG sensors because it requires very high input impedance and small input capacitance for the first-stage amplifier. This circuit characteristic considerably degrades its noise immunity for the power line noise and motion artifact. To address this difficulty, we propose the noise feedback method, which can improve the availability of a capacitively coupled ECG sensor. Noise caused by body movement and the surrounding environment included in the output of the AD converter is extracted by digital filters. DC offset, baseline fluctuation, and low-frequency component of body motion noise are extracted using a variable-gain loop filter. Power line noise is also extracted using a peak filter. Then the noise waveform is estimated from the result of the previous cycle. This noise information is DA converted and given feedback to the first stage amplifier. The proposed method was evaluated using prototype sensor in an actual environment. ECG measurements were confirmed in both two-electrode configuration and single-electrode configuration. Measurement results show that the power line noise can be suppressed to -29.2 dB at maximum.