Design and evaluation of a capacitively coupled sensor readout circuit, toward contact-less ECG and EEG

Electrophysiological signal acquisition such as ECG and EEG play an important part in modern medical monitoring and diagnostics. The measurement of these very low-level, low-frequency signals are normally made from the skin with a directly coupled sensor utilizing a conductive gel to create a low resistance path for the charge. The application of the gel is tedious and time consuming as well as requiring a clinical environment and prevents long period measurements. In this paper, a contact-less, capacitively coupled sensor — without any need for gel — together with an electronic readout circuit using a PCB is presented. A design with a very high input impedance allows for measurements of signals with amplitudes down to a few tens of microvolts and at frequencies between a few hertz to tens of hertz. Measurements show that the circuit could detect an input signal of 25 μV at 10 Hz with an SNR of 9.7 dB.