Preamplifier Design Strategies for Capacitive Sensing of Electrophysiological Signals

Non-contact capacitive electrodes are very suitable for wearable devices to acquire electrophysiological signals in an unobtrusive and long-term way. However, due to its poorly-defined and low electrode capacitance ( $C_{e}$ ), its front-end amplifier requires a large biasing resistance (e.g., $> 100 \mathrm{G}\Omega$ ) to maintain a sub-Hz high-pass frequency and low circuit noise. This paper compares two amplifier structures, i.e., a voltage amplifier (VA) and a charge amplifier (CA), each with three biasing strategies, i.e., pseudo resistors, switched resistors and reset switches. These, in total six, amplifier designs were fabricated in a standard 180nm CMOS technology and compared in terms of their input impedance, transfer function and input-referred noise. As final result, the VA and CA structures have comparable performance. In terms of biasing network, the pseudo resistor can achieve hundreds of $\mathrm{G}\Omega$ , the switched-resistor only reaches several $\mathrm{G}\Omega$ due to parasitics, and the reset switch achieves a $\mathrm{T}\Omega$ resistance when it is shut off, and therefore results in the lowest high-pass frequency and the best noise performance.