A clipping amplifier with passive open-loop baseline cancellation for bio-impedance measurement

A novel clipping amplifier (CAMP) for bio-impedance measurement is presented. Unlike conventional schemes requiring auxiliary current sources, closed-loop control, or active reset circuitry, the proposed approach employs only a passive resistor-diode network to generate an opposite-phase compensation signal. By feeding both the input and compensation signals into a multi-input capacitively coupled instrumentation amplifier (MCCIA), the baseline is directly suppressed, enabling effective amplification of small impedance variations with zero settling time and no phase-alignment requirement. Fabricated in a 180-nm CMOS process occupying 3.0 mm × 2.2 mm, the fabricated chip demonstrates a programmable gain of 34.8–49.2 dB and an input-referred noise floor of 46.7 nVrms/√Hz over 300 Hz–800 Hz. The baseline-cancellation mode achieves a 3.3× improvement in local sensitivity without saturation. The proposed passive open-loop architecture is particularly advantageous in multi-channel systems such as electrical impedance tomography (EIT), where per-channel area and power are tightly constrained.