A Low-Power Opamp-Less Second-Order Delta-Sigma Modulator for Bioelectrical Signals in 0.18 µm CMOS

This article reports on a compact and low-power CMOS readout circuit for bioelectrical signals based on a second-order delta-sigma modulator. The converter uses a voltage-controlled, oscillator-based quantizer, achieving second-order noise shaping with a single opamp-less integrator and minimal analog circuitry. A prototype has been implemented using 0.18 μm CMOS technology and includes two different variants of the same modulator topology. The main modulator has been optimized for low-noise, neural-action-potential detection in the 300 Hz-6 kHz band, with an input-referred noise of 5.0 μV_rms, and occupies an area of 0.0045 mm². An alternative configuration features a larger input stage to reduce low-frequency noise, achieving 8.7 μV_rms in the 1 Hz-10 kHz band, and occupies an area of 0.006 mm². The modulator is powered at 1.8 V with an estimated power consumption of 3.5 μW.