A 12b Dual-Mode SAR ADC for Bio-Medical Applications

This paper presents an ultra-low power 12b successive-approximation register (SAR) analog-to-digital converter (ADC) for bio-medical sensing analog front-end circuits. This ADC used binary-window and capacitor-swapping schemes to improve its linearity. A time-domain comparator was proposed to meet both low-noise and low-supply voltage. Some switched-capacitor circuits were applied with thick-oxide transistors against the leakage issue. Furthermore, a bypass-switching function was applied to deal with the sparse biomedical signals for additional power reduction. The 12b SAR ADC was fabricated using $180-\mathrm{nm}$ CMOS technology. This prototype ADC core consumes only 36 nW from $0.5-\mathrm{V}$ and $1.0-\mathrm{V}$ supplies at $1 \mathrm{kS} / \mathrm{s}$. With the Nyquist rate input, the simulated SNDR and SFDR are 68.7 and 83 dB, respectively. The peak ENOB is maintained at 11.1 bits, equivalent to a figure-of-merit of $18 \mathrm{fJ} /$ conversion-step. The sampling frequency can be scaled from 1 kHz to 10 kHz for EEG signal acquisition.