A 140 dB-DR Light-to-Digital Converter Using Current-Domain Hybrid Zoom for Baseline Cancellation and Interference Compensation

Abstract

Noninvasive optical sensing techniques, such as photoplethysmography (PPG) and functional near-infrared spectroscopy (fNIRS), provide great user comfort and obtain rich hemodynamic information in term of pulse oximetry, blood flow velocity, and blood vessel stiffness. Conventional optical sensor readouts have evolved toward higher dynamic range (DR) and power-efficiency. In addition to quasi-static baseline input signals, fast-varying interferences due to motion artifacts (MAs) or environment changes can also cause saturation. How to compensate or tolerate these disturbances efficiently is one of the prominent properties required in emerging optical sensors. This article presents a high DR, energy-efficient light-to-digital converter (LDC). The proposed LDC utilizes a second-order incremental delta-sigma modulator (I-DSM) for high resolution, while a power-scaling OTA is utilized to reduce power consumption of the main integrator. The current-domain static zoom (SZ) is exploited to cancel baseline currents for high DR, while the dynamic zoom (DZ) tracks and compensates the residual ac input current to prevent fast-varying interference from saturating the analog-front-end (AFE) during fine quantization. Fabricated in a standard $0.18~\mu $ m CMOS process, the LDC achieves a very high DR of 140 dB in a 2 kHz bandwidth. Thanks to the power-scaling OTA, the signal-to-noise and distortion-ratio (SNDR) of the ac path reaches 94.55 dB with only $44~\mu $ W from a 1.2 V supply. The benefits of SZ and DZ have been validated by chest PPG measurement with significant MAs.