A 1-V 8.1-$\mu$ W PPG-Recording Front-End With > 92-dB DR Using Light-to-Digital Conversion With Signal-Aware DC Subtraction and Ambient Light Removal

Abstract

This letter reports on a photoplethysmogram (PPG) readout IC (ROIC), comprising an ultralow-power recording front-end and a 2-channel light-emitting diode (LED) driver. The front-end uniquely employs a continuous-time, first-order, incremental delta–sigma modulator (I– $\Delta \Sigma \text{M}$ ) for direct light-to-digital conversion in a single stage with duty-cycled operation for power savings. Moreover, the I– $\Delta \Sigma \text{M}$ enables reconfigurable operation in intermittent conversion phases that allows the direct measurement of dc and ambient light-induced components of the photodiode current for signal-aware on-the-fly cancellation to recover the small ac PPG signal. Fabricated in 0.18- $\mu \text{m}$ 1P/6M CMOS, the PPG ROIC features a high dynamic range of 92.7 dB, dissipates 8.1 $\mu \text{W}$ and 1.95 mW in the recording front-end and the LED (with its driver), respectively, with ~10% duty cycling, and is validated by high-fidelity PPG recording from a human subject fingertip using an infrared LED.