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

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–<inline-formula> <tex-math notation="LaTeX">$\Delta \Sigma \text{M}$ </tex-math></inline-formula>) for direct light-to-digital conversion in a single stage with duty-cycled operation for power savings. Moreover, the I–<inline-formula> <tex-math notation="LaTeX">$\Delta \Sigma \text{M}$ </tex-math></inline-formula> 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-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> 1P/6M CMOS, the PPG ROIC features a high dynamic range of 92.7 dB, dissipates 8.1 <inline-formula> <tex-math notation="LaTeX">$\mu \text{W}$ </tex-math></inline-formula> 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.

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