A 1.8V 16μA 136.5dB DR PPG/NIRS Recording IC using Noise Shaping Triple Slope Light to Digital Converter

Photoplethysmography (PPG) monitoring can provide vital signs like heart rate (HR), heart rate variability (HRV), and blood oxygen saturation $\left(\mathrm{SpO}_{2}\right)$ non-invasively. Moreover, with other monitoring modalities, the PPG channel is becoming prevalent in wearable devices for fitness, glucose, blood pressure, etc., applications [1]. The PPG channel usually puts a stringent requirement on readout design to cope with the extremely low AC/DC (perfusion index < 0.1 %) [2]. On-chip pinned-photodiodes (PPDs) have been used with very low power consumption $(2.63 \mu \mathrm{W})$ for the PPG readout [3] at the cost of very limited DR and linearity. Direct light-to-digital converters (LDC) have been proposed in [4, 5, 6], achieving more than 130dB DR. However, their SNDR in the AC path is still limited to $\lt 90 \mathrm{~dB}$. A PPG readout with higher EOB in the AC path is required for ambulatory monitoring. Meanwhile, power consumption should be minimized to prolong the battery lifetime.