A 153.4 dB-DR PPG Recording IC with Extended Counting and Hardware Reuse.

Abstract

Photoplethysmogram (PPG) is widely used in wearable devices for health monitoring. High-precision signals are essential for medical diagnostics. However, motion artifacts in these devices can cause significant ambient light variation during PPG recording. This paper presents an accurate PPG recording front end with enhanced ambient light rejection (ALR). Quantization noise in a second-order sigma-delta modulator (SDM), used for direct current conversion, is reduced by extended counting of the modulator's residue. The first integrator of the SDM and the residue analog-to-digital converter (ADC) are reused in ALR circuits. The correlated double sampling (CDS) technique is enhanced by applying a first-order approximation of ambient light. Gain error in the residue ADC is reduced by charge compensation. The PPG front-end, implemented in a 180 nm process, achieves a dynamic range (DR) of 153.4 dB within a bandwidth of 20 Hz. The system operates with a minimum 1.28% duty cycle. Measurements of heart rate and blood oxygen at the fingertip and wrist verify the functionality of the PPG front end.