An Ultralow 6-µ W Analog Front-End for a Flexible OLED-µ LED-OPD PPG Patch by PPG-Gated PDM Emission to Sense Heart Rate

A next-generation 0.5-V-based ultralow-voltage and low-power (ULV-ULP) readout for long-time continuous monitoring of photoplethysmography (PPG) signals is designed by this effort for flexible hybrid organic light-emitting diode (OLED)-micro light-emitting diode ( $\mu $ LED)-organic photodiode (OPD) patches to detect heart rate (HR) in real time. The designed readout consists of a transimpedance amplifier (TIA), a programmable continuousdc cancellation loop, and an autozero-offset cancellation. Toward ULV-ULP, a 0.5-V body-driven (BD) operational amplifier (OPA) with multipath miller zero compensation and PPG-gated pulse density modulation (PDM) to drive OLED are employed as core designs that achieve ultralow power. Also, this design incorporates a bandpass filter (BPF), a delta–sigma modulator, and a comparator. The aforementioned designs are implemented in an IC via the TSMC 0.18- $\mu $ m process in a chip area of 1.6 mm2. The sensing range of the readout circuit reaches from 20 nA to $200 \; \mu $ A. The measured integrated noise over 0.1–25 Hz is 0.8 nArms, while the achievable dynamic range (DR) of the PPG readout is 101.9 dB. The total power consumption of the readout system is as low as only $6 \; \mu $ W, while the power of OLED- $\mu $ LED ranges from $33\mu $ W to 3.8 mW.