A 28μW 134dB DR 2nd-Order Noise-Shaping Slope Light-to-Digital Converter for Chest PPG Monitoring

Photoplethysmography (PPG) is a key non-invasive technique to extract vital signs like heart rate (HR), heart rate variability (HRV), and blood oxygen saturation (SpO2). Integrated in smartwatches, health patches and finger clips, PPG sensors are widely used in fitness, glucose, blood pressure, and medical monitoring with good user comfort. Integrating PPG in medical chest patches is interesting for COVID-19 patients since it can yield, next to HR and respiration, also SpO2, which traditional ECG-based patches cannot provide. However, measuring PPG/SpO 2 on the chest is challenging due to large motion artifacts, small AC/DC perfusion index (PI $\lt 0.1$%), and ambient light. Therefore, it poses a more stringent requirement on dynamic range (DR $\gt 120$ dB) for the PPG readout circuit, while still requiring low power consumption for long battery life. On-chip pinned-photodiodes (PPDs) have been used as passive integrators, resulting in very low power consumption $(2.63 \mu \mathrm{W})$ for the PPG readout [1]. However, the DR and linearity can be limited by the PPDs’ parasitic capacitors. Direct light-todigital converter (LDC) topologies are proposed in [2 - 4], but their DR is still below 120dB. In [5], a digital-controlled TIA is exploited to control the transimpedance gain, which extends the DR to 130dB while consuming $72 \mu \mathrm{W}$ in the analog front-end (AFE).