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PPG and SpO2 Recording Circuit with Ambient Light Cancellation for Trans-Nail Pulse-Wave Monitoring System

In order to efficiently record the photoplethysmography (PPG) and apply it for peripheral oxygen saturation (SpO2) measurement, we developed a trans-nail pulse-wave monitoring system, which is able to record the PPG signal on nail-tip without discomfort and any effects from sweat. This system includes PPG and SpO2 recording circuit (PPG/SpO2-RC) with ambient light cancellation (ALC) function and was fabricated in 1P6M 0.18 μm CMOS technology. It has a small area of about 1.7 mm2. The PPG/SpO2-RC mainly consists of an LED driver circuit, a PPG readout circuit, and a 500 x 500 μm2 photodiode (PD). LED driver circuit selectively drives three different wavelength LEDs. In the PPG readout circuit, an 86/96 dBΩ I/V converter, 20-80 dB programmable gain amplifiers, and LPF ensured the PPG signal being recorded using an appropriate gain for different individuals. On top of that, an ALC circuit was integrated into the PPG readout circuit to reduce noise from ambient light and preventing these signals from saturating the circuit. The PPG/SpO2-RC was evaluated, and the PPG signal and SpO2 were measured and calculated successfully, proving that it worked as it designed. In the future, the circuit will be integrated with other elements such as a Bluetooth module (BLE) in the trans-nail pulse-wave monitoring system.