A Low-Power PPG Readout With an Automatic Switching Between Ambient Light and OLED Driver

A low-power and low-noise photoplethysmography (PPG) readout for a flexible circular organic light-emitting diode (OLED)-organic photodiode (OPD) patch to estimate blood pressure (BP) is designed by this effort. The designed system consists of a current-to-voltage converter, a low-pass filter (LPF), a high-pass filter (HPF), a programmable gain amplifier (PGA), a window-based external OLED driver, and a microcontroller unit (MCU). The front-end circuit of readout is designed to switch automatically between ambient light and OLEDs to be used as a light source for PPG, that is, to reduce or even switch off the OLED during high contrast light to minimize power consumption. The analog front-end circuit is implemented in the integrated chip with an area of 1.44 mm2 and fabricated via Taiwan Semiconductor Manufacturing Company (TSMC) T18 process. With the standard 1.8 V, the experimental result shows that the measured linear current sensing range is 50 nA– $11~\mu $ A. The measured signal-to-noise ratio (SNR) with the OLED-OPD PPG sensing system is over 40 dB (including the sensor). The total power consumption of the designed chip is $54~\mu $ W for the readout, while the power consumption of the OLED driver varies from $3.6~\mu $ W to 5.3 mW@3.3 V.