A Low-Power Photoplethysmography Sensor using Correlated Double Sampling and Reference Readout Circuit

This paper describes a low-power photoplethysmography (PPG) sensor circuit using a current integration circuit. PPG sensors are widely used in modern healthcare products for monitoring cardiovascular information. However, the PPG circuits generally have a large power consumption because the LED driver consumes considerable energy to obtain the required amount of reflected light from the human body. A simple way to reduce the power requirements of PPG circuits is to lower the duty cycle of the LED through intermittent operation of the LED. However, this causes accuracy degradation because the target signal is susceptible to interference from various noises. To reduce the power consumption while maintaining accuracy, the correlated double sampling (CDS) method that we introduced in our previous work was used. In our previous work, wherein the PPG was placed on fingertips, the heartbeat error was 5 ms. This paper presents a signal-to-noise ratio improvement method for CDS by using a PPG sensor with current integration circuits. This enables SNR improvement and measurements to be taken from any part of the body. In the proposed method, the target PPG signal is extracted by canceling noise and DC components using two sensors. The proposed circuit was evaluated using actual measurements and the total consumption current was 26.9 μA. The root mean square error of the heartbeat interval was 4.27 ms, even though the sensor was worn on the wrist during the experiment.