To accommodate multi-modal measurements, photoplethysmography (PPG) sensors have to be placed on some non-standard position, such as chest, which often suffers from signal quality due to unstable sensing. In this case, this paper presented a low-cost consideration of sensing interface that improves the signal quality of reflective PPG. Major designs were raised sensing surface and all-in-one electrode. The raised sensing surface can keep the light-emitting diode (LED) and photodiode (PD) to flexibly attach to the skin. The all-in-one electrode with open window and double-sided tape can provide a stable support for the sensing device. In this paper, two prototypes with different designs were tested to monitor PPG signal at the same time, and the PQR signal quality index (rSQI) was used for signal quality comparison. The results of experiment which was conducted on 9 healthy subjects showed that the device with special hardware designs outperformed the conventional device in reflective PPG measurements, with improvement of average 2.4554 rSQI score. With off-the-shelf parts and processes, these two designs can effectively reduce the low-frequency noise and enhance the quality of PPG signal at a relatively low cost.

reflective PPG,raised sensing surface,all-in-one electrode