AC/DC Ratio Enhancement in Photoplethysmography Using a Pinned Photodiode

Photoplethysmography (PPG) enables non-invasive vitals monitoring. Nevertheless, it is intrinsically limited by the extremely small AC/DC ratio, also called perfusion-index (PI). This increases the dynamic-range requirements to reliably process the PPG wave, particularly for applications requiring stricter specifications and lower noise. We present a PI-enhancement technique exploiting a double transfer gate (TG) pinned-photodiode (PPD) structure. The process takes place at the device level by optimizing the TG control voltage and transfer time. Measurement results show that the PPG PI can be enhanced by a factor 5 by choosing the optimal parameters and without any circuit overhead.