This paper presents a novel scalable CMOS single photon avalanche diode (SPAD) based PPG sensor integrated with direct light to digital conversion using simple CMOS logic gates on a single chip. With single photon detection capability, a SPAD provides much higher light sensitivity than a photodiode (PD) used in typical PPG sensors, reducing LED driving current significantly for the same receiving current than that with a PD. Moreover, the proposed PPG sensor eliminates the typical power and area intensive analog front end readout electronics associated with conventional PPG sensing, which makes it an attractive option for wearable consumer electronics. For this prototype, a 2 X 2 SPAD based pixel array has been implemented in a standard 180 nm CMOS process. Four SPAD based subpixels have been combined to reduce dead time at the pixel level resulting in increased photon detection capability.
