A Monolithic Optical Sensor for IoT Devices Proximity-Detection

Nowadays, low power consumption is one of the key developments in mobile internet-of-things (IoT) devices proximity-detection. A low power monolithic CMOS optical sensor is presented in this article. The prototype sensor uses an auto-zero and correlated double sampling (CDS) based sample and hold (S/H) stage, which reduces low frequency noise as well as the op-amp DC offset and photodiode (PD) dark current and also serves as a single-ended-to-differential converter. A third-order single-bit delta sigma ADC is implemented to attain high resolution proximity sensing. The optical sensor is fabricated with a 0.18 $\mu$m CMOS process. The die size is 0.7*1.2 mm ² and the chip scale package dimensions are only $2\times 1\times 0.7$ mm ³ , showing high industrial design flexibility of sensor apertures sizes and shapes. The sensor draws an average current of 200 $\mu$A from a 1.8 V supply during the active state. Tested under a 90% reflectance panel, the measurement results show a proximity distance of 20 mm and a resolution of 0.5 mm, which makes it a practical approach to proximity sensing in IoT devices.