Wide Dynamic-range Low-power current-mode Optical Sensor Readout

Abstract

Optical sensor has been widely used in daily life. One of the important applications of optical sensors is the ambient light sensor installed in portable devices. The screen brightness of portable devices like smartphones is designed to be automatically adjusted based on the ambient environment. The ambient light detection is realized using optical devices like Photodiode (PD), where the incident light is converted into an analog current. The induced PD current is measured by a readout circuit so that the output of the readout circuit controls the screen brightness accordingly.
This thesis presents an energy-efficient current-mode optical sensor readout circuit that achieves a wide dynamic range. Hourglass ADC is used as the core structure of the readout circuit, applying current-to-frequency conversion. The output of the readout is the digital count within 100ms conversion time. The dark counts of the designed PD readout circuit are less than three counts. The noise count is less than one when output counts are smaller than 100 and less than 1% when output counts are more than 100. Fabricated in a 0.18μm CMOS process, the readout circuit achieves a 160dB dynamic measurement range comparable to the prior-of-art. The entire circuit consumes power of0.113mW and occupies a chip area of 0.056mm2.