A Novel Active Pixel Sensor Architecture with In-Pixel Chopping and Switched Biasing to Reduce the Low-Frequency Noise

In this work, a novel low-noise active pixel sensor (APS) is proposed which deploys in-pixel chopping and switched biasing to reduce the temporal noise. The source follower (SF) of an APS is the major contributor of the temporal noise. The proposed sensor is a modified conventional 3T pixel with an nwell/psub photodiode (PD) as photo-sensing element. To implement in-pixel chopping, two minimal sized additional nMOS switches are used per pixel. The switched biasing is implemented using two column-level (ps and ns) switches without affecting the pixel fill-factor (FF). The noise reduction techniques are validated through measured results obtained from a prototype sensor fabricated in 350 nm OPTO process from AMS. The sensor consists of a 2-D array of 128x128 pixels. The proposed pixel is designed with an area of 11 mu mx11 mu m achieving a FF of 35%. A total noise reduction (and enhancement in dynamic range (DR)) of around 16 dB is achieved from in-pixel chopping with 8 MHz chopping frequency (f(ch)) and switched biasing.