Novel Single-Slope ADC Design for Full Well Capacity Expansion of CMOS Image Sensor

This paper proposes a novel single-slope (SS) ADC design and operation for full well capacity (FWC) expansion of CMOS image sensor to increase the dynamic range for small pixel. With the proposed technique, charges stored in the photodiode and floating diffusion of 4T active pixel sensor are all read out and accumulated by the proposed SS ADC to improve the FWC. Only one A/D conversion is required for each pixel, which decreases chip power consumption compared with the general double A/D conversion operation. A 160×140 CMOS image sensor chip with the proposed SS ADC is fabricated using 0.18-μm CMOS image sensors technology. This chip achieves a sensitivity of 5.33 V/lx·s and a FWC expansion ratio of 2.18 at 38.5 fps. The measured FWC is 47.45 ke^- with 118% boost. The ADC resolution is 8 bits and the resulting differential nonlinearity/integral nonlinearity of proposed column-parallel SS ADC is (+0.16,-0.24)/(+0.28,-0.52) least significant bit. The column-fixed pattern noise is 0.16%.