A Nyquist-rate pixel-level ADC for CMOS image sensors

Abstract

A multichannel bit-serial (MCBS) analog-to-digital converter (ADC) is presented. The ADC is ideally suited to pixel-level implementation in a CMOS image sensor. The ADC uses successive comparisons to output one bit at a time simultaneously from all pixels. It is implemented using a 1-bit comparator/latch pair per pixel or per group of neighboring pixels, and a digital-to-analog-converter/controller shared by all pixels. The comparator/latch pair operates at very slow speeds and can be implemented using simple robust circuits. The ADCs can be fully tested by applying electrical signals without any optics or light sources. A CMOS 320/spl times/256 sensor using the MCBS ADC is described. The chip measures 4.14/spl times/5.16 mm/sup 2/. It achieves 10/spl times/10 /spl mu/m/sup 2/ pixel size at 28% fill factor in 0.35 /spl mu/m CMOS technology. Each 2/spl times/2 pixel block shares an ADC. The pixel block circuit comprises 18 transistors. It operates in subthreshold to maximize gain and minimize power consumption. The power consumed by the sensor array is 20 mW at 30 frames/s. The measured integral nonlinearity is 2.3 LSB, and differential nonlinearity is 1.2 LSB at eight bits of resolution. The standard deviation of the gain and offset fixed pattern noise due to the ADC are 0.24 and 0.2%, respectively.