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

摘要

A multichannel bit-serial (MCBS) analog-to-digital converter (ADC)nis presented. The ADC is ideally suited to pixel-level implementation inna CMOS image sensor. The ADC uses successive comparisons to output onenbit at a time simultaneously from all pixels. It is implemented using an1-bit comparator/latch pair per pixel or per group of neighboringnpixels, and a digital-to-analog-converter/controller shared by allnpixels. The comparator/latch pair operates at very slow speeds and cannbe implemented using simple robust circuits. The ADCs can be fullyntested by applying electrical signals without any optics or lightnsources. A CMOS 320×256 sensor using the MCBS ADC is described.nThe chip measures 4.14×5.16 mm2. It achievesn10×10 Μm2 pixel size at 28% fill factor in 0.35nΜm CMOS technology. Each 2×2 pixel block shares an ADC. Thenpixel block circuit comprises 18 transistors. It operates innsubthreshold to maximize gain and minimize power consumption. The powernconsumed by the sensor array is 20 mW at 30 frames/s. The measurednintegral nonlinearity is 2.3 LSB, and differential nonlinearity is 1.2nLSB at eight bits of resolution. The standard deviation of the gain andnoffset fixed pattern noise due to the ADC are 0.24 and 0.2%,nrespectively