Analog to digital converters for CMOS imagers

Abstract

A/D converters for single chip CMOS imagers have often been designed using the column-parallel approach, employing a slow A/D converter for each column of the sensor array. This thesis investigates a serial approach utilizing a single fast A/D converter to process all of the imager pixels. If power scales linearly with frequency in a given A/D architecture, power dissipation for the two approaches should be comparable. However, the serial approach should occupy less area since only the cost of one A/D converter is incurred. A figure of merit 1/power*area is introduced to verify this theory by comparing previously reported A/D approaches after appropriate technology, speed, and supply scaling. Camera system specifications require a single serial A/D converter to have 10b resolution at a 3MHz sampling rate for a CIF (352x288) imager array running at 30 frames/second Area minimization, power minimization, and the ability to build the A/D in a standard CMOS process are extremely important for consumer product applications. A single slope A/D architecture with a subnanosecond time digitizer shows promise for optimizing figure of merit over pipelined and folding interpolating approaches. This work focuses on the design issues of the 3MHz single-slope based A/D converter. Architectures appropriate for extending this A/D converter to 12MHz for four times CIF image arrays (704x576) are discussed. The 3MHz converter was designed, simulated, and laid out in a 0.35um CMOS technology. At 3.3V supply, 25°C and nominal process conditions, the converter dissipates 29 mW while occupying 0.3 mm2 . A 12MHz trislope extension of this converter is estimated to dissipate 37 mW in 0.4 mm2.