Study On High Dynamic Range CMOS Image Sensor Technique

With the rapid development of microelectronics, the performance of CMOS image sensor has reached and even exceeded CCD's, CMOS image sensors are applied widely in such fields as industry, traffic, multimedia and medicine. Dymanic range is one of the key factors influencing the image quality of CMOS image sensors, and it will determine the development and use of CMOS image sensors, therefore, improving of dynamic range has become one of the the reseach focus on image sensors.This paper introduced CMOS image sensor and explored the factors that affect the dynamic range. The motivation of this paper is to analyze some techniques to enhance DR and present an architecture to improve the dynamic range (DR) of CMOS image sensors. The improved pixel architecture adapts the single pixel integration time to the local illumination condition. The circuit of the high dynamic range pixel contains a high-speed voltage comparator and two capacitors to detect and store the information on the integration time. In addition, the time-coding circuit is designed and the principle of the time-coding circuit has been expatiated. In HDR pixel circuit, the high-speed voltage comparator playes an important role, and its performance directly affects the performance of the entire circuit. Therefor, the OTA-based (operational transconductance amplifier) voltage comparator which is suitable for HDR pixel is designed. The improved voltage comparator is based on the topology of positive feedback comparator. Simulation results show that the comparator has the advantages, such as high gain, small delay and low power consumption. An array of 4×4 pixels has been built.Layout design rule, flow and notice have been introduced. Based on 0.35μm N-well standard CMOS process, the layout of the high dynamic range pixel has been designed.