Wide Dynamic Range CMOS Image Sensor With In-Pixel Double-Exposure And Synthesis

With the development of security surveillance system, the image senor with wide dynamic range is required. However, the dynamic range of the conventional CMOS image sensor(CIS) is lower than 60dB, and the sensor can't capture a scene consisting of both bright and dark regions without losing details. The conventional methods to enlarge dynamic range expand the dynamic range at the cost of complexity of the pixel or capture two or more images in different time periods and synthesize them in a large size memory.A wide-dynamic-range CMOS image sensor based on synthesis of a long-time and a short-time exposure signal in the floating diffusion of five-transistor active pixel is proposed. On basis of analysis of advantages and disadvantages of the conventional methods to enlarge dynamic range, the mathematical model of the wide dynamic range CMOS image sensor with in-pixel double-exposure and synthesis is presented; the pixel operation is discussed with the change of potential barrier and well; based on the require of the pixel operation, the pixel driving circuit and level selection circuit are designed and simulated with Tower 0.18μm CMOS process, including bandgap reference circuit, bias voltage generation circuit, voltage of transfer gate generation circuit, global reset signal driving circuit. The voltage of the transfer gate and the global reset are programmable so that it meets the pixel operation.A prototype wide-dynamic-range CMOS image sensor with in-pixel double-exposure and synthesis was developed with a 0.18μm CIS process. With the optimized pixel operation, the response curve is compressed and a wide dynamic range image is obtained. With the double exposure time 2.4ms and 70ns, the dynamic range of the proposed sensor is 80dB with 30 frames per second. The proposed CMOS image sensor meets the demand of application in the security surveillance system.The key points: The dynamic range of CMOS image sensor is enlarged on basis of in-pixel double-exposure and synthesis without extra transistors in the pixel or memories on chip. Compared to the conventional double exposure technology, the sensor reduces the speed of the readout circuit and analog-digital conversion circuit.