The Research On Readout Circuit Of Active Pixel CMOS Image Sensor

With the rapid development of microelectronics, the process develops from sub-micron to deep sub-micron. The performance of CMOS image sensor has reached andeven exceeded CCD's. CMOS image seneor becomes the focus of image sensor. In this paper, the developing history and present situation of CMOS image sensor inside and outside of China were summarized. The characteristics between CMOS image sensor and CCD image sensor were compared and the research purpose and meaning was pointed out. On the basis of introducing semiconductor photoelectric effect, the characteristics of the PN junction in the state of thermal equilibrium conditions and illumination condition was analyzed in detail. The Photoelectric characteristics of photodiode used in CMOS image sensor were deeply discussed, and its I-V characteristic and equivalent circuit was given. Traditional photodiode active pixel used NMOSFET as the reset transisitor, which existed threshold loss in the process of signal transmission. this made the output-swing of the circuit was restricted. In the paper a pixel structure in which a PMOSFET was used to replace the NMOSFET as the reset transisitor was proposed, and the transisitors of it were all designed optimization. Using software HSPICE to simulate, the results showed that the improved pixel circuit had a high output-swing and a high dynamic range. The readout circuit is an important element of the CMOS image sensor, the traditional CDS circuit was anylyzed and its performance was simulated. The conventional CDS circuit needs twe sample-hold circuits, in which a sample-control MOS transisitor, a sample-hold capacitance and an output buffer are needed. After that, a public differential amplifier is also needed to realize the differential output. The used of large number transistors occupied a lot of the chip area, and increase the consumption of the circuit. In the paper a new structure of the readout circuit CDS structure was presented, the circuit is not only simple in structure, but also low power consumption, low noise and single-ended output. The simulation result showed that at the circumstances of the power supply voltage of 1.8V, the power dissipation of the pixel and the CDS circuit was only 1.277μW, but under the same conditions, the count of the traditional CDS circuit is 3.658μW.