Research On Light-adaptive High-dynamic-range CMOS Image Sensor

Due to adavantages in cost, power dissipation and integration level, CMO image sensor(CIS) has become research focus of modern imaging systems. This paper starts with CIS dynamic range(DR) enhancement, designs and verifies a novel exposure control scheme which can accurately locate the lighting condition of imaging object and adjust exposure time freely.First, this paper introduces the basic structure of CIS, including frequently used integrated photodiodes, pixel readout circuits, analog and digital correlated double sampling(CDS) and analog-to-digital converters(ADC).Next, this paper discusses current DR enhancement measures thoroughly, introduces the operation principles and main drawbacks of auto-reset, image compression and adaptive integration, deduces the DR and signal-to-noise ratio(SNR) equations of dual exposure and multiple exposure, and gives a convincing explanation of DNR dip in segmented integration. Based on these analyses, this paper proposed a novel exposure control scheme.Then, this paper finished the design and simulation of each part on the analog readout chain using HJTC 0.18 μm BiCMOS technology. Readout chain is the whole image processing chain which starts from pixel unit and ends at signal processing circuit. This paper employs N+-Psub photodiode as photodetector. Source follower structure, whose incomplete reset problem is of our prior concern, is chosen as pixel readout circuit. This paper’s signal processing circuits include charge amplifier, single-slope ADC and digital CDS circuit. Charge amplifier is used for improving pixel SNR and serving as sample-and-hold circuit. Single-slope ADC is used for converting analog pixel output to digital codes which will later become the basis of light intensity detection. Digital CDS is a very important noise cancellation technique in CIS.Finally, this paper discusses the design and simulation of timing circuits and then conducted a complete simulation of the entire CIS. Timing circuits, including frequency divider and pixel array scanner, are just auxiliary parts of the analog circuits in traditional CIS. But in our light-adaptive CIS, light intensity detection circuit, based on ripple counters, and exposure time control circuit, especially the programmable frequency divider contained in it, consititutes the most essential part of the design. The complete simulation of CIS includes three lighting conditions: strong light level, weak light level and medium light intensity. Each lighting condition simulation is conducted under two architectures: traditional exposure mode and light-adaptive exposure mode. The final image results meet our initial design aim.