Mohamed I. Ahmed

High dynamic range CMOS image sensor with pixel level ADC and in-situ image enhancement

We describe a CMOS image sensor with pixel level analog to digital conversion (ADC) having high dynamic range (>100db) and the capability of performing many image processing functions at the pixel level during image capture. The sensor has a 102x98 pixel array and is implemented in a 0.18um CMOS process technology. Each pixel is 15.5um x15.5um with 15% fill factor and is comprised of a comparator, two 10 bit memory registers and control logic. A digital to analog converter and system processor are located off-chip. The photodetector produces a photocurrent yielding a photo-voltage proportional to the impinging light intensity. Once the photo-voltage is less than a predetermined global reference voltage; a global code value is latched into the pixel data buffer. This process prevents voltage saturation resulting in high dynamic range imaging. Upon completion of image capture, a digital representation of the image exists at the pixel array, thereby, allowing image data to be accessed in a parallel fashion from the focal plane array. It is demonstrated that by appropriate variation of the global reference voltage with time, it is possible to perform, during image capture, thresholding and image enhancement operations, such as, contrast stretching in a parallel manner.

Average encoding scheme to reduce capacitive coupling effects in mixed-signal integrated circuits

In this paper, we present an encoding scheme, which we call the average encoding scheme, to compensate for the detrimental effects of capacitive coupling between digital metal layers that overlap analog metal layers in a mixed signal integrated circuit. The scheme is not limited for use in integrated circuits and can also be used on circuit boards. In general, a design that has limitations in the number of metal layers for shielding purposes can benefit from our average encoding technique. It does not need any additional bit lines and hence saves on expensive silicon real estate. We present an implementation of the average encoding scheme on the digital pixel sensor (DPS), with measured results. In certain scenarios, the encoding scheme can be adapted after the chip has been fabricated and can be tuned to the particular design. The encoding scheme has the characteristic of averaging and constraining the rate of change of charge accumulation due to coupling capacitance hence avoiding a charge build up on the analog metal lines