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22 March 1999 Characterization of CMOS image sensors with Nyquist rate pixel-level ADC
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Techniques for characterizing CCD imagers have been developed over many years. These techniques have been recently modified and extended to CMOS PPS and APS imagers. With the scaling of CMOS technology, an increasing number of transistors can be added to each pixel. A promising direction to utilize these transistors is to perform pixel level ADC. The authors have designed and prototyped two imagers with pixel level Nyquist rate ADC. The ADCs operate in parallel and output data one bit at a time. The data is read out of the imager array one bit plane at a time in a manner similar to a digital memory. Existing characterization techniques could not be directly used for these imagers, however, since there is no facility to read out the analog pixel values before ADC, and the ADC resolution is limited to only 8 bits. Fortunately, the ADCs are fully testable electrically without the need for any light or optics. This makes it possible obtain the ADC transfer curve, which greatly simplifies characterization. In this paper we describe how we characterize our pixel level ADC imagers. To estimate QE, we measure the imager photon to DN transfer curve and the ADC transfer curve. We find that both curves are quite linear.Using an estimate of the sense node capacitance we then estimate sensitivity, and QE. To estimate FPN we model it as an outcome of the sum of two uncorrelated random processes, one representing the ADC FPN, and the other representing the photodetector FPN, and develop estimators for the model parameters form imager data under uniform illumination. We report characterization result for a 640 by 512 imager, which was fabricated in a 0.35 micrometers standard digital CMOS process.
© (1999) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
David X. D. Yang, Hui Tian, Boyd A. Fowler, Xinqiao Liu, and Abbas El Gamal "Characterization of CMOS image sensors with Nyquist rate pixel-level ADC", Proc. SPIE 3650, Sensors, Cameras, and Applications for Digital Photography, (22 March 1999);

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