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Title: Wide dynamic range CMOS image sensor
Author: Das, Dipayan
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2011
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Abstract:
• Abstract Integrated digital imaging systems are widely used in consumer electronics today. Current digital image sensors have a linear respeiiSt. The limited dynamic range of linear digital image sensors results in saturation when the input dynamic range of the scene is larger than that of the camera. This limitation could be overcome using pixels with an output that is proportional to the logarithm of the detected photocurrent. Conventional CMOS pixels with a logarithmic response, using a transistor operating in the sub-threshold region, are capable of capturing wide dynamic range scenes with more than six decades of illumination intensity. But these pixels suffer from fixed pattern noise, slow response and low sensitivity. A five transistor (5T) pixel circuit for a standard 0.35-fLm CMOS process which integrates the photocurrent linearly and capable of a logarithmic response is described in the thesis. A key component of the 5T pixel is a time-dependent reference voltage. This voltage is applied to the gate of one of the transistors inside each pixel in the array for the duration of the exposure to generate a logarithmic response. A model derived to generate the reference voltage is described. Improvements were made to the reference voltage model to take into consideration the non-ideal effects such as charge feedthrough and threshold voltage variations. A potential problem associated with successfully tonemapping low photo currents with the 5T pixel has been described and a method to calculate the optimal value of reference current Iret proposed. This was shown to lead to an optimum photoresponse. Measurement results from fabricated 1-D and 2-D arrays of 5T pixels are presented and analysed. An overall DR of 97-dB (almost 5 decades) has been achieved from 100 mlux to 6.7 Klux. The slope of the logarithmic photoresponse was shown to be adjustable and controlled by the slope parameter S in the reference voltage model. A large output swing of over 1 V due to the large photoresponse slope in the logarithmic region results in greater signal-to-noise ratio compared to the conventional logarithmic pixel based on the subthreshold transistor operation (60 m V/decade). Digital and analogue reference voltage generating techniques are described with circuits implemented in 0.35-fLm CM OS process. Finally, a 5T NMOS pixel that is capable of WDR imaging with superior low-light performance (23 mlux) and greater DR (1l0-dB) than the 5T PMOS pixel is described. [ a
Supervisor: Collins, Stephen Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.572608  DOI: Not available
Keywords: Image converters ; Digital images ; Metal oxide semiconductors, Complementary
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