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Title: Investigations of time-interpolated single-slope analog-to-digital converters for CMOS image sensors
Author: Levski, Deyan
ISNI:       0000 0004 7430 6344
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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This thesis presents a study on solutions to high-speed analog-to-digital conversion in CMOS image sensors using time-interpolation methods. Data conversion is one of the few remaining speed bottlenecks in conventional 2D imagers. At the same time, as pixel dark current continues to improve, the resolution requirements on imaging data converters impose very high system-level design challenges. The focus of the presented investigations here is to shed light on methods in Time-to-Digital Converter interpolation of single-slope ADCs. By using high-factor time-interpolation, the resolution of single-slope converters can be increased without sacrificing conversion time or power. This work emphasizes on solutions for improvement of multiphase clock interpolation schemes, following an all-digital design paradigm. Presented is a digital calibration scheme which allows a complete elimination of analog clock generation blocks, such as PLL or DLL in Flash TDC-interpolated single-slope converters. To match the multiphase clocks in time-interpolated single-slope ADCs, the latter are generated by a conventional open-loop delay line. In order to correct the process voltage and temperature drift of the delay line, a digital backend calibration has been developed. It is also executed online, in-column, and at the end of each sample conversion. The introduced concept has been tested in silicon, and has showed promising results for its introduction in practical mass-production scenarios. Methods for reference voltage generation in single-slope ADCs have also been looked at. The origins of error and noise phenomenona, which occur during both the discrete and continuous-time conversion phases in a single-slope ADC have been mathematically formalized. A method for practical measurement of noise on the ramp reference voltage has also been presented. Multiphase clock interpolation schemes are difficult for implementation when high interpolation factors are used, due to their quadratic clock phase growth with resolution. To allow high interpolation factors a time-domain binary search concept with error calibration has been introduced. Although the study being conceptual, it shows promising results for highly efficient implementations, if a solution to stable column-level unit delays can be found. The latter is listed as a matter of future investigations.
Supervisor: Choubey, Bhaskar ; Sheard, Steve ; Dzahini, Daniel ; Waeny, Martin Sponsor: European Commission
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Analog to Digital Converters ; Engineering Science ; Microelectronics ; Analog Integrated Circuit Design ; Photonics Devices ; CMOS Image Sensors ; Time to Digital Converter ; Pixel ; Column Parallel ; Analog to Digital Converter