PhD Thesis on Analog Signal Processing for CMOS Image Sensors

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The very first PhD thesis that came out of Albert Theuwissen's group at TU Delft is now freely available as a pdf. This seems like a great educational resource for people interested in image sensors.

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Title: Analog Signal Processing for CMOS Image Sensors
Author: Martijn Snoeij
Year: 2007

This thesis describes the development of low-noise power-efficient analog interface circuitry for CMOS image sensors. It focuses on improving two aspects of the interface circuitry: firstly, lowering the noise in the front-end readout circuit, and secondly the realization of more power-efficient analog-to-digital converters (ADCs) that are capable of reading out high-resolution imaging arrays. 

Chapter 2 provides an overview of the analog signal processing chain in conventional, commercially-available CMOS imagers. First of all, the different photo-sensitive elements that form the input to the analog signal chain are briefly discussed. This is followed by a discussion of the analog signal processing chain itself, which will be divided into two parts. Firstly, the analog front-end, consisting of in-pixel circuitry and column-level circuitry, is discussed. Second, the analog back-end, consisting of variable gain amplification and A/D conversion is discussed. Finally, a brief overview of advanced readout circuit techniques is provided.

In chapter 3, the performance of the analog front-end is analyzed in detail. It is shown that its noise performance is the most important parameter of the front-end. An overview of front-end noise sources is given and their relative importance is discussed. It will be shown that 1/f noise is the limiting noise source in current CMOS imagers. A relatively unknown 1/f noise reduction technique, called switched-biasing or large signal excitation (LSE), is introduced and its applicability to CMOS imagers is explored. Measurement results on this 1/f noise reduction technique are presented. Finally, at the end of the chapter, a preliminary conclusion on CMOS imager noise performance is presented. 

The main function of the back-end analog signal chain is analog-to-digital conversion, which is described in chapter 4. First of all, the conventional approach of a single chip-level ADC is compared to a massively-parallel, column-level ADC, and the advantages of the latter will be shown. Next, the existing column-level ADC architectures will be briefly discussed, in particular the column-parallel single-slope ADC. Furthermore, a new architecture, the multiple-ramp single-slope ADC will be proposed. Finally, two circuit techniques are introduced that can improve ADC performance. Firstly, it will be shown that the presence of photon shot noise in an imager can be used to significantly decrease ADC power consumption. Secondly, an column FPN reduction technique, called Dynamic Column Switching (DCS) is introduced.

Chapter 5 and 6 present two realisations of imagers with column-level ADCs. In chapter 5, a CMOS imager with single-slope ADC is presented that consumes only 3.2µW per column. The circuit details of the comparator achieving this low power consumption are described, as well as the digital column circuitry. The ADC uses the dynamic column switching technique introduced in chapter 4 to reduce the perceptional effects of column FPN. Chapter 6 presents an imager with a multiple-ramp single-slope architecture, which was proposed in chapter 4. The column comparator used in this design is taken from a commercially available CMOS imager. The multiple ramps are generated on chip with a low power ladder DAC structure. The ADC uses an auto-calibration scheme to compensate for offset and delay of the ramp drivers.

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