CMOS Image Sensor Readout Circuit for High-Speed HDR Capture
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Solution Overview
Problem
Standard image sensors have a limited dynamic range, struggling to capture details in both bright highlights and dim shadows due to their restricted ability to handle the broader luminance range of natural scenes, which is addressed by incorporating high dynamic range (HDR) technologies.
Innovation Solution
The implementation of a high dynamic range shared pixel CMOS image sensor with a readout circuit that includes sample and hold circuitry, enabling parallel operations and utilizing a pixel array with subpixels configured as small and large photodiodes, along with a lateral overflow integration capacitor, to enhance dynamic range and facilitate high-speed readouts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If standard image sensors are used to capture images, then the device complexity remains low, but the dynamic range is limited to approximately 60 to 70 dB, unable to capture details in both bright highlights and dim shadows
Solution Approach 1:
The pixel array is divided into multiple pixel types within the same pixel cell, including first pixels with small photodiodes for bright regions, second pixels with large photodiodes for dark regions, and third pixels with overflow integration capacitors for very bright regions. This segmentation allows different regions of the same scene to be captured with optimal sensitivity, expanding the dynamic range from 60-70 dB to over 90 dB while maintaining a relatively compact sensor structure.
Solution Approach 2:
Different pixel types are strategically positioned within the pixel array to match the spatial distribution of light intensities in the scene. Small photodiodes are used in regions expecting bright light, large photodiodes in regions expecting dim light, and overflow capacitors in regions with extreme highlights. This local optimization of pixel characteristics maximizes the overall dynamic range capture capability of the sensor.
2Adaptability or versatility
If multiple exposures are merged to increase dynamic range, then the dynamic range increases to capture broader luminance ranges, but the readout time increases due to sequential processing requirements
Solution Approach 1:
The sensor performs preliminary parallel readout of image charges from all pixel types simultaneously during a single exposure. The readout circuit is designed to read out signals from small photodiodes, large photodiodes, and overflow integration capacitors at the same time, eliminating the need for sequential multi-exposure capture and processing, thus reducing readout time while maintaining high dynamic range capability.
Solution Approach 2:
The patent merges the functionality of multiple pixel types (small photodiodes, large photodiodes, and overflow capacitors) into a single integrated readout circuit that processes all signals simultaneously. This combining of readout operations into a unified parallel process eliminates the time penalty associated with sequential processing of multiple exposures while achieving HDR results.
3Productivity
If parallel readout operations are implemented to reduce readout time, then the readout speed increases, but the device complexity increases due to additional circuitry requirements
Solution Approach 1:
The readout circuit is designed with universal, multi-functional components that can handle signals from all pixel types through a single integrated pathway. The same readout circuitry processes signals from small photodiodes, large photodiodes, and overflow capacitors, eliminating the need for separate dedicated readout circuits for each pixel type. This multi-functionality achieves parallel readout capability while minimizing the increase in device complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution significantly expands the dynamic range of image sensors, allowing for the simultaneous capture of bright highlights and dim shadows, resulting in improved image quality and reduced readout times through parallel analog to digital conversions.
Implementation Method 1
The image sensor includes an array of pixels having photosensitive elements (e.g., photodiodes) that absorb a portion of the incident image light and generate image charge upon absorption of the image light
Data Source
AI summary
A readout circuit for use in an image sensor includes a first sample and hold (SH) circuit coupled to a bitline that is coupled to a pixel array. A second SH circuit is coupled to the bitline. A bypass switch is coupled to the bitline, the first SH circuit, and the second SH circuit. An analog to digital converter (ADC) is coupled to the bypass switch. The bypass switch is configured to provide an image charge value from the pixel array to the ADC through the bitline, or through one of the first SH circuit or the second SH circuit in response to a switch select signal.