Global Shutter Image Sensor Charge Overflow Management
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Solution Overview
Problem
Existing image sensors face challenges in achieving high global shutter efficiency due to non-uniform exposure periods and motion blur, especially in applications like augmented reality and virtual reality, where a global shutter operation is necessary to reduce image distortion.
Innovation Solution
The image sensor employs a pixel cell structure with a photodiode, a charge sensing unit comprising a first and second charge storage device, and a controller that manages charge transfer and quantization within a global exposure period, allowing for simultaneous exposure and accurate measurement of light intensity, thereby reducing parasitic light contributions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a global shutter operation is performed to reduce image distortion, then image quality is improved, but parasitic light contributions increase due to non-uniform exposure periods
Solution Approach 1:
The pixel cell is segmented into multiple functional regions: a photodiode for light sensing, a first charge storage device for storing charge during exposure, and a second charge storage device for storing overflow charge. This segmentation allows different parts of the pixel to perform specialized functions, improving charge management and reducing parasitic light effects while maintaining global shutter operation.
Solution Approach 2:
The first charge storage device acts as an intermediary between the photodiode and the second charge storage device. It temporarily holds charge during the exposure period and transfers overflow charge to the second charge storage device, enabling precise control of charge flow and reducing parasitic light contributions while maintaining measurement precision.
2Adaptability or versatility
If charge storage capacity is increased to handle overflow charge, then dynamic range is improved, but device complexity increases
Solution Approach 1:
The charge storage function is segmented across two separate charge storage devices rather than using a single large storage device. The first charge storage device handles charge during exposure, while the second charge storage device handles overflow charge, achieving extended dynamic range through functional segmentation while keeping individual device complexity manageable.
Solution Approach 2:
The pixel cell employs dynamic switching mechanisms controlled by control signals that enable or disable connections between the photodiode, first charge storage device, and second charge storage device. This dynamic reconfiguration allows the same hardware structure to adapt to different operating conditions (normal exposure vs. overflow conditions), achieving high adaptability without permanent structural complexity.
3Productivity
If simultaneous exposure of all pixel cells is implemented, then global shutter efficiency is improved, but measurement accuracy decreases due to charge saturation
Solution Approach 1:
The first charge storage device is prepared in advance to receive and hold charge from the photodiode during the exposure period. By pre-establishing this charge storage path, the system can simultaneously expose all pixel cells without charge loss, maintaining global shutter efficiency while preventing saturation through timely charge transfer to the second charge storage device.
Solution Approach 2:
The first charge storage device serves as an intermediary buffer that temporarily holds charge during simultaneous exposure of all pixel cells. This intermediary structure prevents charge saturation in the photodiode while maintaining synchronization across the array, thereby preserving both global shutter efficiency and measurement accuracy.
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 approach enhances global shutter efficiency and reduces image distortion, enabling high-quality image capture in motion-sensitive applications by ensuring all pixel cells measure light within the same global exposure period.
Implementation Method 1
A typical image sensor includes a photodiode to measure the intensity of incident light by converting photons into charge (e.g., electrons or holes)
Data Source
AI summary
In one example, an apparatus is provided. The apparatus includes a photodiode, a charge sensing unit, an analog-to-digital converter (ADC), and a controller. The controller is configured to: enable the photodiode to generate charge in response to incident light, accumulate at least a portion of the charge as residual charge until the photodiode becomes saturated by the residual charge, and transfer the remaining portion of the charge to the charge sensing unit as overflow charge if the photodiode becomes saturated by the residual charge. The controller is further configured to: generate, using the ADC, a first digital output based on the residual charge; after generating the first digital output, generate, using the ADC, a second digital output based on the overflow charge; and generate a digital representation of an intensity of the incident light based on at least one of the first digital output or the second digital output.