CMOS Image Sensor AD Conversion Timing Control
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Solution Overview
Problem
Existing photoelectric conversion devices, such as CMOS image sensors, face challenges in improving image quality due to noise caused by the simultaneous inversion of comparator output levels, particularly in horizontal OB pixels, which are not adequately distinguished from effective pixels in previous techniques.
Innovation Solution
A photoelectric conversion device is designed with a pixel array unit and AD conversion circuits, where a first pixel is shielded from light and a second pixel receives light, with comparators having different slew rates to control the timing of AD conversion results, ensuring that the result for the shielded pixel is determined before that of the light-receiving pixel for signals of the same level.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If AD conversion is performed simultaneously for all pixels including OB pixels and effective pixels, then processing efficiency is maintained, but noise is generated due to simultaneous inversion of comparator output levels
Solution Approach 1:
The patent segments the pixel array into two distinct groups: OB pixels (optical black pixels) and effective pixels. Each group is processed separately through dedicated column circuits, allowing independent control of AD conversion timing. This segmentation enables the system to process OB pixels first without affecting effective pixel processing, thereby eliminating noise from simultaneous inversion while maintaining overall processing efficiency.
Solution Approach 2:
The patent implements preliminary action by performing AD conversion for OB pixels before processing effective pixels. The control circuit is configured to complete AD conversion for OB pixels first, storing their digital values in memory, and then proceeding with effective pixel conversion. This sequential approach ensures that comparator inversion noise does not interfere with effective pixel data, as the harmful inversion occurs only during the already-completed OB pixel processing phase.
2Device complexity
If horizontal OB pixels are processed together with effective pixels, then circuit simplicity is maintained, but image quality deteriorates due to noise influence
Solution Approach 1:
The patent applies segmentation by creating separate column circuit paths for OB pixels and effective pixels. Horizontal OB pixels are routed through dedicated column circuits that are distinct from those processing effective pixels. This physical and functional separation allows the system to maintain relatively simple circuit structures while achieving noise-free processing, as each segment can be optimized independently without requiring complex inter-segment coordination.
3Speed
If comparator output levels are inverted simultaneously across all columns, then AD conversion speed is maintained, but noise propagation occurs affecting multiple pixels
Solution Approach 1:
The patent extracts OB pixels from the main effective pixel processing flow and handles them separately through dedicated column circuits. By removing OB pixels from the simultaneous inversion process, the system allows comparator output levels to be inverted for effective pixels without propagating noise to OB pixel data, since the OB pixel conversion is already complete and stored. This extraction maintains AD conversion speed for effective pixels while eliminating noise propagation issues.
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 configuration effectively suppresses noise caused by comparator inversion, leading to improved image quality by ensuring that the AD conversion of the optical black pixel is completed before that of the effective pixels, thereby reducing errors and enhancing image fidelity.
Implementation Method 1
a first pixel arranged in a first column of the plurality of columns and in which the photoelectric conversion element is shielded from light, and a second pixel arranged in a second column of the plurality of columns and in which light is incident on the photoelectric conversion element
Data Source
AI summary
The photoelectric conversion device includes a plurality of pixels arranged to form a plurality of columns, a plurality of AD conversion circuits provided corresponding to the plurality of columns, and a control circuit configured to control the AD conversion circuits. The plurality of pixels includes an OB pixel arranged in a first column and an effective pixel arranged in a second column. The plurality of AD conversion circuits each include a first AD conversion circuit including a first comparator receiving a signal of the OB pixel, and a second AD conversion circuit including a second comparator receiving a signal of the effective pixel. The control circuit controls the first and second comparators such that the result of the AD conversion by the first AD conversion circuit is determined earlier than the result of the AD conversion by the second AD conversion circuit for signals of the same level.