CMOS Image Sensor Column Source Follower Noise Elimination
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Solution Overview
Problem
The noise elimination unit in conventional CMOS image sensors requires two capacitors per pixel column, limiting miniaturization and causing signal gain reduction and image quality deterioration due to leakage currents in column selection transistors, which result in shading and uneven luminance.
Innovation Solution
The proposed imaging element employs a column source follower with a gate connected to a power supply voltage and a potential setting unit that sets the source end potential, eliminating the need for a sampling capacitor and reducing the noise elimination unit's size by using a transfer capacitor reset unit and a drive unit to manage noise signals effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a noise elimination unit with two capacitors (AC coupling capacitor and sampling capacitor) is provided for each pixel column, then noise elimination function is achieved, but device size increases and miniaturization becomes difficult
Solution Approach 1:
The patent extracts and eliminates the sampling capacitor from the noise elimination unit, retaining only the AC coupling capacitor. This reduction in components directly addresses the size issue while maintaining the essential noise elimination function through the remaining capacitor and modified circuit configuration.
Solution Approach 2:
The patent changes the operational parameters of the noise elimination unit by modifying how the AC coupling capacitor is used and adjusting the timing of operations. By changing the operational mode rather than keeping the same two-capacitor structure, the patent achieves noise elimination with reduced component count and smaller device size.
2Area of stationary object
If the capacity of the sampling capacitor is decreased to enable miniaturization, then device size is reduced, but leakage current effects increase causing shading and uneven luminance
Solution Approach 1:
By removing the sampling capacitor entirely, the patent eliminates the source of leakage current problems associated with capacitor capacity reduction. The harmful leakage effects are avoided by not including the component that would generate them, while device size is simultaneously reduced.
Solution Approach 2:
The patent converts the potential harm of having any capacitor (leakage current) into a benefit by using only the AC coupling capacitor with optimized timing. The single capacitor configuration, when operated with proper timing control, achieves noise elimination without the leakage current penalties of reduced-capacity sampling capacitors.
3Reliability
If the capacity of the AC coupling capacitor is increased to suppress leakage current effects, then image quality is improved, but device size increases further
Solution Approach 1:
The patent uses a partial action approach by employing only one capacitor (the AC coupling capacitor) rather than the full two-capacitor configuration. This partial implementation achieves sufficient noise elimination and image quality while avoiding the size penalty of having both capacitors at optimal sizes.
Solution Approach 2:
The patent employs periodic action through timing control of the noise elimination operations. By operating the AC coupling capacitor at specific timing intervals and using transfer transistors to control signal flow periodically, the system achieves effective noise elimination and image quality without requiring excessive capacitor capacity, thus maintaining compact device size.
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 allows for miniaturization of the imaging device without compromising image quality, as it suppresses noise and leakage current effects, enabling efficient noise elimination and improved image fidelity.
Implementation Method 1
a photoelectric conversion element, which accumulates a signal charge depending on the amount of incident light
Data Source
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AI summary
An imaging element includes: a plurality of pixels arranged in a two-dimensional matrix form, configured to receive light from outside, and configured to generate and output an imaging signal depending on an amount of the light received; a first transfer line connected to the pixel and configured to transfer the imaging signal; a second transfer line to which the imaging signal transferred by the first transfer line is output; a column selection switch configured to select one pixel column among the two-dimensional matrix, and output the imaging signal transferred by the first transfer line to the second transfer line; a column source follower including a gate to which the imaging signal transferred by the first transfer line is input, a drain end being connected to a power supply voltage, and a source end being connected to the column selection switch; and a potential setting unit configured to set a potential of the source end side of the column source follower to a predetermined potential.