Image Sensor Sample and Hold Circuit for Time Variant Noise Reduction
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Solution Overview
Problem
Image sensors with pinned photodiode pixels in CMOS architecture face challenges in reducing noise from support circuitry, particularly time variant noise that affects entire rows or columns of pixels, making it more noticeable to the human eye.
Innovation Solution
The implementation of sample and hold circuitry to maintain a proportion of support circuitry noise as time invariant, combined with readout circuitry that samples and holds noise levels and performs analog to digital conversion to minimize output noise, using techniques such as Correlated Double Sampling and continuous time architecture.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If support circuitry is used to enable pixel array operation, then pixel array functionality is achieved, but time variant noise is introduced that degrades output quality
Solution Approach 1:
The sample and hold circuit samples the noise from support circuitry at a first time before the pixel readout occurs, and holds this sampled noise level constant during the subsequent pixel signal readout at a second time. This preliminary sampling and holding action allows the noise to be captured and maintained at a constant level, enabling subsequent noise subtraction without introducing additional time variant noise during the critical measurement period.
2Measurement precision
If noise from support circuitry is reduced, then output quality improves, but circuit complexity increases
Solution Approach 1:
The sample and hold circuit acts as an intermediary between the noisy support circuitry and the pixel readout circuitry. It captures the noise from the support circuitry, holds it at a constant level, and provides this held noise as a reference signal for subtraction from the pixel signals. This intermediary approach reduces the time variant noise in the final output without requiring complete redesign of the support circuitry, thus limiting the increase in overall system complexity.
3Measurement precision
If noise is addressed on a pixel-to-pixel basis, then individual pixel quality improves, but row/column noise affecting all pixels remains problematic
Solution Approach 1:
The noise reduction approach segments the noise handling into two components: pixel-specific noise addressed through conventional per-pixel techniques, and support circuitry noise affecting entire rows or columns addressed through the sample and hold circuit. The sample and hold circuit specifically targets the row/column noise by sampling and holding the noise from support circuitry that affects multiple pixels, allowing this common-mode noise to be subtracted from all pixels in the row or column uniformly.
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 significantly reduces time variant noise from support circuitry, improving image sensor output quality by making noise contributions time invariant, thereby enhancing image quality and relaxing noise specifications for the preceding stages.
Implementation Method 1
an array of pixels, each pixel having at least one photo-sensitive element
Implementation Method 2
it may further comprise a sampling capacitor for said maintaining substantially level said at least a proportion of said support circuitry noise
Data Source
AI summary
An image sensor includes an array of pixels. Each pixel has at least one photo-sensitive element. Readout circuitry receives an analog signal from each pixel at a first time and at a second time, between which the analog signal changes. The image sensor further includes associated support circuitry which is a source of time variant noise. The signal level at both first and second times includes pixel noise. Sample and hold circuitry is provided to maintain substantially level at least a proportion of this support circuitry noise time invariant at the sensor output between the first time and the second time.