Image Sensor Dynamic Range Capacitor Charge Management
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Solution Overview
Problem
Conventional image sensors with narrow dynamic range struggle to capture clear images across a wide illuminance range, leading to signal-to-noise ratio (SNR) dips when merging low-illuminance and high-illuminance images, resulting in distorted or saturated colors.
Innovation Solution
The implementation of a wide dynamic range (WDR) image sensor that integrates overflowed charge from a photodiode into a dynamic range capacitor, allowing for expanded capacity and separate conversion gains in high- and low-illuminance modes, using a cylinder-type capacitor and transistors to manage charge transfer and reset levels effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a narrow dynamic range is used in the image sensor, then the device complexity is reduced, but the image quality deteriorates due to saturation and color distortion in high-illuminance conditions
Solution Approach 1:
The image sensor is segmented into two distinct photodiodes: a first photodiode for capturing low-illuminance images and a second photodiode for capturing high-illuminance images. This segmentation allows each photodiode to be optimized for its specific illuminance range, preventing saturation and color distortion while maintaining manageable device complexity through functional division.
2Manufacturing precision
If a wide dynamic range is implemented using dual photodiodes, then the image quality is improved across illuminance ranges, but the device complexity increases
Solution Approach 1:
The image sensor merges the functionality of two specialized photodiodes (one for low-illuminance, one for high-illuminance) into a single integrated device. By combining these elements and using a switching mechanism to select between them, the system achieves wide dynamic range performance without proportionally increasing overall device complexity, as the two photodiodes share common circuitry and processing pathways.
3Quantity of substance
If overflowed charge is not integrated in high-illuminance mode, then the full well capacity is limited, but the signal-to-noise ratio deteriorates due to saturation
Solution Approach 1:
A third node is introduced as an intermediary to collect and hold overflowed charge from the second photodiode in high-illuminance mode. This intermediary node prevents charge saturation in the floating diffusion area while maintaining the full well capacity, thereby preserving signal-to-noise ratio. The third node acts as a buffer that mediates between the photodiode and the readout circuitry.
4Device complexity
If the same floating diffusion area is used for both low- and high-illuminance modes, then the device complexity is reduced, but the measurement precision deteriorates due to SNR dips when merging images
Solution Approach 1:
The measurement function is segmented by using separate photodiodes for low- and high-illuminance conditions. The first photodiode is optimized for low-illuminance measurement with appropriate full well capacity, while the second photodiode is optimized for high-illuminance measurement. This segmentation eliminates SNR dips that would occur when merging images from a single photodiode operating across both ranges, as each photodiode operates in its optimal performance regime.
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 eliminates SNR dips and enables high-quality image capture across a wide illuminance range without saturation, providing a clear and linear representation of both low- and high-illuminance images by efficiently managing charge accumulation and conversion gains.
Implementation Method 1
a photodiode that generates a charge in response to an incident light
Data Source
AI summary
A method of driving an image sensor includes integrating an overflowed charge from a photodiode in the floating diffusion area and a dynamic range capacitor. The dynamic range capacitor is formed between the floating diffusion area and a power supply voltage. The method further includes sampling a first voltage formed in the floating diffusion area by the integrated overflowed charge, resetting the photodiode, the floating diffusion area, and the dynamic range capacitor, sampling a reset level of the reset floating diffusion area, transferring a charge accumulated in the photodiode to the floating diffusion area, and sampling a second voltage formed in the floating diffusion area.