CMOS Pixel Readout Circuit Dual Floating Diffusion Dynamic Range
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Solution Overview
Problem
Designing a CMOS image sensor that balances high conversion gain with high full well capacity is challenging due to the trade-off between noise reduction and voltage limitations, leading to difficulties in achieving both high sensitivity and wide dynamic range.
Innovation Solution
A method involving a pixel photodiode with a floating diffusion region and an overflow diffusion region, where electrons are transferred from the floating diffusion region to the overflow region when saturated, allowing for readout of total charge without loss of information, and utilizing additional transistors and capacitors to manage noise and sensitivity levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a large conversion factor (CVF) is used to reduce noise, then low noise performance is improved, but full well capacity is limited due to voltage constraints on the conversion capacitor
Solution Approach 1:
The pixel is divided into two separate floating diffusion regions: a first floating diffusion region with high conversion gain for low-light signals, and a second floating diffusion region with low conversion gain for bright-light signals. This segmentation allows each region to be optimized for its specific signal range, resolving the contradiction between noise performance and full well capacity.
Solution Approach 2:
The system dynamically selects which floating diffusion region to use based on the illumination level. A transfer gate controls the routing of photo-generated charges to either the first or second floating diffusion region, enabling adaptive optimization of noise performance and dynamic range for different lighting conditions.
2Measurement precision
If a small conversion capacitor is used to achieve high conversion gain, then low noise is achieved, but the voltage on the capacitor is limited by supply voltage, reducing full well capacity
Solution Approach 1:
Different regions of the pixel have different capacitance values tailored to their specific functions. The first floating diffusion region has small capacitance for high gain, while the second floating diffusion region has large capacitance for high capacity, allowing each local region to have optimal properties for its purpose.
Solution Approach 2:
The system changes the effective capacitance parameter by selecting different floating diffusion regions based on signal level. This allows the conversion gain to be adjusted dynamically - high gain when needed for low light, and low gain (effectively higher capacity) for bright light conditions.
3Device complexity
If a single floating diffusion region is used, then device complexity is low, but the pixel cannot achieve both high sensitivity and wide dynamic range
Solution Approach 1:
The pixel structure achieves multi-functionality by incorporating two floating diffusion regions that can handle different signal ranges. The first region handles low-light signals with high sensitivity, while the second region handles bright-light signals with high capacity, together providing both high sensitivity and wide dynamic range in a single pixel.
Solution Approach 2:
A transfer gate acts as an intermediary element that routes photo-generated charges to the appropriate floating diffusion region based on illumination conditions. This mediator enables the pixel to adaptively select the optimal readout path, achieving both high sensitivity and wide dynamic range without excessive complexity.
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
Enables high pixel full well capacity while maintaining high sensitivity, with the ability to read out total charge without information loss, and provides a higher dynamic range with minimal additional cost and complexity.
Implementation Method 1
In a typical 3T or pinned photodiode (PPD) 4T pixel structure, the electrons are converted into voltage using a capacitor
Data Source
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AI summary
Described herein is a pixel readout circuit (100) which provides readout at two sensitivity levels depending on the amount of electrons generated by a pixel photodiode (110) in the circuit. A floating diffusion capacitor (145) operates to store charge up to a saturation value determined by its capacitance and an overflow capacitor (190) is provided in an overflow region (170) for storing charge above the saturation value of the floating diffusion capacitor (145). Readout at a high sensitivity level is provided when the floating diffusion capacitor (145) is not saturated and readout at a lower sensitivity level is provided when there is saturation and subsequent overflow to the overflow region (170). Connection of the floating diffusion capacitor (145) to the overflow capacitor (190) shares the charge over the combined capacitance of the two capacitors and provides readout at a lower sensitivity without loss of charge.