Dual-Photodiode Pixel Circuit for Faster Image Sensor Reset
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current image sensors face challenges in reducing reset time and time constant, which affects their operational efficiency and image quality.
Innovation Solution
The proposed solution involves an image sensor design that electrically connects voltage nodes, incorporating a unit pixel circuit with a photodiode, floating diffusion nodes, capacitors, and transistors to accumulate and convert charges, and a logic circuit to control these components, allowing for improved reset time and reduced time constant.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a capacitor is added to the floating diffusion node to increase dynamic range, then the dynamic range is improved, but the reset time and time constant increase
Solution Approach 1:
The pixel circuit is divided into two separate photodiodes (first and second photodiodes) with separate floating diffusion nodes. The capacitor is connected to only one floating diffusion node (first floating diffusion node), segmenting the capacitance addition to affect only one pixel circuit path, thereby limiting the impact on overall reset time while maintaining dynamic range enhancement for specific pixels.
Solution Approach 2:
The first and second photodiodes are combined within the same pixel, with their respective floating diffusion nodes connected to a common third floating diffusion node through switch transistors. This merging allows charge from either photodiode to be accumulated, effectively doubling the charge accumulation capacity and dynamic range without requiring a large capacitor that would increase reset time.
2Reliability
If a capacitor is added to the floating diffusion node to mitigate LED flicker, then the LED flicker is reduced, but the reset time and time constant increase
Solution Approach 1:
The capacitor is segmented and connected to only one of the two floating diffusion nodes (first floating diffusion node), rather than both. This segmentation allows LED flicker mitigation to be achieved in one pixel circuit path without the full reset time penalty affecting the entire pixel array, as the second photodiode path remains faster.
Solution Approach 2:
The third floating diffusion node acts as an intermediary that receives charges from both first and second floating diffusion nodes through switch transistors. This intermediary structure allows the capacitor's flicker-mitigating effect to be applied selectively while the common readout path maintains consistent timing characteristics across all pixels.
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 design enhances the operational performance of image sensors by reducing reset time and time constant, leading to improved image quality and efficiency.
Implementation Method 1
a first photodiode, a second photodiode different from the first photodiode
Data Source
AI summary
A unit pixel circuit includes a first photodiode, a second photodiode different from the first photodiode, a first floating diffusion node in which charges generated in the first photodiode are accumulated, a second floating diffusion node in which charges generated in the second photodiode are accumulated, a capacitor connected to the first floating diffusion node and a first voltage node, and accumulating overflowed charges of the first photodiode, a first switch transistor connecting the first floating diffusion node to a third floating diffusion node, a reset transistor connecting the third floating diffusion node to a second voltage node, a gain control transistor connecting the second floating diffusion node to the third floating diffusion node, and a second switch transistor connected to the first voltage node and the second voltage node.