Split-Readout Image Sensor Pixel Architecture for High Dynamic Range
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Solution Overview
Problem
Modern CMOS image sensors face challenges in achieving high dynamic range due to constraints in pixel full well capacity in bright light conditions and input-referred read noise in low-light conditions.
Innovation Solution
The implementation of a split-readout pixel architecture with multiple readout channels, including a primary channel with high conversion gain for low-light sensitivity and one or more secondary channels for residual photocharge transfer, allowing for complete photocharge transfer up to the full well capacity of the photodetection element.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conversion gain is increased to reduce input-referred read noise in low-light conditions, then low-light sensitivity is improved, but full well capacity is reduced compromising bright light sensitivity
Solution Approach 1:
The pixel is divided into two separate readout channels: a first readout channel with high conversion gain for low-light conditions, and a second readout channel with lower conversion gain for bright light conditions. This segmentation allows each channel to be optimized for its specific operating range, resolving the contradiction between low-light sensitivity and bright light capacity.
Solution Approach 2:
The system dynamically selects which readout channel to use based on the lighting conditions. The pixel can switch between high conversion gain mode for low-light scenes and lower conversion gain mode for bright light scenes, allowing the conversion gain to be adapted to the specific operating conditions rather than being fixed.
2Measurement precision
If floating diffusion capacitance is reduced to increase conversion gain, then low-light sensitivity is improved, but bright light performance deteriorates
Solution Approach 1:
The pixel architecture segments the readout function into two channels with different floating diffusion capacitance values. The first channel uses reduced capacitance for high conversion gain in low-light, while the second channel uses increased capacitance for lower conversion gain in bright light, resolving the contradiction between read noise and bright light performance.
Solution Approach 2:
Different parts of the pixel (the two readout channels) have different local qualities - specifically different floating diffusion capacitance values - allowing each region to be optimized for its specific function: one for low-noise low-light readout, another for high-capacity bright light readout.
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 enables high dynamic range imaging by maintaining maximum bright light performance while enhancing low-light sensitivity, effectively addressing the limitations of conventional pixel architectures.
Implementation Method 1
a photodetection element and two readout channels coupled to the photodetection element
Data Source
AI summary
First and second readout circuits, each having a respective floating diffusion node, are coupled to a photodetection element within a pixel of an integrated-circuit image sensor. Following an exposure interval in which photocharge is accumulated within the photodetection element, a first portion of the accumulated photocharge is transferred from the photodetection element to the first floating diffusion node to enable generation of a first output signal within the first readout circuit, and a second portion of the accumulated photocharge is transferred from the photodetection element to the second floating diffusion node to enable generation of a second output signal within the second readout circuit. A digital pixel value is generated based on the first and second output signals.