Dual ADC Readout Circuit for CMOS Image Sensor Gain Trade-off
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Solution Overview
Problem
CMOS image sensors face a trade-off between high signal-to-noise ratio (SNR) and low light performance due to the limitations of pixel capacitance, where reducing capacitance improves low light performance but decreases full well capacity and SNR, and existing solutions compromise between these two.
Innovation Solution
A readout circuit with dual analog-to-digital converters (ADCs) and a control circuit that provide different conversion gains for each pixel, allowing for separate conversion of reference and captured voltage levels based on two distinct capacitances, enabling high and low gain modes depending on light conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the capacitance at the sense node is reduced to increase conversion gain, then low light performance is improved, but full well capacity decreases and signal-to-noise ratio deteriorates
Solution Approach 1:
The patent implements dynamic selection between two conversion gain modes (first and second ADCs with different gains) based on the captured voltage level. The system switches between high gain mode for low light conditions and low gain mode for brighter conditions, allowing the conversion gain to be dynamically adjusted rather than fixed. This resolves the contradiction by enabling high conversion gain only when needed for low light performance, while maintaining full well capacity and better SNR in brighter conditions.
2Adaptability or versatility
If a single conversion gain is used for all pixels, then device complexity is reduced, but the ability to capture images with both high SNR and good low light performance is compromised
Solution Approach 1:
The patent divides the pixel array into multiple segments, where different groups of pixels (e.g., odd and even rows or columns) are assigned to different ADCs with different conversion gains. This segmentation allows simultaneous reading of pixels with different gain requirements, providing adaptability across different light conditions while keeping the complexity manageable through structured organization of the readout circuit.
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 allows for improved SNR across a broad range of light levels by selectively using high or low gain modes based on pixel values, enhancing image quality in both bright and low-light conditions without compromising full well capacity.
Implementation Method 1
CMOS image sensors are based on arrays of pixels each comprising a photodiode that collects a charge during an integration period
Data Source
AI summary
The present disclosure relates to a readout circuit of a pixel array comprising: a first analog to digital converter coupled to a first column line of the pixel array and configured to convert a reference voltage level and a captured voltage level of a first pixel based on a first conversion gain of the first pixel; and a second analog to digital converter coupled to the first column line of the pixel array and configured to convert a reference voltage level and a captured voltage level of the first pixel based on a second conversion gain of the first pixel, the first and second conversion gains being different from each other.