Image Sensor ADC Comparator Layout for Low-Light Noise Reduction
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Solution Overview
Problem
Existing analog-digital converting circuits in CMOS image sensors face challenges in reducing noise levels, particularly in low-illumination environments, which affect image quality.
Innovation Solution
The implementation of a comparator with a reduced noise level in the analog-digital converting circuit, utilizing a first comparator with a differential amplifier structure and a second comparator for amplification, along with a column parallel correlated double sampling (CDS) circuit to compensate for pixel-specific deviations and reduce noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a conventional comparator is used in the analog-digital converting circuit, then the circuit structure is simple, but the noise level is high which degrades image quality in low-illumination environments
Solution Approach 1:
The comparator is divided into two separate comparators: a first comparator that performs initial comparison and a second comparator that performs final comparison with higher precision. This segmentation allows each comparator to be optimized for its specific function, with the second comparator having enhanced noise reduction capabilities through correlated double sampling, thereby reducing overall noise level while maintaining manageable circuit complexity through functional specialization.
Solution Approach 2:
A column parallel correlated double sampling (CDS) circuit is introduced as an intermediary component between the pixel array and the comparators. This CDS circuit compensates for pixel-specific deviations and reduces noise before the signals reach the comparators. The intermediary CDS circuit processes multiple pixel signals in parallel, effectively reducing noise levels without significantly increasing the complexity of the comparator structures themselves.
2Measurement precision
If noise reduction techniques are applied to improve image quality in low-illumination conditions, then the measurement precision is improved, but the device complexity increases
Solution Approach 1:
The analog-digital converting circuit employs dynamic element matching (DEM) techniques where capacitor values and transistor parameters are dynamically adjusted based on operating conditions. The circuit switches between different capacitor configurations and transistor biasing states to optimize performance for different illumination levels. This dynamic adaptation improves measurement precision across varying light conditions while avoiding the need for completely separate circuit designs for each condition, thereby controlling overall device complexity.
Solution Approach 2:
The circuit utilizes parameter changes in capacitor values and transistor dimensions to achieve different gain levels and noise reduction characteristics. By varying the capacitance values in the integrators and comparators, and adjusting transistor width-to-length ratios, the circuit can optimize its transfer function and noise performance for different operating conditions. These parameter changes enable high measurement precision without requiring fundamentally different circuit topologies, thus managing device complexity.
Data Source
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AI summary
Provided are analog-digital converting circuits including a comparator, and an image sensor. The analog-digital converting circuit include a counter and a comparator, the comparator including a first P-type transistor including a gate connected to a first input node of the comparator, a second P-type transistor including a gate connected to a second input node of the comparator, a first N-type transistor including a gate connected to the first input node and a drain connected to the first P-type transistor, a second N-type transistor including a gate connected to the second input node and a drain connected to the second P-type transistor, and a transistor including a gate connected to the drain of the first N-type transistor and a source to which ground voltage or power supply voltage is applied.