Dual-Ramp Pixel Readout for Low-Noise High-Dynamic-Range Sensing
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Solution Overview
Problem
Modern CMOS image sensors face challenges in achieving high dynamic range with low noise readout, particularly in reading out pixels with varying electron counts, which affects image quality.
Innovation Solution
The implementation of a dual-ramp generator in CMOS image sensors, which uses two consecutive ramps during both the reset conversion phase and the signal measurement phase, with one ramp being slow and having a limited swing and the other being fast and covering the full range of pixel outputs, to relax ADC read noise without significant impact on total noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single ramp is used for ADC conversion, then the circuit complexity is low, but the dynamic range is limited and read noise cannot be sufficiently reduced
Solution Approach 1:
The patent divides the ADC conversion process into two separate ramp operations: a first ramp for reset conversion and a second ramp for signal measurement. This segmentation allows each ramp to be optimized independently - the first ramp can be slower with lower noise for reset, while the second ramp handles the full dynamic range for signal measurement, thereby reducing overall read noise without requiring a completely complex new circuit architecture
Solution Approach 2:
The patent implements dynamic ramp rate selection by using different ramp rates for different conversion phases. The dual-ramp generator can operate at different speeds depending on whether it's performing reset conversion or signal measurement, allowing the system to adapt its timing characteristics to the specific requirements of each phase while maintaining manageable circuit complexity
2Productivity
If a fast ramp is used for ADC conversion, then the conversion time is reduced, but the read noise increases
Solution Approach 1:
The conversion process is segmented into two phases with different ramp rates. The first ramp (reset conversion) uses a slower rate optimized for low noise, while the second ramp (signal measurement) uses a faster rate optimized for speed. This segmentation allows the system to achieve both low noise and fast conversion by applying the appropriate ramp rate to the appropriate phase
Solution Approach 2:
Different qualities (ramp rates) are applied to different parts of the conversion process. The first ramp has a slower rate with lower noise characteristics suitable for reset conversion, while the second ramp has a faster rate suitable for signal measurement. This local optimization of ramp characteristics resolves the contradiction between speed and noise
3Measurement precision
If a slow ramp with limited swing is used, then the read noise is reduced, but the dynamic range is limited
Solution Approach 1:
The patent segments the measurement task into two parts: reset conversion using a slow ramp with limited swing for low noise, and signal measurement using a fast ramp with full swing for extended dynamic range. The final signal is obtained by differencing these two measurements, thereby achieving both low noise and full dynamic range
Solution Approach 2:
The system dynamically switches between different ramp configurations based on the conversion phase. During reset conversion, a slow ramp with limited swing is used to minimize noise. During signal measurement, a fast ramp with full swing capability is used to capture the complete dynamic range. This dynamic adaptation resolves the contradiction between noise reduction and dynamic range coverage
Data Source
AI summary
An image sensor includes a plurality of pixel columns and a plurality of readout circuits. Each readout circuit is coupled to one of the plurality of pixel columns and includes an ADC for receiving a first analog signal of a pixel in a reset conversion phase and a second analog signal of the pixel in a signal measurement phase, a dual-ramp generator for generating a first ramp having a first ramp rate and a second ramp having a second ramp rate greater than the first ramp rate and providing the first ramp to the readout circuits in the reset conversion phase and the second ramp to the plurality of readout circuits in the signal measurement phase, and a controller configured to provide control signals to the readout circuits and the dual-ramp generator.