CMOS Image Sensor ADC Multiple Sampling for Low-Noise Readout
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Solution Overview
Problem
CMOS image sensors face challenges in achieving high-speed operation with reduced noise and settling time, particularly in analog-to-digital conversion processes.
Innovation Solution
A method and system for operating CMOS image sensors using a single slope analog-to-digital conversion with multiple sampling, where an analog pixel signal is generated based on incident light, with the reset component sampled during a first time interval and the image component sampled during a subsequent time interval, utilizing a ramp signal with a constant slope and offset adjustments to reduce noise and settling time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If high-speed analog-to-digital conversion is implemented in CMOS image sensors, then conversion speed is improved, but noise and settling time increase
Solution Approach 1:
The patent applies periodic action by using a ramp signal that periodically increases over time to sample the analog pixel signal multiple times. This periodic sampling approach allows the system to perform multiple conversions at different time points, enabling noise reduction through averaging while maintaining high conversion speed. The ramp signal's continuous increase creates natural periodic sampling opportunities without requiring additional complex circuitry.
Solution Approach 2:
The patent implements continuity of useful action by using a continuously increasing ramp signal for analog-to-digital conversion. Instead of discrete, intermittent conversion cycles, the ramp signal provides continuous conversion opportunities throughout the integration period, maximizing the utilization of the analog pixel signal and reducing idle time. This continuous action improves both conversion speed and noise performance by ensuring constant engagement of the conversion mechanism.
2Measurement precision
If multiple sampling is performed during analog-to-digital conversion, then noise is reduced, but conversion time increases
Solution Approach 1:
The patent applies preliminary action by performing multiple samplings of the analog pixel signal during the integration period before the final conversion is needed. The ramp signal enables these preliminary samplings to occur naturally as it increases over time, capturing multiple data points that can be averaged to reduce noise. This preliminary data collection happens concurrently with the integration process, not after, thereby reducing the total conversion time while achieving noise reduction.
3Loss of time
If ramp signal offset is adjusted dynamically, then settling time is reduced, but circuit complexity increases
Solution Approach 1:
The patent implements self-service by designing the ramp signal generator to automatically adjust its own offset without requiring external intervention or complex control circuits. The ramp signal's offset is dynamically adjusted based on the integration period and sampling requirements, allowing the system to adapt to different operating conditions autonomously. This self-adjusting mechanism reduces settling time while avoiding the need for additional complex offset control circuitry.
Data Source
AI summary
A method of operating an image sensor includes generating an analog pixel signal, including a reset component and an image component, based on incident light received by a pixel in the image sensor. Operations are performed to repeatedly sample the reset component of the analog pixel signal using a ramp signal, during a first time interval, and then repeatedly sample the image component of the analog pixel signal using the ramp signal, during a second time interval subsequent to the first time interval. A digital signal corresponding to an effective image component of the incident light is then generated. This digital signal is based on the repeatedly sampled reset component of the analog pixel signal and the repeatedly sampled image component of the analog pixel signal. In addition, during both the first and second time intervals, the ramp signal decreases in magnitude and increases in magnitude.