CMOS Image Sensor ADC with Correlated Double Sampling Noise Reduction
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in effectively reducing power noise and ramp noise, which degrade image quality due to the need for separate reference voltage generators and numerous dummy pixels, leading to reduced accuracy and incomplete noise removal.
Innovation Solution
The proposed solution involves an analog-to-digital converter with a correlated double sampling (CDS) array that uses a one-to-one matching operation between active and dummy pixels in the row direction to generate and compare sampling signals, efficiently removing power noise and ramp noise using shared circuitry and matching noise characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a separate reference voltage generator is used to reduce power noise, then power noise reduction is improved, but device complexity increases and additional noise sources are introduced
Solution Approach 1:
The patent extracts the reference voltage generation function from a separate reference voltage generator and integrates it into the dummy pixel array itself. Each dummy pixel generates its own reference voltage locally, eliminating the need for a centralized reference voltage generator and its associated complexity while maintaining effective power noise reduction.
Solution Approach 2:
The dummy pixel array serves multiple functions: it acts as both the source of reference voltages for noise reduction and as functional dummy pixels for horizontal noise reduction. This multi-functionality eliminates the need for separate reference voltage generation circuitry while achieving both power noise and horizontal noise reduction goals.
2Object-affected harmful factors
If numerous dummy pixels are used for horizontal noise reduction, then horizontal noise reduction is improved, but device complexity and area increase
Solution Approach 1:
The dummy pixel array is designed to simultaneously serve as the reference voltage source for power noise reduction and as the horizontal noise reduction mechanism. By making the dummy pixels multi-functional, the patent achieves both noise reduction goals without requiring separate structures, thereby reducing overall device complexity.
3Productivity
If a conventional column parallel ADC scheme is used, then conversion capability is provided, but power noise and ramp noise degrade image quality
Solution Approach 1:
The patent performs preliminary noise reduction by having each dummy pixel generate its reference voltage in advance, before the actual conversion process. This preliminary action of local reference voltage generation ensures that power noise is already compensated for when the conversion occurs, preventing noise degradation of image quality.
Solution Approach 2:
The patent segments the reference voltage generation process into individual dummy pixel units, with each dummy pixel independently generating its own reference voltage. This segmentation allows for localized noise compensation without the need for a centralized reference voltage generator, thereby maintaining conversion capability while reducing noise impact on image quality.
Data Source
AI summary
A CMOS image sensor includes an active pixel array suitable for generating an active pixel signal, a dummy pixel array suitable for generating a dummy pixel signal, a row driver suitable for controlling the active pixel array and the dummy pixel array to simultaneously operate at a same column, and a correlated double sampling (CDS) array suitable for generating an active sampling signal and a dummy sampling signal, which are sampled from the active pixel signal and the dummy pixel signal by using a correlated double sampling, respectively, based on a first ramp signal and a second ramp signal, and comparing the active sampling signal with the dummy sampling signal.