Image Sensor Ramp Signal Calibration for Stable SNR
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Solution Overview
Problem
CMOS image sensors face deterioration in signal-to-noise ratio (SNR) due to fluctuations in the ramp signal caused by manufacturing variations and clock frequency changes, affecting image quality and display consistency under constant illumination.
Innovation Solution
An adaptive ramp signal calibration method using a second ADC to generate a feedback reference code, which is compared with a digital target code to adjust the ramp signal, ensuring it remains constant across color channels, thereby stabilizing the image signal output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional analog BISC scheme is used to calibrate ramp signal, then manufacturing process variations and offset calibration are improved, but signal-to-noise ratio deteriorates due to ramp signal fluctuations
Solution Approach 1:
The patent implements a feedback mechanism where the calibrated ramp signal is fed back to a second ADC to generate a feedback reference code, which is then compared with a digital target code. The comparison result is used to adjust the ramp signal calibration, creating a closed-loop system that continuously maintains accurate calibration while suppressing noise and fluctuations.
2Measurement precision
If ramp signal is adaptively calibrated using analog comparator, then target voltage coincidence is improved, but calibration stability worsens due to clock frequency changes and noise
Solution Approach 1:
The patent replaces the conventional analog comparator-based calibration system with a digital calibration system using two ADCs. The first ADC converts the image signal, while the second ADC converts the ramp signal to generate a feedback reference code. This digital substitution eliminates the instability caused by analog components and clock frequency variations, providing more stable and accurate calibration.
3Device complexity
If single-slope ADC is used for image signal conversion, then device complexity is reduced, but image quality worsens due to ramp signal variations
Solution Approach 1:
The patent maintains the simple single-slope ADC structure for image signal conversion while adding a feedback mechanism using a second ADC. The second ADC generates a feedback reference code from the ramp signal, which is compared with the digital target code to calibrate the ramp signal. This feedback approach preserves the simplicity of the single-slope ADC while eliminating image quality deterioration caused by ramp signal variations.
Data Source
AI summary
An image sensor comprises an active pixel sensor (APS) array, a first analog-to-digital converter (ADC) and a ramp signal generator. The APS array includes a plurality of pixels arranged in a two-dimensional matrix, wherein the APS array generates a reset signal and an image signal for each selected column of the APS array. The first ADC includes a correlated double sampling (CDS) circuit array comprising CDS circuits that are arranged for each column of the APS array, wherein the first ADC generates a digital code from a signal corresponding to the difference between the reset signal and the image signal which are generated by the CDS circuit using a ramp signal. The ramp signal generator generates the ramp signal, wherein a second ADC receives a feedback of the generated ramp signal and generates a feedback reference code, and wherein the ramp signal generator calibrates the ramp signal based on a comparison using the feedback reference code.