Continuous Ramp Generator Calibration for CMOS Image Sensor ADCs
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Solution Overview
Problem
Conventional ADCs in CMOS image sensors face challenges with high resolution and high speed imaging due to staircase ramp signals, which are affected by temperature and supply variations, leading to gain errors and color distortions in images.
Innovation Solution
A continuous ramp generator design with calibration using a first current generator for coarse gain, a second current generator for integer gain, and an integrator for fine gain, controlled by specific control signals to generate a non-staircase, linear ramp signal, allowing for precise calibration of the ADC gain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a staircase ramp signal is used in single-slope ADCs, then the circuit implementation is simple, but the conversion speed is limited and gain errors occur due to temperature and supply variations
Solution Approach 1:
The patent transitions from a static staircase ramp signal to a dynamic continuous ramp signal generated by an integrator circuit. This dynamic approach allows the ramp to be continuously adjusted and calibrated, enabling higher conversion speeds while maintaining circuit simplicity through the use of standard integrator components.
Solution Approach 2:
The patent introduces calibration mechanisms that dynamically adjust the ramp signal parameters (slope and offset) based on temperature and supply voltage conditions. By changing these parameters in real-time, the system maintains accurate conversion speed and gain across varying operating conditions without complicating the basic integrator circuit.
2Speed
If the ramp signal slope is increased to improve conversion speed, then high speed operation is achieved, but gain errors increase due to temperature and supply variations
Solution Approach 1:
The patent implements calibration circuits that use feedback mechanisms to measure and adjust the ramp signal slope based on actual operating conditions. The calibration process monitors temperature and supply voltage, then adjusts the integrator components to maintain accurate gain regardless of the ramp slope required for high-speed operation.
Solution Approach 2:
The patent performs preliminary calibration of the ramp signal parameters before actual conversion operations. By pre-adjusting the integrator components based on expected operating conditions, the system ensures accurate gain measurement is maintained even when operating at high conversion speeds with steeper ramp slopes.
3Speed
If a non-staircase continuous ramp signal is used, then high speed operation is achieved, but the slope becomes independent of operating frequency and susceptible to variations
Solution Approach 1:
The patent makes the integrator circuit dynamically adjustable by introducing calibration control signals that modify the integrator components based on operating frequency. This allows the ramp signal slope to adapt to different frequencies while maintaining continuous operation, resolving the conflict between speed and frequency adaptability.
4Measurement precision
If gain calibration is performed to correct errors, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent divides the gain calibration into multiple independent stages (coarse calibration and fine calibration) that can be performed separately. This segmentation allows each calibration stage to use simpler circuits optimized for its specific function, reducing overall complexity while achieving high precision through the combination of stages.
Data Source
AI summary
Aspects of the invention provide a continuous ramp generator design and its calibration for CMOS image sensors using single-ramp ADCs. An embodiment of the invention comprises controlling a coarse gain, integer gain, and fine gain of the analog-to-digital converter. Gain of the analog-to-digital converter may be calibrated by tuning the integer gain based on reference voltages converted to equivalent digital values.