CMOS Pixel Circuit Threshold Voltage Compensation
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Solution Overview
Problem
CMOS image sensors experience image distortion due to voltage drift in threshold voltage, leading to different output signals for the same incident light from the same or different CMOS devices over time.
Innovation Solution
A pixel circuit with a compensation circuit that acquires and stores the threshold voltage, allowing the output signal generation circuit to generate consistent output signals by compensating for voltage drift, comprising initialization, photoelectric conversion, storage, and output control circuits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a CMOS device is used to generate output signals based on incident light, then the image sensor can efficiently convert light to electrical signals with lower power consumption, but the threshold voltage drift causes different output signals for the same incident light over time, resulting in image distortion
Solution Approach 1:
The patent applies preliminary action by measuring and storing the threshold voltage of the transistor before it drifts away from the reference value. The compensation circuit proactively captures the threshold voltage at different times and stores it in a storage circuit, enabling future compensation without waiting for distortion to occur. This prevents the reliability issue by preparing compensation data in advance.
Solution Approach 2:
The patent implements feedback by comparing the stored threshold voltage with a reference threshold voltage and using the difference to compensate for output signal variations. The compensation circuit continuously monitors threshold voltage changes and adjusts the output signal accordingly, creating a closed-loop system that maintains signal consistency despite transistor aging or environmental changes.
2Device complexity
If the threshold voltage drift is not compensated, then the pixel circuit structure remains simple, but image distortion occurs due to varying output signals from the same or different CMOS devices over time
Solution Approach 1:
The patent applies segmentation by dividing the pixel circuit into distinct functional modules: a photoelectric conversion unit, a compensation circuit, a storage circuit, and an output control circuit. This modular approach allows each component to perform its specific function independently, making the overall system manageable despite increased complexity. The compensation circuit is further segmented into threshold voltage measurement and compensation signal generation sub-functions.
Solution Approach 2:
The patent introduces an intermediary storage circuit that holds the measured threshold voltage for later use. This storage circuit acts as a mediator between the threshold voltage measurement and the final compensation process, allowing the system to decouple the measurement timing from the compensation timing. The intermediary enables flexible compensation without requiring real-time processing.
3Reliability
If compensation for threshold voltage drift is implemented, then output signal consistency is improved across different pixel circuits and over time, but the pixel circuit structure and operation become more complex
Solution Approach 1:
The patent merges the compensation functionality into the existing pixel circuit architecture by integrating the compensation circuit with the photoelectric conversion and output control circuits. Rather than adding a completely separate compensation system, the patent combines multiple functions (photoelectric conversion, threshold voltage measurement, storage, and compensation) into a unified circuit structure, reducing overall system complexity.
Solution Approach 2:
The patent applies universality by designing the compensation circuit to handle multiple functions: measuring threshold voltage, storing compensation data, and generating compensation signals for different pixel circuits. The storage circuit can serve multiple pixel circuits, and the compensation mechanism works for both same-pixel temporal drift and different-pixel manufacturing variations, making the system highly versatile.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The compensation circuit ensures that different pixel circuits or the same pixel circuit at different times produce the same output signal for the same incident light, mitigating or eliminating image distortion.
Implementation Method 1
a photoelectric conversion circuit configured to convert incident light into an electrical signal
Data Source
AI summary
A pixel circuit includes: an initialization circuit that initializes a storage circuit; a photoelectric conversion circuit that converts incident light into an electrical signal; a photoelectric conversion control circuit that controls the photoelectric conversion circuit to convert the incident light into the electrical signal; an output signal generation circuit that generates an output signal of the pixel circuit corresponding to the incident light in dependence on the electrical signal converted by the photoelectric conversion circuit; a compensation circuit that acquires a threshold voltage of the output signal generation circuit; a storage circuit that stores the electrical signal converted by the photoelectric conversion circuit and the threshold voltage of the output signal generation circuit; and an output control circuit that controls the output of the output signal of the pixel circuit corresponding to the incident light.