Photodiode Limiter Circuit for CMOS Image Sensor Streaking
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Solution Overview
Problem
Current CMOS image sensors experience issues with streaking due to overload conditions during integration, leading to image artifacts and errors, particularly in scenes with both highlight and darker areas, as the voltage on the photodiode can drop below the threshold, causing the transfer gate to turn on erroneously and result in charge loss.
Innovation Solution
The image sensor incorporates a photodiode limiter mechanism, where a shutter control circuit supplies a pulsed voltage to maintain the photodiode voltage above the threshold during integration, preventing the transfer gate from activating erroneously and using a second charge holding capacity to limit voltage swing and prevent overload in the output circuit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of moving object
If the photodiode integrates charges for a longer period to improve image brightness, then the integration time increases, but the voltage on the photodiode drops below the threshold causing erroneous transfer gate activation and charge loss
Solution Approach 1:
The shutter control circuit applies a preliminary counteracting voltage to the photodiode during integration to prevent the voltage from dropping below the transfer gate threshold. This preliminary anti-action counterbalances the voltage decrease caused by charge accumulation, allowing longer integration times without erroneous transfer gate activation.
Solution Approach 2:
The system dynamically changes the voltage parameter of the photodiode by applying an additional voltage through the shutter control circuit. This parameter change maintains the photodiode voltage above the threshold level throughout the integration period, preventing premature transfer gate activation while allowing extended integration times.
2Reliability
If the shutter control circuit applies a high voltage to reset the photodiode before integration, then the photodiode voltage is properly initialized, but the voltage swing during integration causes overload conditions in the output circuit
Solution Approach 1:
The shutter control circuit applies a compensating voltage beforehand during integration to cushion against the voltage swing that would otherwise cause output circuit overload. This prior cushioning prevents the voltage from dropping too low while maintaining proper photodiode reset conditions, eliminating streaking artifacts.
3Productivity
If the transfer gate is allowed to activate freely during integration to capture all generated charges, then charge transfer efficiency improves, but the transfer gate activates erroneously when photodiode voltage drops below threshold, causing charge loss
Solution Approach 1:
The shutter control circuit continuously monitors the photodiode voltage during integration and provides feedback by adjusting the applied voltage to maintain it above the transfer gate threshold. This feedback mechanism ensures the transfer gate only activates at the intended time, preventing charge loss while maintaining transfer efficiency.
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
This solution effectively addresses the streaking issue by maintaining the photodiode voltage above the threshold, preventing transfer gate activation during integration and reducing image artifacts, thereby enhancing image quality by limiting voltage swing and preventing overload conditions.
Implementation Method 1
a photodiode that generates electrons in response to light detected during an integration time
Data Source
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AI summary
An image sensor is provided. In one aspect, the image sensor includes a pixel coupled to an output line. The pixel includes a photodiode configured to generate electrical charges in response to light and a supply circuit configured to supply a voltage to the photodiode to keep a voltage of the photodiode at or above a threshold level in an integration time. In another aspect, the pixel includes a supply circuit configured to selectively supply voltage to the photodiode in a first charge holding capacity and a second charge holding capacity.