Image Sensor Pixel Circuit Dynamic Conversion Gain Adjustment
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Solution Overview
Problem
Image sensors face challenges in dynamically adjusting their conversion gain to optimize image quality across varying illumination conditions, leading to noise properties and dynamic range issues.
Innovation Solution
The image sensor dynamically adjusts the capacitance of the floating diffusion in its pixel circuit by turning on or off a switch device based on exposure time, allowing for different conversion gains to be applied during different exposure periods, thereby improving noise properties and dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the conversion gain is increased to improve noise properties in low-light conditions, then the dynamic range is reduced causing saturation in bright areas
Solution Approach 1:
The patent implements dynamic conversion gain adjustment by controlling the capacitance of the floating diffusion node through a control signal. The conversion gain is changed during the readout period based on the exposure time, allowing the system to adapt to different lighting conditions. This resolves the contradiction by making the conversion gain a variable parameter rather than a fixed value, enabling optimal noise performance in low-light while maintaining dynamic range in bright conditions.
Solution Approach 2:
The patent changes the electrical parameter (capacitance) of the floating diffusion node to adjust the conversion gain. By controlling the capacitance value through a switchable configuration, the system can switch between different conversion gain modes (e.g., high conversion gain for long exposure/low light, low conversion gain for short exposure/bright light). This parameter change approach allows the system to optimize noise properties without permanently sacrificing dynamic range.
2Device complexity
If a single conversion gain is used for all exposure times, then the circuit design is simplified, but image quality deteriorates under varying illumination conditions
Solution Approach 1:
The patent introduces dynamic control of the conversion gain through a control signal that adjusts the capacitance of the floating diffusion node. This dynamic adjustment capability allows the system to optimize image quality for different exposure times and illumination conditions while adding minimal circuit complexity. The control mechanism uses existing circuit elements (transistors, capacitors) configured in a switchable manner rather than requiring completely separate circuit paths.
3Illumination intensity
If the capacitance of the floating diffusion is increased to reduce conversion gain for bright areas, then the noise properties in dark areas worsen
Solution Approach 1:
The patent uses dynamic conversion gain adjustment where the capacitance of the floating diffusion node is controlled based on the exposure time and illumination conditions. For short exposure times (bright areas), the capacitance is increased to reduce conversion gain and prevent saturation. For long exposure times (dark areas), the capacitance is decreased to increase conversion gain and improve signal-to-noise ratio. This time-dependent dynamic control resolves the contradiction by applying different capacitance values appropriate for different lighting scenarios.
Solution Approach 2:
The patent implements periodic adjustment of the conversion gain during the readout period, switching between different gain modes based on the exposure time category. The control signal periodically changes the capacitance configuration to match the illumination conditions, ensuring optimal performance for both bright and dark areas at different times during the imaging sequence.
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 approach enhances image quality by optimizing conversion gain according to exposure time, effectively reducing noise and improving the dynamic range of the image sensor.
Implementation Method 1
a photodiode for generating an electric charge in response to light
Data Source
AI summary
An image sensor including: a pixel array including a plurality of pixels connected to a plurality of row lines and a plurality of column lines, each of the plurality of pixels including a photodiode for generating an electric charge in response to light, and a pixel circuit having a floating diffusion for storing the electric charge; and a controller configured to adjust a capacitance of the floating diffusion to a first value and obtain a first pixel signal from the pixel circuit during a first time period, adjust the capacitance of the floating diffusion to a second value greater than the first value and obtain a second pixel signal from the pixel circuit during a second time period subsequent to the first time period, and generate a result image using the first pixel signal and the second pixel signal.