CMOS Image Sensor Floating Diffusion Capacitance Control
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Solution Overview
Problem
CMOS image sensors face challenges in maintaining image quality due to noise issues when dynamically changing the number of connected switches to adjust conversion gain for wide dynamic range, particularly in video exposure periods, leading to a low signal-to-noise ratio.
Innovation Solution
The implementation of a solid-state imaging device with a capacitance changing part that adjusts the capacitance of the floating diffusion by using binning switches, allowing for multiple conversion gain modes within a single reading period, optimizing the signal-to-noise ratio and reducing noise through optimal capacitance adjustment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the number of connected switches is increased to increase the capacitance of the floating diffusion for wide dynamic range, then the dynamic range is expanded, but the conversion gain becomes too low for small signals
Solution Approach 1:
The patent implements dynamic switching of switch connections during the reading period to adjust the capacitance of the floating diffusion. The control unit selectively connects or disconnects switches based on signal intensity requirements, enabling the system to adapt between high conversion gain mode (fewer switches connected) for small signals and low conversion gain mode (more switches connected) for large signals within the same exposure period.
Solution Approach 2:
The patent changes the capacitance parameter of the floating diffusion by controlling the connection state of switches. The control unit adjusts the number of connected switches to modify the total capacitance value, thereby changing the conversion gain parameter to match the signal intensity requirements for different imaging scenarios.
2Adaptability or versatility
If switches are switched during the reading period to adjust conversion gain dynamically, then the dynamic range performance is enhanced, but large noises occur during switching that degrade the signal-to-noise ratio
Solution Approach 1:
The patent performs preliminary separation of signal reading operations into distinct high gain reading periods and low gain reading periods. The control unit pre-configures the switch connection states before each reading operation, ensuring that all signals are read with the same capacitance setting to avoid switching noises during the reading process itself.
Solution Approach 2:
The patent segments the reading period into multiple distinct reading operations, each dedicated to a specific gain mode. By separating high gain readings from low gain readings in time and using different switch configurations for each segment, the system avoids the need to switch capacitance settings during active signal reading, thereby eliminating switching-induced noises.
3Measurement precision
If the capacitance of the floating diffusion is adjusted to optimize signal-to-noise ratio, then image quality is improved, but the device complexity increases
Solution Approach 1:
The patent designs the switch network and control unit to serve multiple functions: they control both the capacitance adjustment for conversion gain optimization and the signal routing for different reading operations. This multi-functional design reduces the need for separate control mechanisms, thereby limiting the increase in device complexity while achieving improved signal-to-noise ratio through capacitance adjustment.
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 the signal-to-noise ratio and expanding the dynamic range without degrading optical characteristics, enabling the simultaneous output of bright and dark signals in a single reading period.
Implementation Method 1
a photodiode (a photoelectric conversion element) and a floating diffusion (FD) amplifier
Data Source
Figure 1
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Figure 3A
AI summary
In a solid-state imaging device 10, the first binning switch 81 is formed such that a MOS capacitance and a wire capacitance of a wire connected to the binning switch 81, each having a value in accordance with an ON or OFF state, are added to a capacitance of a floating diffusion FD of a pixel PXL to be read, so as to optimize the capacitance of the floating diffusion FD and optimally adjust a conversion gain in accordance with a mode. This operation increases an image quality.