CMOS Pixel Sensor Cell with Dynamic Gain Reducing Capacitance
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Solution Overview
Problem
Conventional CMOS image sensors face limitations in sensitivity and dynamic range, with existing noise reduction techniques either not being linear, requiring multiple integration times, or increasing noise due to additional transistors.
Innovation Solution
A pixel sensor cell design incorporating a photodiode, first and second transfer transistors, and a gain reducing capacitance, allowing for a multi-step read-out scheme that isolates the sense node from the first transfer transistor and additional components, reducing noise and enhancing saturation levels by varying capacitance during read-out.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If additional transistors are added to vary sense node capacitance for high dynamic range, then dynamic range is improved, but readout noise increases due to increased sense node capacitance
Solution Approach 1:
The patent divides the readout process into multiple phases using a multi-phase readout scheme. The sense node capacitance is segmented into different levels (first capacitance level with additional capacitance connected, second capacitance level with additional capacitance disconnected) that are switched during different readout phases. This allows the system to achieve high dynamic range by connecting the additional capacitance for high-light conditions while maintaining low readout noise by disconnecting it for low-light conditions.
Solution Approach 2:
The patent implements dynamic variation of sense node capacitance during the readout process. The additional capacitance is dynamically connected or disconnected from the sense node based on the light intensity conditions being measured. This dynamic adjustment allows the sensor to adapt its conversion gain - using lower conversion gain (higher capacitance) for high dynamic range measurements and higher conversion gain (lower capacitance) for low-light measurements, thereby resolving the contradiction between dynamic range and readout noise.
2Adaptability or versatility
If multiple integration times are used to achieve high dynamic range, then dynamic range is improved, but device complexity and operation complexity increase
Solution Approach 1:
The patent maintains continuous single-integration-time operation while achieving high dynamic range through continuous adjustment of the sense node capacitance during the readout phase. Instead of requiring multiple discrete integration times, the system continuously varies the capacitance level to match the light intensity, allowing a single integration period to capture the full dynamic range. This eliminates the complexity of coordinating multiple integration cycles while maintaining the beneficial high dynamic range capability.
3Quantity of substance
If sense node capacitance is increased to reduce conversion gain for high dynamic signals, then saturation level is improved, but readout noise increases
Solution Approach 1:
The patent employs periodic switching of the additional capacitance connection during the readout process. The capacitance is periodically adjusted between two levels: a first level (higher capacitance) for handling high dynamic signals approaching saturation, and a second level (lower capacitance) for maintaining low readout noise during low-light conditions. This periodic adjustment of capacitance during the readout phases allows the system to optimize both saturation level and readout noise performance across varying light conditions without requiring permanent high capacitance.
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 solution achieves a wide dynamic range with low noise read-out for both high-dynamic and low-light conditions, maintaining sensitivity by reducing sense node capacitance and allowing for different conversion gains during read-out.
Implementation Method 1
a photodiode for generating photoelectrons
Data Source
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AI summary
The present invention relates to a pixel sensor cell (1) for a CMOS sensor device comprising: - a photodiode (11) for generating photoelectrons; - a first transfer transistor (12) coupling the photodiode (11) with an intermediate node (IN) and configured to be controlled by a first control signal (TX1); - a gain reducing capacitance (CHD) applied on the intermediate node (IN); - a second transfer transistor (14) coupling the intermediate node (IN) with a sense node (SN) and configured to be controlled by a second control signal (TX2); - an output buffer (15) coupled with the sense node (SN) and configured to amplify a potential on the sense node (SN).