Time-Based CMOS Image Sensor Reset Potential Control
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Solution Overview
Problem
Conventional time-based CMOS image sensors are not operable at low illumination due to insufficient difference in reset electron potential between the photodiode and the floating diffusion node, limiting their ability to detect fine light signals and making them sensitive to noise.
Innovation Solution
A method for resetting a time-based CMOS image sensor where the reset electron potential of the photodiode is set higher than that of the floating diffusion node by applying a voltage lower than the power voltage, allowing for effective detection of low light signals by ensuring the difference in reset electron potential exceeds the comparator offset, enabling operation at low illumination conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the photodiode and floating diffusion node are reset to the same reset voltage (VDD-VT), then the resetting process is simple, but the sensor cannot detect fine light signals at low illumination due to insufficient potential difference
Solution Approach 1:
The patent changes the reset voltage parameter applied to the photodiode and floating diffusion node. Specifically, it applies a first reset voltage (VDD-VT-Δ) to the photodiode and a second reset voltage (VDD-VT) to the floating diffusion node, creating a controlled potential difference Δ that exceeds the comparator offset. This parameter change enables the sensor to detect fine light signals at low illumination while maintaining a straightforward resetting process.
2Measurement precision
If the reset electron potential difference between photodiode and floating diffusion node is increased to overcome comparator offset, then low illumination detection becomes possible, but the resetting process becomes more complex
Solution Approach 1:
The patent implements a dual reset voltage scheme where the photodiode is reset to VDD-VT-Δ and the floating diffusion node is reset to VDD-VT. This controlled parameter change creates the necessary potential difference Δ to overcome the comparator offset, enabling low illumination detection. The complexity is managed by systematically applying different reset voltages to different nodes rather than modifying the comparator or other components.
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
Enables the time-based CMOS image sensor to detect fine light signals effectively even at low illumination, reducing noise sensitivity and allowing for precise measurement of light signals using digital correlated double sampling.
Implementation Method 1
a photodiode PD, a transfer transistor Tx transferring photo-generated charges generated in the photodiode PD
Data Source
AI summary
A method of resetting a time-based CMOS image sensor may be provided, where the time-based CMOS image sensor may include a photodiode, a transfer transistor transferring photo-generated charges generated in the photodiode to a floating diffusion node and having a gate to which a ramp signal is input, and a reset transistor resetting the photodiode and the floating diffusion node. The method may include generating photo-generated charges at the photodiode, transferring the photo-generated charges to the floating diffusion node in response to a ramp signal; and resetting a reset electron potential of the photodiode to be higher than a reset electron potential of the floating diffusion node.