Multilevel Reset Voltage for Multi-Gain Image Sensor
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional CMOS image sensors face challenges in achieving high conversion gain for low-light sensitivity while avoiding pixel saturation in bright environments, as the reduction of floating diffusion node capacitance leads to decreased conversion gain and increased pixel size or slower clock speeds.
Innovation Solution
Implementing a dual conversion gain (DCG) system that adjusts the capacitance of the floating diffusion node based on ambient light conditions, using a shared reset generator with adjustable capacitance to switch between high and low conversion gain modes, thereby optimizing reset voltage and capacitance for improved dynamic range and frame rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the capacitance of the floating diffusion node is reduced to increase conversion gain, then low-light sensitivity is improved, but the pixel size increases or clock speed must be reduced
Solution Approach 1:
The patent implements a dual conversion gain system where the capacitance of the floating diffusion node is dynamically adjusted based on ambient light conditions. A shared reset generator with adjustable capacitance switches between high and low conversion gain modes, allowing the system to optimize for either low-light sensitivity or bright environment performance as needed, rather than being fixed at a single capacitance value.
2Measurement precision
If the capacitance of the floating diffusion node is reduced to increase conversion gain, then low-light sensitivity is improved, but pixel saturation occurs in bright environments
Solution Approach 1:
The system dynamically switches between high and low conversion gain modes depending on ambient light conditions. In bright environments, the system operates in low conversion gain mode with higher capacitance to prevent pixel saturation, while in low-light conditions, it switches to high conversion gain mode with lower capacitance to improve sensitivity.
Solution Approach 2:
The patent changes the capacitance parameter of the floating diffusion node based on operating conditions. By adjusting the reset voltage and capacitance values, the system optimizes performance for different lighting scenarios, preventing pixel saturation in bright environments while maintaining low-light sensitivity when needed.
3Adaptability or versatility
If a single reset voltage is used, then device complexity is reduced, but the system cannot optimize for both low-light sensitivity and bright environment performance
Solution Approach 1:
The patent implements a shared reset generator that serves multiple functions by generating both high and low reset voltages. This single reset generator with adjustable capacitance can switch between high and low conversion gain modes, eliminating the need for separate reset generators for each mode and reducing overall device complexity while maintaining adaptability.
Data Source
AI summary
A method of operating an image sensor includes adjusting a capacitance coupled to a circuit node within a pixel cell. The circuit node is coupled to selectively receive an image charge acquired by a photo-sensor of the pixel cell. A conversion gain is selected from multiple conversion gains for the pixel cell by adjusting the capacitance. A voltage level from multiple voltage levels is selected for use as a reset signal when the reset signal is asserted. The reset signal controls resetting of the circuit node during operation of the pixel cell. The voltage level is selected dependent upon which of the multiple conversion gains is selected by adjusting the capacitance. The reset signal is asserted to reset a voltage at the circuit node.