CMOS Active Pixel Voltage Readout for High Dynamic Range Imaging
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional CMOS cameras face challenges in dynamic lighting conditions, leading to image saturation and loss of relevant information due to limited dynamic range and signal-to-noise ratio, particularly in applications like intelligent car airbags and video surveillance.
Innovation Solution
A CMOS active pixel structure that allows reading voltage of any polarity during photodiode evolution, utilizing a depletion NMOS read transistor and capacitive coupling to improve signal-to-noise ratio and exploit the entire evolution range of the photodiode, including linear, mixed, and logarithmic zones.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional CMOS cameras use linear response photodiodes, then manufacturing is simple and compact, but dynamic range is limited and saturation occurs in varying lighting conditions
Solution Approach 1:
The pixel structure dynamically switches between photovoltaic mode and photoconductive mode based on lighting conditions. The transfer transistor controls the connection between photodiode and floating diffusion, enabling the system to adapt its operating mode to maximize dynamic range while maintaining compactness
Solution Approach 2:
The invention changes the operational parameters of the photodiode by switching between two distinct modes: photovoltaic mode (open circuit) for low light conditions and photoconductive mode (short circuit) for bright light conditions. This parameter switching expands the usable dynamic range without increasing physical pixel size
2Measurement precision
If photodiode operates only in photovoltaic mode, then structure is simple, but signal amplitude is very low and signal-to-noise ratio is limited
Solution Approach 1:
The reading circuit dynamically selects between photovoltaic and photoconductive modes based on signal amplitude requirements. The transfer transistor enables switching to photoconductive mode when higher signal amplitude is needed, improving signal-to-noise ratio without requiring permanently complex circuitry
Solution Approach 2:
The same photodiode and reading circuit structure serves multiple functions: it operates in photovoltaic mode for compact simplicity and in photoconductive mode for high signal-to-noise ratio performance. The dual-mode capability makes the circuit universally applicable to various lighting conditions
3Adaptability or versatility
If photodiode enters mixed or logarithmic zones, then dynamic range is extended, but image quality deteriorates due to strong compression and low amplitude signals
Solution Approach 1:
The system dynamically operates in the linear zone of photodiode evolution by switching between photovoltaic and photoconductive modes, avoiding the compressed mixed and logarithmic zones. This dynamic mode switching maintains high image quality while achieving extended operating dynamics through dual-mode operation
Solution Approach 2:
The invention changes the operational parameter from single-mode (photovoltaic only) to dual-mode (photovoltaic and photoconductive), enabling the photodiode to operate in the linear evolution zone across a broader range of lighting conditions without entering the problematic mixed or logarithmic zones
4Reliability
If isolation box is used to separate NMOS and PMOS transistors, then device operation is stable, but useful surface area is reduced and photoelectric performance decreases
Solution Approach 1:
The invention extracts the PMOS transistor from the pixel structure, using only NMOS transistors for all functions (transfer transistor, read transistor, reset transistor). This eliminates the need for isolation boxes and increases the photodiode surface area while maintaining stable device operation through careful NMOS circuit design
Solution Approach 2:
The pixel structure uses homogeneous NMOS transistors for all switching and reading functions, eliminating the need for PMOS transistors and their associated isolation requirements. This homogeneous NMOS approach increases photodiode area while maintaining operational stability
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 provides a high dynamic range and improved signal-to-noise ratio, enabling better image quality and reduced saturation, while maintaining physical compactness and enhancing photoelectric performance.
Implementation Method 1
a photodiode (10) having a junction capacitance CPD
Implementation Method 2
capacitive coupling to improve signal-to-noise ratio and exploit the entire evolution range of the photodiode
Data Source
Figure 1a~3b
Figure 4a~5
Figure 6a~6e
AI summary
The invention relates to a structure of an active pixel of the CMOS type (1) that comprises: at least one photodiode (10), characterised in that it comprises means for reading any bias voltage in the evolution phase of the photodiode (10) upon exposure.