Dual Conversion Gain Pixel Circuit for Dynamic Range Extension
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Solution Overview
Problem
Three-transistor pixel circuits suffer from high thermal noise, which limits dynamic range and signal-to-noise ratio, especially at high light levels, and existing solutions like feedback amplifiers require additional power and are not effectively implemented.
Innovation Solution
The introduction of an overflow capacitor and a dual conversion gain pixel circuit that allows high conversion gain at low light levels to reduce thermal noise, while switching to low conversion gain at high light levels to extend dynamic range and improve signal-to-noise ratio, using a correlated double-sampling circuit and single slope analog-to-digital converter with automatic gain control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high conversion gain is used in 3T pixel circuit, then thermal noise is reduced, but dynamic range deteriorates
Solution Approach 1:
The patent implements dynamic conversion gain adjustment by switching between high conversion gain mode (for low light conditions with reduced thermal noise) and low conversion gain mode (for high light conditions with extended dynamic range). This is achieved through a dual conversion gain pixel circuit that can adaptively change its operating state based on scene requirements, resolving the contradiction between noise performance and dynamic range.
Solution Approach 2:
The patent changes the conversion gain parameter dynamically by using different capacitor configurations (reset capacitor Cr and overflow capacitor Cof) to achieve high conversion gain (q/Cr) or low conversion gain (q/(Cr+Cof)). This parameter switching allows the system to optimize for either noise reduction or dynamic range extension depending on lighting conditions.
2Measurement precision
If feedback amplifiers are used to reduce kTC noise, then thermal noise is reduced, but power consumption increases
Solution Approach 1:
The patent extracts and eliminates the feedback amplifier component from the pixel circuit, replacing it with a simpler dual conversion gain architecture using capacitor switching. This removes the source of high power consumption while achieving the same noise reduction goal through alternative means (conversion gain adjustment rather than active feedback amplification).
Solution Approach 2:
The patent replaces the expensive and power-hungry feedback amplifier with a simple capacitor-based conversion gain mechanism that consumes minimal power. The solution uses passive components (capacitors Cr and Cof) instead of active amplifiers, achieving noise reduction through architectural design rather than power-intensive active compensation.
3Measurement precision
If parametric reset with voltage dependent capacitor is used, then kTC noise is reduced, but device complexity increases
Solution Approach 1:
The patent replaces the complex and unimplemented voltage-dependent capacitor approach with simple fixed capacitors (Cr and Cof) that can be switched using standard transistor switches. This dramatically simplifies the circuit implementation while achieving the desired noise reduction through conversion gain adjustment, making the solution practical and manufacturable.
Solution Approach 2:
The patent extracts and removes the complex voltage-dependent capacitor component from the circuit, replacing it with a straightforward capacitor switching mechanism using standard transistors. This simplifies the overall circuit architecture while maintaining the ability to adjust conversion gain for noise reduction.
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 effectively reduces thermal noise and extends intra-scene dynamic range without requiring additional power or complex feedback amplifiers, maintaining a linear response and improving signal-to-noise ratio across varying light conditions.
Implementation Method 1
A major drawback of a typical three transistor (3T) pixel circuit is high thermal (e.g., kTC) noise
Implementation Method 2
A major drawback of a typical three transistor (3T) pixel circuit is high thermal (e.g., kTC) noise
Data Source
AI summary
In various embodiments, an electronic image sensor having extended dynamic range comprises, for example, a pixel circuit and a column readout circuit. The column readout circuit includes, for example, a correlated double-sampling (CDS) capacitor, one or more CDS clamp switches, a single slope analog-to-digital converter (ADC) circuit, and a column memory. Other devices and methods are disclosed.