Global Shutter Pixel With Segmented Capacitors For Correlated Double Sampling
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Solution Overview
Problem
Global shutter image sensors face challenges with existing pixels and control methods, particularly in implementing correlated double sampling effectively while being compatible with existing readout circuits.
Innovation Solution
A global shutter pixel design featuring a first transistor, switches, capacitors, and a control method that allows for correlated double sampling, compatible with 4T pixel readout circuits, where the source of the first transistor is not coupled to a constant bias current, and a control circuit applies different power supply levels to manage the pixel's operating phases.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a global shutter pixel uses a conventional design with constant bias current, then the circuit is simpler, but the dynamic range and signal-to-noise ratio are limited
Solution Approach 1:
The pixel circuit is segmented into multiple independent assemblies, each with its own transistor and capacitor. This segmentation allows each assembly to be controlled independently during correlated double sampling, improving signal-to-noise ratio by enabling precise subtraction of reference signals while maintaining manageable circuit complexity through modular design.
Solution Approach 2:
The invention replaces constant bias current with dynamic voltage control. The first transistor's source is not coupled to constant bias current, allowing the circuit to dynamically adjust operating points during different phases (integration, readout, reset). This dynamic operation expands the usable voltage range, improving dynamic range while the controlled complexity of voltage switching maintains acceptable circuit design.
2Reliability
If correlated double sampling is implemented in global shutter pixels, then the signal-to-noise ratio improves, but the compatibility with existing readout circuits deteriorates
Solution Approach 1:
The pixel circuit is designed with universal interfaces that can work with existing readout circuits. The first transistor, switches, and capacitors are configured to produce standard voltage signals at the output node that are compatible with conventional 4T pixel readout circuits, while internally supporting correlated double sampling operations. This multi-functionality allows the pixel to perform advanced sampling without requiring complete readout circuit redesign.
Solution Approach 2:
The invention uses intermediate voltage nodes and control circuits as mediators between the correlated double sampling mechanism and the existing readout circuitry. The control circuit applies potentials to the first node and controls switch timing, acting as an intermediary layer that enables CDS functionality while presenting a compatible interface to legacy readout circuits.
3Measurement precision
If multiple assemblies with capacitors are used for voltage storage, then the correlated double sampling precision improves, but the manufacturing complexity increases
Solution Approach 1:
Multiple assemblies are designed with homogeneous structures - each assembly consists of a capacitor and switch configured identically. This homogeneity ensures consistent voltage storage characteristics across all assemblies, improving measurement precision for correlated double sampling. The repetitive, uniform design also simplifies manufacturing processes and quality control, as the same fabrication steps can be applied to all assemblies.
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 efficient correlated double sampling, improving the dynamic range and signal-to-noise ratio of the image sensor by maintaining identical control over identical assemblies, thus overcoming the limitations of existing global shutter pixels and control methods.
Implementation Method 1
a photoconversion area, preferably a photodiode, preferably pinned
Data Source
AI summary
A global shutter pixel includes a first transistor and a first switch series-connected between a first node of application of a potential and an internal node of the pixel. A control terminal of the first transistor is coupled to a floating diffusion node of the pixel. At least two assemblies are coupled to the internal node, where each assembly is formed of a capacitor series-connected with a second switch coupling the capacitor to the internal node. A second transistor has a control terminal connected to the internal node and a first conduction terminal coupled to an output node of the pixel. The pixel operation is controlled to store an initialization voltage from the floating diffusion on one of the capacitors and a pixel integration voltage from the floating diffusion on another of the capacitors.