Image Sensor Pixel Reset Voltage Correction for Thermal Noise
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Solution Overview
Problem
Existing image sensor pixel circuits face challenges in reducing thermal noise during the reset operation, which degrades the sensitivity and signal-to-noise ratio, and current solutions like correlated double sampling either prolong read operations or require additional memory or complex pixel architectures.
Innovation Solution
A pixel circuit with a reset voltage correction circuit that includes a first switch to couple the input node to a correction node capacitively coupled to the sense node, and a second switch to apply the reset voltage, allowing for correction of the reset voltage level and reduction of thermal noise without the need for additional memory or complex pixel structures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If correlated double sampling technique is used to reduce thermal noise, then thermal noise is reduced, but read operation time increases or additional memory capacitors are required
Solution Approach 1:
The patent applies preliminary action by performing the reset operation and capturing the reset noise level before the actual light integration begins. The reset noise is sampled and stored in a temporary holding capacitor during the integration period, then subtracted from the final signal. This allows noise reduction without extending the read operation time because the noise correction data was collected in advance during the reset phase.
Solution Approach 2:
The patent merges the reset operation with the signal readout operation by using the same readout path and timing circuitry for both purposes. The reset noise sampling and signal reading share common hardware resources including the sense node, readout amplifier, and timing control logic. This integration eliminates the need for separate memory capacitors or additional readout paths, achieving noise reduction without increasing device complexity or read time.
2Object-affected harmful factors
If correlated double sampling technique is used to reduce thermal noise, then thermal noise is reduced, but additional memory capacitors or complex pixel structures are required
Solution Approach 1:
The patent applies universality by designing the readout circuitry to perform multiple functions: it serves as both the reset operation circuit and the signal readout circuit, and also functions as the noise sampling and subtraction circuit. The same transistors, capacitors, and amplifiers are reused across different operational phases (reset, integration, readout, noise correction). This multi-functionality achieves thermal noise reduction without requiring additional dedicated components for each function, thereby maintaining simple pixel structure.
Solution Approach 2:
The patent implements self-service by having the pixel's own readout circuitry perform the noise correction function without requiring external assistance. The pixel circuit autonomously samples its own reset noise, stores it temporarily, and subtracts it from the final signal using its internal components. This self-contained approach eliminates the need for external memory capacitors or complex additional structures, achieving noise reduction while maintaining device simplicity.
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 effectively reduces thermal noise, improving the sensitivity and signal-to-noise ratio of the image sensor while maintaining a compact and efficient pixel circuit design, avoiding the drawbacks of existing methods.
Implementation Method 1
a photodiode, or other type of photodetector, that generates a current by integrating incident light received from the image scene
Implementation Method 2
the correction node being coupled by a capacitance to the sense node
Data Source
AI summary
A pixel circuit comprising: a light-sensing element; a first transistor having its control node coupled to a sense node and its source coupled to a readout path of the pixel circuit; and a reset voltage correction circuit comprising: a first switch configured to selectively couple an input node of the reset voltage correction circuit to a correction node, the input node being connected to the sense node or to the source of the first transistor, the correction node being coupled by a capacitance to the sense node; and a second switch configured to selectively couple the correction node to a reset voltage.