Capacitive Coupling in X-Ray Image Sensors
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Solution Overview
Problem
Existing image sensors, particularly those using thin-film transistor (TFT) panels, face challenges with noise and voltage threshold variations, which limit their dynamic range and increase power dissipation, especially in active pixel arrays for X-ray imaging.
Innovation Solution
The implementation of capacitive coupling between the input node and the column line allows for voltage shifting within the operational range of the readout circuitry, enabling the use of TFT technology while isolating high voltages from semiconductor dies, thereby reducing noise and threshold voltage variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If active pixel arrays are implemented on TFT panels, then the dynamic range is improved, but noise and threshold voltage variations increase
Solution Approach 1:
A capacitive coupling element is introduced between the pixel array output and the readout circuitry input. This capacitor acts as an intermediary that blocks DC voltage variations and threshold voltage drift from the TFT panel while allowing AC signal transmission, thereby reducing noise and stabilizing the readout circuit operation without compromising dynamic range
Solution Approach 2:
The system is divided into two distinct parts: the pixel array on the TFT panel and the readout circuitry on a separate semiconductor die. This segmentation isolates the noise-sensitive readout circuit from the noisy TFT panel, allowing each to be optimized independently while maintaining their functional connection through the capacitive coupling
2Reliability
If high current is used to bias the input stage of the amplifier, then noise is reduced, but power dissipation increases
Solution Approach 1:
The biasing parameters of the amplifier input stage are optimized to achieve low noise performance with reduced current consumption. Additionally, the capacitive coupling allows for AC-coupled operation that reduces the need for high DC bias currents, thereby lowering power dissipation while maintaining noise performance through careful parameter selection
3Reliability
If the bandwidth of the CDS unit is reduced, then noise is reduced, but readout speed decreases
Solution Approach 1:
The CDS unit bandwidth is made dynamically adjustable rather than fixed. The system can adapt the bandwidth according to operating conditions, allowing high bandwidth for fast readout when noise is not critical and low bandwidth for noise reduction when speed is less important, thereby resolving the trade-off between noise performance and readout speed
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 enhances the dynamic range and reduces noise in image sensors, maintaining low power dissipation and improving the stability of active pixel arrays on TFT panels for X-ray imaging.
Implementation Method 1
a photosensitive element, such as a photodiode, that is configured for absorbing incoming X-ray photons and to generate a photocurrent
Implementation Method 2
a scintillator layer that converts incoming X-ray photons into visible light photons
Implementation Method 3
an image sensor according to the present invention comprises a pixel array, readout circuitry and a capacitive unit. The readout circuitry comprises a plurality of readout units, each readout unit having an input node. The capacitive unit is configured for coupling the input node to a column line
Data Source
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AI summary
The present invention relates to an image sensor and to an imaging system comprising the same. The present invention particularly relates to X-ray image sensors and imaging systems. The image sensor according to the invention comprises a pixel array that includes a plurality of active pixels arranged in a matrix of rows and columns, and a plurality of column lines to which outputs of pixels in the same column are coupled for the purpose of outputting pixel signals. The image sensor further comprises readout circuitry that includes a plurality of readout units, each readout unit being configured for reading out a respective column line through an input node of the readout unit. The image sensor is characterized in that the image sensor further comprises capacitive units, such as capacitors, for capacitively coupling each input node to its corresponding column line.