Image Sensor Row Selection Signal Timing for CDS
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Solution Overview
Problem
Conventional X-ray image sensors face challenges in achieving high signal-to-noise ratio (S/N ratio) for low radiation exposure and high definition, particularly due to threshold voltage variations in thin film transistors, which affect the reliability and light sensitivity, and require complex signal processing and increased manufacturing costs.
Innovation Solution
An image sensor design with a photoelectric conversion device, amplification transistor, selection transistor, and reset transistor, where the row selection signal is applied for an extended period, overlapping with adjacent pixel rows, allowing for correlated double sampling (CDS) without dedicated reset wiring, thereby compensating for threshold voltage variations without reducing light sensitivity or increasing manufacturing costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If a thin film transistor is used as a switching device in each pixel, then the FPD can be prepared on a large area substrate using thin film semiconductor technology, but threshold voltage variations occur in the transistor, affecting reliability and light sensitivity
Solution Approach 1:
The patent applies preliminary action by performing correlated double sampling (CDS) to measure and compensate for threshold voltage variations before they affect the final signal. The system samples the signal at two different times (when the reset transistor is on and when it is off) and uses the difference to eliminate the effect of threshold voltage shifts, thereby maintaining reliability despite the inherent variations in thin film transistors.
2Measurement precision
If the pixel size is decreased to achieve high definition, then the resolution is improved, but signal electric charge is decreased, degrading the S/N ratio
Solution Approach 1:
The patent performs preliminary sampling of the signal state before the main signal acquisition. By sampling the pixel state when the reset transistor is conductive and when it is non-conductive, the system can compensate for signal loss due to small pixel size. The CDS technique allows the system to maintain accurate signal measurement even when pixel area is reduced, thereby preserving S/N ratio while achieving high definition.
Solution Approach 2:
The patent implements feedback by using the sampled signal values to correct the final output. The system measures the signal at multiple points and uses this information to compensate for threshold voltage variations and signal losses, effectively feedback-adjusting the final signal to maintain high S/N ratio despite reduced pixel size.
3Object-affected harmful factors
If X-ray radiation amount is decreased to achieve low radiation exposure, then patient safety is improved, but signal electric charge detected by the FPD is decreased, degrading the S/N ratio
Solution Approach 1:
The patent applies preliminary sampling to capture signal characteristics before the main X-ray signal is fully processed. By sampling the pixel state at different phases (during and after the radiation pulse), the system can compensate for reduced signal charge caused by lower radiation exposure. The CDS technique enhances the detectable signal strength, allowing high S/N ratio even at reduced radiation doses.
Solution Approach 2:
The system uses feedback from the sampled signal values to compensate for signal loss due to reduced radiation exposure. The CDS process measures the signal at multiple time points and uses this feedback information to correct the final output, thereby maintaining high S/N ratio even when the incident X-ray radiation amount is decreased.
4Reliability
If dedicated reset wiring is added to each pixel, then threshold voltage variations can be compensated, but device complexity and manufacturing costs increase
Solution Approach 1:
The patent applies universality by making the row selection line serve multiple functions: it acts as both the signal selection line for reading pixel data and as the reset control line. By timing the row selection signal to overlap with the reset operation, the same wiring infrastructure performs both signal reading and reset functions, eliminating the need for separate dedicated reset wiring and reducing device complexity.
Solution Approach 2:
The patent merges the reset function into the existing row selection line circuitry. Instead of adding separate reset wiring, the system combines the reset transistor control with the row selection signal timing. The row selection line is activated long enough to serve as both the signal readout control and the reset control, merging two functions into a single wiring infrastructure.
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 compensates for threshold voltage variations and maintains light sensitivity while reducing manufacturing costs by eliminating the need for dedicated reset wiring and simplifying the drive circuit, thus enhancing the S/N ratio without decreasing light sensitivity.
Implementation Method 1
a photoelectric conversion section converting an X ray into electric charge
Data Source
AI summary
As a control signal used for resetting a photodiode, a control signal for selecting an adjacent pixel row is used. Accordingly, the number of kinds of used control signals decreases, and a decrease in the area of the photodiode is prevented. In addition, a period in which all of a plurality of control signals selecting an adjacent pixel row are active is provided by setting an active period of a control signal selecting a pixel row to a period longer than a period in which a signal of one pixel row is read. Therefore, a CDS operation is realized.