Capacitive Biometric Sensor with Integrated Readout Circuit
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Solution Overview
Problem
Current capacitive touch sensors face limitations in achieving high resolution and large area coverage due to parasitic capacitance and sensitivity to environmental noise, making them less effective for secure biometric authentication.
Innovation Solution
A capacitive biometric skin contact sensor with an array of sensor pixels, each comprising a thin film transistor and a capacitive sensing electrode, utilizing a current multiplexer and current mirror assembly to dynamically control read-out currents and gain, allowing for flexible operation in different modes to optimize resolution, sensitivity, and energy consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If passive matrix capacitive touch sensing systems are used with external driving circuits, then the sensor can detect touch events, but the system becomes sensitive to environmental noise and interference
Solution Approach 1:
The patent combines the driving circuit and readout circuit into a single integrated circuit (IC) that directly interfaces with the electrode matrix. This merging eliminates the need for separate external driving circuits while maintaining touch detection capability and reducing environmental noise sensitivity through integrated signal processing.
Solution Approach 2:
The patent introduces a specialized IC as an intermediary between the electrode matrix and the external environment. This IC contains both driving and readout functionality, acting as a mediator that processes signals internally and reduces exposure to external electromagnetic interference and noise.
2Measurement precision
If active matrix capacitive touch sensors are used with switching elements in each pixel, then the sensor can control conduction paths, but the parasitic capacitance combines additively limiting the number of pixels per column
Solution Approach 1:
The patent transitions from a column-parallel readout architecture to a row-parallel readout architecture. By switching elements connected to row lines instead of column lines, the patent changes the dimensional organization of signal paths, allowing larger sensor arrays without the parasitic capacitance accumulation that limits column-based designs.
Solution Approach 2:
The patent segments the sensor array into independently controllable rows, with each row having its own switching elements and readout path. This segmentation allows the sensor to scale to larger sizes by adding more rows without the parasitic capacitance of multiple columns combining additively, thus removing the inherent limit on pixel count.
3Measurement precision
If high resolution fingerprint sensing is implemented with hundreds of pixels per inch, then authentication security is improved, but manufacturing becomes difficult for integration into mobile devices
Solution Approach 1:
The patent changes the fundamental operating parameters of the sensor by using a lower pixel density (e.g., 100 pixels per inch) combined with advanced signal processing techniques in the integrated circuit. This parameter change maintains sufficient fingerprint authentication capability while dramatically simplifying manufacturing and enabling easy integration into mobile devices compared to high-resolution sensors requiring hundreds of pixels per inch.
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 increased flexibility in controlling operational parameters, such as resolution and sensitivity, while reducing environmental noise interference, thereby enhancing the effectiveness of biometric authentication.
Implementation Method 1
Capacitance sensors such as these use capacitive effects associated with the surface contours of the fingerprint. The sensor array pixels each include an electrode which acts as one plate of a capacitor, the dermal layer (which is electrically conductive) acts as the other plate, and the non-conductive epidermal layer acts as a dielectric.
Data Source
AI summary
A capacitive biometric skin contact sensor configured to resolve the contours of skin in contact with the sensor, wherein the sensor comprises: a contact sensing area comprising an array of sensor pixels, wherein each sensor pixel comprises a thin film transistor and a capacitive sensing electrode connected to the thin film transistor; a plurality of gate drive channels, wherein each gate drive channel is arranged to provide a gate drive signal to one or more of the sensor pixels; a plurality of read-out channels, wherein each read-out channel is arranged to receive a read-out current from one or more of the sensor pixels, each read-out current being indicative of a proximity to a respective capacitive sensing electrode of a conductive object to be sensed; a current multiplexer connected to a plurality of the read-out channels to receive read-out currents therefrom; and an analog to digital converter configured to provide a digital signal based on said read-out currents; wherein the sensor is configured to control at least one of the current multiplexer and the ADC to switch between operation in a first sensing mode and a second sensing mode.