Capacitive Fingerprint Sensor Readout Circuit Noise Reduction
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Solution Overview
Problem
Capacitive fingerprint sensors embedded in image panels face challenges in accurately distinguishing between ridge and valley capacitances due to environmental noise and cross-talk, affecting fingerprint recognition accuracy.
Innovation Solution
The capacitive fingerprint sensor system incorporates a readout circuit with transistors, an operational amplifier, a reference capacitor, and a multiplexer to differentiate between ridge and valley capacitances by precharging and redistributing charges, using a control circuit to identify capacitance types through voltage selection and comparison.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a simple capacitor structure is used for fingerprint sensing, then the device complexity is reduced, but the measurement precision of ridge and valley capacitance difference deteriorates
Solution Approach 1:
The sensing area is divided into multiple independent sensing units, each with its own readout circuit. This segmentation allows parallel processing of multiple fingerprint points, improving measurement precision through dedicated circuits while managing overall device complexity through modular design
Solution Approach 2:
A compensation capacitor is introduced as an intermediary element to bridge the difference between ridge and valley capacitances. This intermediary component enables accurate measurement of capacitance differences by providing a reference that compensates for environmental noise and cross-talk effects
2Reliability
If environmental noise and cross-talk are present, then the reliability of fingerprint recognition is reduced, but adding complex noise filtering circuits increases device complexity
Solution Approach 1:
The readout circuit performs preliminary actions by pre-charging capacitors and establishing reference voltage levels before actual fingerprint measurement. This preliminary preparation reduces the impact of environmental noise and cross-talk during the actual sensing operation, improving reliability without requiring complex real-time filtering circuits
Solution Approach 2:
The circuit implements feedback mechanisms where the measured capacitance values are compared against reference values, and compensation is applied based on the difference. This feedback loop continuously corrects for noise and cross-talk effects, maintaining high recognition accuracy while using standard circuit components
3Measurement precision
If the capacitance difference between ridge and valley is small, then the measurement precision is improved, but the difficulty of detecting and measuring increases
Solution Approach 1:
The readout circuit is designed with multi-functionality, serving both as a charge transfer mechanism and as a measurement instrument. The same circuit components perform multiple functions including charging, transferring, comparing, and compensating capacitance values, reducing the need for specialized high-complexity measurement equipment while maintaining high detection accuracy
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 accuracy of fingerprint recognition by effectively distinguishing between ridge and valley capacitances, improving the sensor's ability to discern fingerprint patterns amidst noise and interference.
Implementation Method 1
The fingerprint capacitor has a capacitance which is either a valley capacitance CFV or a ridge capacitance CFR, and CFV is smaller than CFR
Implementation Method 2
One input end of the operational amplifier is connected to the output terminal of the second transistor. The reference capacitor Cfb is connected to input and output ends of the operational amplifier
Data Source
AI summary
A capacitive fingerprint sensor comprises a fingerprint capacitor, an integrator, a first transistor, a second transistor and a multiplexer. The fingerprint capacitor has a capacitance that is either a valley capacitance CFV or a ridge capacitance CFR, wherein CFV is smaller than CFR. The integrator has a reference capacitor Cfb. The first transistor is configured to control the fingerprint capacitor during a scan line period. The second transistor is configured to precharge the fingerprint capacitor and to redistribute the charges between the fingerprint capacitor and the reference capacitor Cfb. The multiplexer is connected to the integrator for providing a first voltage VA and a second voltage VB, wherein the first voltage VA is greater than the second voltage VB.