Fingerprint Sensing Circuit Capacitance Voltage Conversion
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Solution Overview
Problem
Existing fingerprint sensing circuits have low sensing accuracy due to small variations in coupling capacitances caused by fingerprint ridges and valleys, which are not effectively detected.
Innovation Solution
A fingerprint sensing circuit that includes a sensing electrode, a first converting circuit to convert coupling capacitance into a drive voltage, and a second converting circuit to generate a sensing current based on the drive voltage, enhancing the accuracy of fingerprint recognition.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional fingerprint sensing circuits are used, then the device structure remains simple, but the sensing accuracy is low due to small variations in coupling capacitances
Solution Approach 1:
The converting circuit is divided into two independent modules: a first converting circuit that converts coupling capacitance to drive voltage, and a second converting circuit that converts drive voltage to sensing current. This segmentation allows each module to be optimized independently for its specific function, improving overall sensing accuracy while maintaining manageable circuit complexity through modular design.
Solution Approach 2:
The first converting circuit acts as an intermediary between the sensing electrode and the second converting circuit. It transforms the difficult-to-detect coupling capacitance variations into drive voltage signals, which are then further converted to sensing current. This intermediate transformation step enhances the detectability of small capacitance changes without requiring direct high-precision capacitance measurement.
2Measurement precision
If direct capacitance measurement is used, then the circuit remains simple, but small variations in coupling capacitances caused by fingerprint ridges and valleys cannot be effectively detected
Solution Approach 1:
The circuit transforms the measurement parameter from direct capacitance measurement to current measurement through a two-stage conversion process. The coupling capacitance is first converted to drive voltage, then to sensing current. This parameter transformation makes small capacitance variations caused by fingerprint ridges and valleys more detectable, as current measurements can achieve higher precision for small signal variations.
Solution Approach 2:
The patent replaces direct electrical capacitance measurement with a voltage-to-current conversion system. Instead of directly measuring and comparing capacitance values, the system converts capacitance to voltage and then to current, leveraging the higher sensitivity and lower noise characteristics of current measurement for detecting small variations.
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 proposed solution improves fingerprint sensing accuracy by converting coupling capacitance into a drive voltage and then generating a sensing current, which is used by the fingerprint signal processor for more precise recognition.
Implementation Method 1
a first converting circuit connected to the sensing electrode and configured to convert a coupling capacitance sensed by the sensing electrode into a drive voltage
Implementation Method 2
a second converting circuit configured to generate a sensing current based on the drive voltage, and send the sensing current to a fingerprint signal processor, where the sensing current is equal to a product of a transconductance gain of the second converting circuit and the voltage variation
Data Source
AI summary
A fingerprint sensing circuit and a fingerprint sensing apparatus are provided. The fingerprint sensing circuit includes a sensing electrode; a first converting circuit connected to the sensing electrode and configured to convert a coupling capacitance sensed by the sensing electrode into a drive voltage, where the drive voltage is equal to a sum of a voltage variation converted from the coupling capacitance and a reference voltage; and a second converting circuit configured to generate a sensing current based on the drive voltage, and send the sensing current to a fingerprint signal processor, where the sensing current is equal to a product of a transconductance gain of the second converting circuit and the voltage variation, and the fingerprint signal processor performs fingerprint sensing based on the sensing current. With the fingerprint sensing circuit and the fingerprint sensing apparatus, the detection accuracy can be improved.