Capacitance Sensing Circuit With Adaptive Integration for Fingerprint Accuracy
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Solution Overview
Problem
Fingerprint authentication mechanisms in electronic devices can be compromised by stolen fingerprint patterns being replicated, leading to unauthorized access, and existing capacitance detecting circuits may operate outside their linear region, causing erroneous identification due to manufacturing variations and noise issues.
Innovation Solution
A capacitance detecting circuit that consistently operates within a linear operating region, using integrators and pixel sensing circuits to enhance signal-to-noise ratio and prevent saturation, thereby accurately detecting and authenticating fingerprints.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional capacitance detecting circuits are used, then the circuit may operate outside linear region due to manufacturing variations, but this causes erroneous identification and reduces measurement precision
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the integration time period based on the detected capacitance value. When the integrating output voltage approaches saturation, the integration time is reduced, allowing the circuit to operate within its linear region despite manufacturing variations. This adaptive parameter adjustment resolves the contradiction by maintaining both reliability and measurement precision.
Solution Approach 2:
The patent implements feedback control where the integration time period is continuously adjusted based on the output voltage level. The system monitors the integrating output voltage and modifies the integration time accordingly, creating a closed-loop control that prevents saturation and ensures accurate fingerprint authentication while accommodating manufacturing variations.
2Measurement precision
If the integration time period is increased to improve signal detection, then the signal-to-noise ratio improves, but the circuit may saturate and operate outside linear region
Solution Approach 1:
The patent applies dynamics by making the integration time period variable rather than fixed. The system dynamically adjusts the integration time based on real-time feedback from the integrating output voltage, allowing the circuit to optimize signal-to-noise ratio while preventing saturation. This dynamic adjustment resolves the contradiction between improving measurement precision and maintaining linear operation reliability.
Solution Approach 2:
The patent changes the integration time parameter adaptively based on operating conditions. When the output voltage indicates approaching saturation, the integration time is reduced; when there is sufficient headroom, the integration time can be increased to improve signal detection. This parameter change strategy simultaneously achieves high signal-to-noise ratio and reliable linear operation.
3Device complexity
If fixed integration time is used to simplify circuit operation, then device complexity is reduced, but manufacturing variations cause the circuit to operate outside linear region
Solution Approach 1:
The patent implements self-service control where the circuit automatically adjusts its own integration time based on its output state. The system monitors its own integrating output voltage and modifies the integration time without external intervention, enabling the circuit to maintain accurate operation despite manufacturing variations while keeping the control logic integrated and relatively simple.
Solution Approach 2:
The patent uses feedback control to automatically adjust the integration time based on the circuit's own output voltage level. This self-regulating mechanism allows the circuit to compensate for manufacturing variations and maintain accurate capacitance detection without requiring complex external control systems.
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 prevents unauthorized access by ensuring accurate fingerprint detection and authentication, maintaining the circuit within a linear operating region to enhance the reliability of fingerprint recognition systems.
Implementation Method 1
A capacitance detecting circuit detects a capacitance of a capacitor formed by a user's finger in a pixel sensing circuit
Data Source
Figure 1A~1B
Figure 1C~1D
Figure 1E~1F
AI summary
Capacitance detecting circuit(10) is disclosed for fingerprint sensing and other applications. The capacitance detecting circuit(10) includes a first capacitor(C1), an integrator(100), a second capacitor(C2), a comparator(102), and a counter(104). The integrator(100) can generate an integrating output voltage and includes a first single-ended amplifier(106) and at least one integration capacitor(Cint_1-Cint_4). The first single-ended amplifier(106) includes a first input terminal and an integrating output terminal. The comparator(102) can generate a comparing output and include a negative input terminal coupled to the integrating output terminal of the first single-ended amplifier(106), a positive input terminal to receive a reference voltage(Vr), and a comparing output terminal to output the comparing output voltage(Vo_cmp). The counter(104) is coupled to the comparing output terminal and can generate a counter output(Vo_cnt). A connection between the second capacitor and the first input terminal is controlled to be conducted or cutoff according to the comparing output.