Capacitive Fingerprint Sensing Circuit Without ADC Readout
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Solution Overview
Problem
Existing capacitive sensing circuits for fingerprint identification have complex circuit structures, large area requirements, high production costs, and high power consumption due to the use of analog-to-digital converters.
Innovation Solution
A capacitive sensing circuit that converts charge stored in a contact capacitor into a time signal using a sample-and-hold circuit, integrating circuit, comparator, and logic circuit, eliminating the need for an analog-to-digital converter by determining capacitance values based on integration time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an analog-to-digital converter is used to convert the analog voltage signal into a digital voltage signal, then the fingerprint identification accuracy is improved, but the circuit structure becomes complex and the circuit area increases
Solution Approach 1:
The patent extracts and removes the analog-to-digital converter from the capacitive sensing circuit, replacing it with a simplified timing-based measurement approach. The capacitance value is determined by measuring the integration time required for the integrator output to reach a reference voltage level, eliminating the need for complex ADC hardware while maintaining measurement capability.
Solution Approach 2:
The patent substitutes the electronic analog-to-digital conversion mechanism with a timing-based measurement system. Instead of using an ADC to convert analog voltage to digital values, the system uses a comparator and timer to measure how long it takes for an integrating capacitor to charge to a reference voltage, thereby inferring the original capacitance value through time measurement.
2Measurement precision
If an analog-to-digital converter is used to convert the analog voltage signal into a digital voltage signal, then the fingerprint identification accuracy is improved, but the circuit area occupied increases
Solution Approach 1:
The patent extracts and removes the analog-to-digital converter from the capacitive sensing circuit, replacing it with a simplified timing-based measurement approach. The capacitance value is determined by measuring the integration time required for the integrator output to reach a reference voltage level, eliminating the need for complex ADC hardware while maintaining measurement capability.
3Measurement precision
If an analog-to-digital converter is used to convert the analog voltage signal into a digital voltage signal, then the fingerprint identification accuracy is improved, but the production cost increases
Solution Approach 1:
The patent extracts and removes the analog-to-digital converter from the capacitive sensing circuit, replacing it with a simplified timing-based measurement approach. The capacitance value is determined by measuring the integration time required for the integrator output to reach a reference voltage level, eliminating the need for complex ADC hardware while maintaining measurement capability.
4Measurement precision
If an analog-to-digital converter is used to convert the analog voltage signal into a digital voltage signal, then the fingerprint identification accuracy is improved, but the power consumption increases
Solution Approach 1:
The patent extracts and removes the analog-to-digital converter from the capacitive sensing circuit, replacing it with a simplified timing-based measurement approach. The capacitance value is determined by measuring the integration time required for the integrator output to reach a reference voltage level, eliminating the need for complex ADC hardware while maintaining measurement capability.
Solution Approach 2:
The patent employs periodic sampling and integration cycles to measure capacitance values. The circuit periodically charges the integrating capacitor through a known current source, then uses a comparator to detect when the integration voltage reaches a reference level, generating a time-based measurement result. This periodic operation reduces average power consumption compared to continuous ADC conversion.
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 results in a simpler circuit structure, reduced area, lower costs, and lower power consumption while accurately interpreting capacitance values for fingerprint identification.
Implementation Method 1
an integrating circuit including an integrating input terminal coupled to the sample-and-hold circuit and an integrating output terminal
Implementation Method 2
a comparator including a first input terminal coupled to the integrating output terminal, a second input terminal for receiving a reference voltage, and a comparison output terminal for outputting a comparison output voltage
Data Source
AI summary
A capacitive sensing circuit (10) includes a sample-and-hold circuit (SH) coupled to a contact capacitor (Cf); an integrating circuit (100) coupled to the sample-and-hold circuit (SH); a comparator (comp) including a first input terminal coupled to the integrating circuit (100), a second input terminal for receiving a reference voltage (VREF), and a comparison output terminal for outputting a comparison output voltage (VCMP); a logic circuit (102) coupled to the comparison output terminal, where the logic circuit (102) outputs an integration time (TOUT) of the integrating circuit (100) when the comparison output voltage (VCMP) indicates that the comparator (comp) performs a transition, where the integration time (TOUT) correlates with a capacitance value of the contact capacitor (Cf).