Capacitive Fingerprint Sensor Noise Reduction via Frequency Modulation
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Solution Overview
Problem
Capacitive fingerprint detection devices are prone to external noise interference, particularly from AC power sources, which can lead to suboptimal detection sensitivity and stability due to differences in ground potential, causing signal overflow and inaccurate data acquisition.
Innovation Solution
A detecting device with a first electrode selection circuit that changes the frequency of drive signals for each period within a frame period, combined with a detection circuit to detect capacitance between electrodes, and a fingerprint pattern generator to produce patterns based on output signals, effectively reducing noise interference by varying the clock frequency and replacing excessive data values with '0'.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If code division multiplexing drive is used to obtain signals from small electrodes, then detection sensitivity is improved, but the system becomes more vulnerable to external noise interference
Solution Approach 1:
The patent applies periodic action by using code division multiplexing to sequentially select and drive multiple electrodes in a periodic manner. Each electrode is activated in a specific time slot according to a code sequence, allowing the system to obtain signals from multiple small electrodes without simultaneous excitation. This periodic selection reduces the impact of external noise by ensuring that noise affecting one electrode does not interfere with signals from other electrodes being measured.
2Ease of manufacture
If capacitive detecting device is coupled to AC power source, then power supply is simplified, but noise from AC power source propagates to detecting device due to ground potential difference
Solution Approach 1:
The patent introduces an intermediary approach by implementing a noise cancellation mechanism that acts as a mediator between the AC-powered detection device and the external noise environment. The system uses a noise cancellation circuit that generates a signal opposite in phase to the external noise and combines it with the detection signal, thereby canceling out the noise propagation from the AC power source while maintaining the simplified power supply connection.
3Productivity
If external noise synchronized with clock frequency is superimposed on detection signals, then data overflow occurs, but increasing clock frequency improves detection speed
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the clock frequency based on detection conditions. The system monitors the detection signals for signs of noise synchronization and automatically modifies the clock frequency to avoid resonant conditions that cause data overflow. This allows the system to maintain high detection speed when conditions permit while ensuring data accuracy by reducing the clock frequency when noise interference is detected.
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 significantly reduces the impact of external noise, ensuring stable and sensitive fingerprint detection by minimizing noise effects and preventing data overflow, resulting in improved operational performance.
Implementation Method 1
a detection circuit configured to detect capacitance generated between the first electrodes and the second electrodes due to the drive signal
Data Source
AI summary
A detecting device includes a plurality of first electrodes extending in a first direction and a plurality of second electrodes extending in a second direction intersecting the first direction, the first electrodes and the second electrodes being disposed facing each other with an insulating layer interposed therebetween, a first electrode selection circuit configured to change a combination of a plurality of selected first electrodes for each of a plurality of periods obtained by dividing one frame period and supply a drive signal to the selected first electrodes, and a detection circuit configured to detect capacitance generated between the first electrodes and the second electrodes due to the drive signal. The first electrode selection circuit changes a frequency of the drive signal for each of the periods.