Capacitive Fingerprint Sensor Signal Clipping
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Solution Overview
Problem
Existing fingerprint detection systems using capacitive sensors face signal clipping issues due to increased capacitance values when multiple drive electrodes are selected, leading to reduced detection accuracy.
Innovation Solution
A detecting device with a first electrode selection circuit that alternates between two drive signals of different potentials during code division multiplexing, ensuring optimal signal detection by managing capacitance values across multiple periods.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If the number of simultaneously selected drive electrodes is increased to improve detection coverage, then the capacitance value increases, but detection signals may be clipped and detection accuracy decreases
Solution Approach 1:
The detection period is divided into a first period and a second period. In the first period, a first code is assigned to all first electrodes to perform comprehensive detection. In the second period, the first code is assigned to only some first electrodes to reduce capacitance and prevent signal clipping. This segmentation allows the system to achieve both wide detection coverage and high detection accuracy at different times.
Solution Approach 2:
The electrode selection circuit operates periodically by alternating between the first period and the second period. This periodic action allows the system to cycle between comprehensive detection mode (first period) and low-capacitance mode (second period), thereby preventing signal clipping while maintaining overall detection coverage.
2Reliability
If the gain is designed to match a state with relatively large number of simultaneously selected drive electrodes, then signal clipping is prevented, but detection signals become small when the number of selected electrodes is small, reducing detection accuracy
Solution Approach 1:
The gain is dynamically adjusted based on the detection period. In the first period, a first gain is applied, and in the second period, a second gain is applied. This dynamic gain adjustment ensures that detection signals are amplified appropriately in each period, preventing signal clipping when many electrodes are selected while maintaining high detection accuracy when fewer electrodes are selected.
Solution Approach 2:
The system changes the gain parameter according to the detection period. By switching between different gain values (first gain and second gain) corresponding to different numbers of selected drive electrodes, the system optimizes both signal stability and detection accuracy for each operating condition.
3Measurement precision
If code division multiplexing drive is used to simultaneously select multiple drive electrodes, then detection sensitivity is improved, but capacitance value increases causing signal clipping
Solution Approach 1:
The code assignment is segmented into different periods. In the first period, the first code is assigned to all first electrodes to achieve comprehensive detection with high sensitivity. In the second period, the first code is assigned to only some first electrodes to reduce total capacitance and prevent signal clipping. This segmentation allows the system to enjoy the benefits of code division multiplexing while avoiding its harmful effects.
Solution Approach 2:
The system periodically alternates between comprehensive code assignment (first period) and selective code assignment (second period). This periodic action allows the system to maintain high detection sensitivity when needed while preventing signal clipping by reducing capacitance during other periods.
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 prevents signal clipping and enhances fingerprint detection accuracy by adjusting drive signals based on code assignments to electrodes, improving sensitivity and reliability.
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 supply a first drive signal or a second drive signal with a lower potential than that of the first drive signal to the first electrodes according to a supplied signal of a first code or a second code, and a detection circuit configured to detect capacitance generated between the first electrodes and the second electrodes due to the drive signal.