Capacitive Fingerprint Sensor Baseline Subtraction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Capacitive fingerprint sensors face significant challenges due to fixed pattern noise, which degrades the signal-to-noise ratio and affects the accuracy of fingerprint detection, leading to reduced security in biometric systems.
Innovation Solution
The implementation of a capacitive fingerprint sensor system that includes an array of capacitive sensing elements, readout circuitry, and digital-to-analog converters (DACs) to subtract baseline voltages from pixel voltages, thereby reducing fixed pattern noise and improving the signal-to-noise ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If signal amplification is applied to detect small electric field differences, then detection sensitivity is improved, but fixed pattern noise increases significantly
Solution Approach 1:
The patent applies preliminary action by performing baseline subtraction before signal amplification. The readout circuitry subtracts baseline voltages from pixel voltages to remove fixed pattern components, and only then is the remaining signal amplified. This prevents the amplification of fixed pattern noise while still achieving sufficient signal strength for detection.
Solution Approach 2:
The patent extracts the harmful fixed pattern noise component from the total signal by measuring and subtracting baseline voltages. The baseline measurement is performed separately without a finger present, and this extracted baseline component is then removed from the actual fingerprint signals, leaving only the relevant fingerprint information to be amplified.
2Power
If baseline signal is amplified along with the difference signal, then signal strength is improved, but signal to noise ratio deteriorates
Solution Approach 1:
The patent extracts the baseline signal component from the total pixel voltage using dedicated baseline measurement circuitry. By separating the baseline measurement (performed without a finger) from the fingerprint signal measurement, the system can subtract the extracted baseline from the pixel voltage, amplifying only the remaining fingerprint signal without amplifying the fixed pattern noise.
Solution Approach 2:
The patent performs preliminary baseline subtraction before the amplification stage. The readout circuitry subtracts the baseline voltage from each pixel voltage in advance, and only the difference signal (containing fingerprint information) is then amplified. This preliminary action ensures that the amplification process strengthens the signal without proportionally increasing the fixed pattern noise.
3Measurement precision
If fixed pattern noise is reduced through baseline subtraction, then signal to noise ratio is improved, but device complexity increases
Solution Approach 1:
The patent merges the baseline measurement and subtraction functions directly into the readout circuitry of the capacitive sensing array. Rather than using separate external circuitry, the baseline measurement, storage, and subtraction operations are integrated into the on-chip readout circuit, sharing common circuit elements and reducing overall device complexity.
Solution Approach 2:
The readout circuitry performs its own baseline measurement and subtraction operations autonomously without requiring external processing. The circuit measures baseline voltages, stores them in memory, and automatically subtracts them from pixel voltages during normal operation, making the noise reduction function self-contained and reducing system complexity.
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 effectively reduces fixed pattern noise, enhancing the accuracy of fingerprint detection and improving the security of biometric systems by increasing the signal-to-noise ratio and maintaining the fingerprint information of interest.
Implementation Method 1
capacitive sensing elements to detect fingerprint images for collection. Such sensors are able to detect electric field differences between ridges and valleys of the fingerprint of a finger in contact with a contact surface of the consumer electronic device adapted for this purpose by measuring charge accumulated by the capacitive sensing elements
Implementation Method 2
a block first digital to analog converter (DAC); at least one second DACs. The first DAC is adapted to provide a block baseline voltage for each block of capacitive sensing elements. The at least one second DAC is adapted to provide a pixel baseline voltage difference
Data Source
AI summary
A capacitive fingerprint sensor includes an array of capacitive sensing elements, readout circuitry electrically coupled to the array of capacitive sensing elements, a block first digital to analog converter (DAC), at least one second DAC, and at least one summing junction electrically coupled to the readout circuitry, the first DAC, and the at least one second DAC. The readout circuitry is adapted to read out pixel voltages from a group of each block of capacitive sensing elements. The first DAC is adapted to provide a block baseline voltage for each block of capacitive sensing elements. The second DAC is adapted to provide a pixel baseline voltage difference for one capacitive sensing element of each group of each block. The summing junction is adapted to subtract the received block baseline voltage and the received pixel baseline voltage difference from the corresponding pixel voltage of each row of each block.