Analog Sampling Circuitry for Noise Suppression in Capacitive Fingerprint Sensors
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Solution Overview
Problem
Capacitive fingerprint sensing devices face challenges in accurately capturing fingerprint images due to common mode noise, particularly from external sources like chargers, which can corrupt measurements and reduce image quality.
Innovation Solution
The implementation of an analog sampling circuitry with at least three sample and hold circuits, controlled to capture samples at specific times relative to a drive signal's voltage levels, allowing for the suppression of noise components before analog-to-digital conversion, enabling real-time filtering and improved noise reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple digital readings are averaged to reduce common mode noise, then noise suppression is improved, but the fingerprint image capture time increases due to multiple ADC conversions
Solution Approach 1:
The patent replaces digital noise reduction methods (requiring multiple ADC conversions) with an analog noise cancellation circuit that performs differential amplification in the analog domain. This substitution of the signal processing domain from digital to analog allows noise suppression to occur before ADC conversion, dramatically reducing the time required for fingerprint capture while maintaining effective common mode noise rejection.
2Reliability
If post-processing methods are used to subtract common mode noise, then noise reduction is improved, but measurement accuracy deteriorates when saturation occurs during analog sampling
Solution Approach 1:
The patent implements noise cancellation in the analog domain before ADC conversion by using a differential amplifier configuration. This preliminary action of subtracting common mode noise at the analog stage prevents saturation during sampling, ensuring that the full dynamic range of the ADC is utilized for capturing valid fingerprint data rather than being wasted on saturated values that cannot be recovered through post-processing.
3Measurement precision
If the sensing device detects smaller capacitive differences for accurate fingerprint capture, then measurement precision is improved, but sensitivity to common mode noise increases
Solution Approach 1:
The patent converts the harmful common mode noise signal into a useful differential signal by using the noise itself as part of the measurement process. The differential amplifier configuration captures both the fingerprint signal and common mode noise, then uses the known characteristics of the drive signal to mathematically eliminate the noise component, thereby transforming the noise from a harmful interference into a manageable parameter that can be subtracted out.
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 suppresses common mode noise, enhancing the accuracy and speed of fingerprint image capture by performing analog filtering prior to AD-conversion, thereby improving image quality and reducing the time required for fingerprint sensing.
Implementation Method 1
analog sampling circuitry comprising at least three analog sample and hold circuits arranged to sample the sensing signal, and a sampling control unit for individually controlling each of the at least three analog sample and hold circuits to capture a sample at a specified time
Implementation Method 2
All capacitive fingerprint sensors provide a measure indicative of the capacitance between each of several sensing structures and a finger placed on or moved across the surface of the fingerprint sensor
Data Source
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Figure 3
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AI summary
There is provided a capacitive fingerprint sensing device comprising a plurality of sensing elements, sensing circuitry for providing an analog sensing signal, drive signal circuitry providing a drive signal comprising a drive pulse having a maximum level and a minimum level, providing a change in potential difference between the finger and the sensing structure, analog sampling circuitry comprising at least three analog sample and hold circuits arranged to sample the sensing signal, and a sampling control unit for individually controlling the sample and hold circuits to capture a sample at a specified time, wherein the samples comprises one sample captured when the drive signal is at a first voltage level Vi and one sample captured when the drive signal is at a second voltage level V2, different from V-i; and an analog-to-digital converter, ADC, configured to convert a combination of the samples into a digital signal, wherein the three samples are captured times such that a noise component is suppressed from the sensing signal when the combination is formed.