Capacitive Fingerprint Sensor Noise Detection Circuit
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Solution Overview
Problem
Capacitive fingerprint sensing devices are susceptible to common mode noise, which affects image quality and is challenging to suppress effectively without increasing processing time or risking saturation during analog sampling.
Innovation Solution
The device senses capacitive coupling for a predetermined period without a drive signal to characterize noise, allowing for the selection of optimal sensor settings that minimize noise, thereby improving image capture and dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If noise reduction techniques such as averaging multiple digital readings are implemented, then common mode noise suppression is improved, but analog to digital conversion time increases
Solution Approach 1:
The patent applies preliminary action by performing noise characterization before actual fingerprint image capture. The system measures noise properties during a predetermined period before sampling, allowing the analog-to-digital converter to be pre-configured with optimal settings that minimize noise without requiring multiple conversions during the actual capture process.
Solution Approach 2:
The patent utilizes parameter changes by dynamically adjusting the analog-to-digital converter settings based on the characterized noise properties. The system modifies conversion parameters such as gain, sampling rate, or filtering characteristics according to the measured noise level and frequency content, thereby achieving noise suppression without increasing conversion time.
2Object-affected harmful factors
If post-processing methods are used to subtract common mode noise, then noise reduction is improved, but measurement values may be corrupted in case of saturation during analog sampling
Solution Approach 1:
The patent applies preliminary action by characterizing noise properties before the actual measurement process. By measuring noise during a predetermined period before sampling and storing these characteristics, the system can apply targeted noise reduction during post-processing without affecting the integrity of the actual fingerprint measurement values.
Solution Approach 2:
The patent applies segmentation by separating the noise measurement process from the fingerprint image capture process. The system divides the operation into distinct phases: noise characterization phase (before sampling) and image capture phase (during sampling). This segmentation ensures that noise measurements do not corrupt the actual fingerprint data, as each phase uses dedicated sampling periods.
3Productivity
If the drive signal is used to excite the capacitive sensor, then fingerprint image capture is enabled, but common mode noise can make the system ground to swing looking like an additional drive signal
Solution Approach 1:
The patent applies feedback by using the characterized common mode noise properties to adjust the drive signal parameters or the sampling timing. The system measures the noise frequency and amplitude, then uses this information to optimize the drive signal frequency or phase, or to select optimal sampling windows that avoid noise peaks, thereby reducing noise interference while maintaining image capture capability.
Solution Approach 2:
The patent utilizes parameter changes by modifying drive signal characteristics such as frequency, amplitude, or waveform based on the characterized noise properties. The system adjusts these parameters to operate at frequencies or levels that minimize the impact of common mode noise, thereby maintaining productivity while reducing harmful factors.
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 enables robust noise suppression during image sampling, avoiding saturation and enhancing image quality by setting up sensor settings to reduce noise, allowing for higher gain and effective post-processing.
Implementation Method 1
sense a capacitive coupling between the finger and at least one sensing structure and provide a sensing signal indicative of the capacitive coupling
Data Source
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AI summary
A capacitive fingerprint sensing device and method therein for noise detection are disclosed. The capacitive fingerprint sensing device comprises a plurality of sensing elements, each comprises a sensing structure and is configured to sense a capacitive coupling between the sensing structure and a finger. The capacitive fingerprint sensing device further comprises sensing circuitry configured to provide a sensing signal indicative of the capacitive coupling between the finger and the sensing structures. The capacitive fingerprint sensing device comprises timing circuitry configured to control a timing of a drive signal. The fingerprint sensing device is controllable to operate in a noise-detection mode and in a fingerprint mode. When the fingerprint sensing device operates in the noise-detection mode, the fingerprint sensing device is configured to control the timing circuitry such that no drive signal is provided. The fingerprint sensing device senses a capacitive coupling between the finger and at least one sensing structure and provides a sensing signal indicative of the capacitive coupling between the finger and the sensing structures by means of said sensing circuitry.