Differential Capacitive Image Sensing Circuit for Power Noise Rejection
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Solution Overview
Problem
Capacitive image sensing devices, such as fingerprint sensors, are prone to failures due to interference from power noise caused by system voltage, which affects the accuracy of sensing signals.
Innovation Solution
A symmetric readout circuit structure is implemented using two charge amplifiers with similar circuit characteristics, where one charge amplifier receives sensing signals from sensing electrodes and the other provides a reference voltage signal to a differential amplifier, effectively eliminating common-mode noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a typical readout circuit is used in the capacitive image sensing device, then the device can operate with a simple circuit structure, but the sensing signal accuracy deteriorates due to interference from power noise of system voltage
Solution Approach 1:
The readout circuit is segmented into two separate charge amplifiers with identical structures, where one amplifier processes the sensing signal and the other generates a reference signal. This segmentation allows the circuit to handle both the sensing signal and reference signal through symmetric paths, thereby eliminating the impact of power noise on sensing accuracy while maintaining operational functionality.
2Device complexity
If a single charge amplifier is used to process sensing signals, then the circuit remains simple, but the sensing signal becomes vulnerable to power noise interference from system voltage
Solution Approach 1:
A second charge amplifier is designed as an exact copy of the first charge amplifier, but instead of processing sensing signals, it generates a reference signal that mirrors the power noise characteristics. This copied structure ensures that both amplifiers experience identical power noise interference, allowing the differential amplifier to subtract the noise component and retrieve the clean sensing signal.
3Ease of manufacture
If asymmetric readout circuit configuration is used, then circuit design is simpler, but common-mode noise from power supply cannot be effectively rejected
Solution Approach 1:
While the individual charge amplifier structures are symmetric, their functional assignments create a controlled asymmetry: one amplifier processes the sensing signal and the other generates the reference signal. This functional asymmetry within a symmetric structural framework enables the differential amplifier to reject common-mode noise while maintaining ease of manufacture through standardized amplifier design.
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 configuration significantly reduces interference from power noise, enhancing the accuracy and reliability of sensing signals by offsetting common-mode noise, thereby improving the performance of capacitive image sensing devices.
Implementation Method 1
The differential amplifier has a first input terminal, a second input terminal and a differential output terminal pair. A first input terminal of the differential amplifier is coupled to the first charge amplifier. A second input terminal of the differential amplifier is coupled to the second charge amplifier.
Data Source
AI summary
A capacitive image sensing device is provided. The capacitive image sensing device includes a first charge amplifier, a second charge amplifier and a differential amplifier. The first charge amplifier is coupled to one of sensing electrodes of a sensor array. The differential amplifier has a first input terminal, a second input terminal and a differential output terminal pair. The first input terminal of the differential amplifier is coupled to the first charge amplifier. The second input terminal of the differential amplifier is coupled to the second charge amplifier.