Capacitive Sensing Circuit With Filtering for Low-SNR Detection
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Solution Overview
Problem
Capacitive sensing devices face challenges in extracting the signal of interest from noise, particularly in touch devices, where the signal-to-noise ratio (SNR) is low due to noise components introduced during capacitance measurement, making it difficult to accurately detect the presence or proximity of objects.
Innovation Solution
The implementation of a capacitance to voltage sensing circuit that utilizes a higher reference voltage (Vdd) and incorporates amplifier and integrator circuits with filtering structures to enhance the signal magnitude and reduce noise interference, improving the SNR by amplifying the capacitive charge and integrating it effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional capacitance sensing circuitry is used, then the device structure is simple, but the signal-to-noise ratio is low making it difficult to extract the signal of interest
Solution Approach 1:
The sensing circuit is divided into multiple functional blocks: a capacitive-to-voltage conversion block that converts capacitance changes to voltage signals, a filtering block with high-pass and low-pass filters that separates different frequency components, and an analysis block that processes the filtered signals. This segmentation allows each block to optimize for its specific function, improving overall signal-to-noise ratio while keeping individual blocks relatively simple.
Solution Approach 2:
The patent introduces intermediate filtering stages between the capacitance sensing element and the final measurement. High-pass and low-pass filters act as intermediaries that selectively pass desired signal frequency components while blocking noise components. This intermediary approach enables precise signal extraction without requiring the entire circuit to be complex.
2Measurement precision
If higher reference voltage is used to enhance signal amplification, then the desired signal is boosted, but noise components may also be amplified
Solution Approach 1:
The patent employs periodic switching of the reference voltage applied to the measurement capacitor. By periodically modulating the reference voltage at a specific frequency, the desired signal is encoded at that frequency. The filtering block then uses frequency-selective filtering to extract only the modulated signal component while rejecting noise at other frequencies. This periodic action enables signal amplification without proportionally amplifying noise.
Solution Approach 2:
The circuit performs preliminary filtering of the capacitive signal before final amplification and measurement. High-pass and low-pass filters are applied in advance to remove out-of-band noise components before the signal undergoes full amplification. This preliminary action prevents noise amplification while still achieving the desired signal boost in the final measurement stage.
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 solution significantly improves the signal-to-noise ratio, allowing for better extraction of the desired signal from noise, enhancing the accuracy of touch detection and proximity sensing in capacitive devices.
Implementation Method 1
The touch device is comprised, generally speaking, of a capacitance whose value is modulated by the presence of the touching or approaching object
Implementation Method 2
The operational amplifier 12 and holding capacitor Ch function, as will be described in more detail below, as an integrator circuit
Data Source
AI summary
A circuit for converting a measured variable capacitance to an output voltage signal includes a charge amplifier circuit selectively coupled to an integrator circuit. The charge amplifier circuit, in one implementation, is configured as a high pass filter. In another implementation, the charge amplifier circuit is configured as a combination high pass and low pass filter. The charge amplifier circuit is selectively coupled to the integrator circuit when the circuit forces a switch in voltage across a measurement capacitor.