Active Integrator Circuit for Capacitive Touch Sensing
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Solution Overview
Problem
Conventional capacitive sensing receiving circuits face challenges in accurately measuring small capacitance changes caused by touch due to a large DC component and noise interference, resulting in a low signal-to-noise ratio.
Innovation Solution
An active integrator circuit is employed, which continuously integrates the response signal from the touch sense array using a switched-capacitive approach, featuring operational amplifiers with variable feedback capacitors for sensitivity calibration and a sample-and-hold circuit for full-wave demodulation, improving the signal-to-noise ratio by matching the switching frequency with the fundamental frequency of the response signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional capacitive sensing receiving circuits are used to measure capacitance changes, then the measurement process is simple, but the signal-to-noise ratio is low due to large DC component and noise interference
Solution Approach 1:
The patent extracts and removes the large DC component from the capacitance measurement signal through differential measurement techniques. By measuring the difference between two capacitive sensors instead of absolute capacitance values, the common-mode DC component is rejected, leaving only the small AC signal containing the touch information.
Solution Approach 2:
The patent employs periodic switching of the capacitive sensors between different measurement states (e.g., connected to different reference voltages or each other). This periodic action converts the static capacitance measurement into a dynamic signal that can be processed to extract the small capacitance change while rejecting noise and DC components.
2Measurement precision
If conventional charge-to-voltage converters are used, then the circuit complexity is low, but the signal-to-noise ratio remains low
Solution Approach 1:
The patent merges multiple functions into the capacitive sensor structure itself. The sensors are configured to perform both sensing and signal conditioning functions, with the differential pair of sensors acting as both the measurement element and the noise-rejection mechanism, reducing the need for separate complex signal processing circuits.
Solution Approach 2:
The patent changes the measurement parameter from absolute capacitance to differential capacitance change. By measuring the difference in capacitance between two sensors rather than absolute values, the system achieves better signal-to-noise ratio while using relatively simple circuitry to detect the differential signal.
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 enhances the signal-to-noise ratio and improves the accuracy of capacitance measurements, allowing for more precise detection of touch locations and force magnitude on capacitive touch sense arrays.
Implementation Method 1
an active integrator receiving circuit for a touch sense array... continuously integrates the response signal from the touch sense array using a switched-capacitive approach
Implementation Method 2
A first feedback capacitor is configured to store a charge responsive to the positive portion of the response signal, and a second feedback capacitor is configured to store a charge responsive to the negative portion of the response signal
Data Source
AI summary
An active integrator for sensing capacitance of a touch sense array is disclosed. The active integrator is configured to receive from the touch sense array a response signal having a positive portion and a negative portion. The response signal is representative of a presence or an absence of a conductive object on the touch sense array. The active integrator is configured to continuously integrate the response signal.