Capacitive Sensing Analog Front End Noise Cancellation
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Solution Overview
Problem
Capacitive touch screen systems face challenges in accurately detecting touch events due to noise interference from a user's body, which affects the signal-to-noise ratio (SNR) and makes it difficult to extract precise touch information.
Innovation Solution
A capacitive sensing analog front end with a capacitance-to-voltage converter, summer, low pass filter, and sample-and-hold circuit that extracts the DC shift of a touch signal during a monitoring period and subtracts it before integration, improving the SNR by analog cancellation of noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional capacitive sensing is used to detect touch events, then the system can detect touch presence, but the signal-to-noise ratio deteriorates due to body noise interference
Solution Approach 1:
The patent extracts and separates the noise signal from the touch signal by using a dedicated noise measurement path. The system measures noise independently through the same capacitor being tested, then subtracts this noise component from the total measured signal, effectively removing the harmful body noise interference from the touch detection measurement.
Solution Approach 2:
The patent introduces an intermediary measurement process that uses the capacitor itself as a noise sensor during specific time periods. By measuring the capacitor's response to noise separately and using this as a reference, the system creates an intermediary signal that helps cancel out the body noise from the actual touch measurement, improving overall detection accuracy.
2Measurement precision
If measurement time is increased to improve accuracy, then touch information can be extracted more precisely, but the response time increases
Solution Approach 1:
The patent implements periodic switching between noise measurement mode and touch measurement mode. During specific time intervals, the system periodically measures noise through the capacitor, then periodically measures the actual touch signal. This periodic action allows accurate noise characterization without continuously occupying the measurement channel, balancing precision with response time.
Solution Approach 2:
The patent performs preliminary noise measurement before the actual touch measurement. By measuring and storing the noise baseline in advance through the same capacitor, the system prepares a reference signal that can be subtracted from subsequent touch measurements. This preliminary action improves accuracy by accounting for noise while minimizing the time required during actual touch detection.
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 enhances the SNR, allowing for more accurate detection of touch events by effectively canceling low-frequency noise and filtering high-frequency noise, resulting in improved touch sensitivity and precision.
Implementation Method 1
measuring the Ci and Rj capacitance (Cs) variations
Implementation Method 2
low pass filter having an input coupled to an output of the summer and an output for providing an output signal
Data Source
AI summary
A capacitive sensing analog front end for a touchscreen system having an improved signal-to-noise ratio includes a capacitance-to-voltage converter having an input for coupling to an external sampling capacitor, a summer having a first input coupled to an output of the capacitance-to-voltage converter, a low pass filter having an input coupled to an output of the summer and an output for providing an output signal; and a sample-and-hold circuit having an input coupled to the output of the low pass filter and an output coupled to a second input of the summer. The signal-to-noise ratio of the touchscreen system is improved by extracting the DC shift of a touch signal during a monitoring period and then subtracting the DC shift before integrating the touch signal.