Capacitive Touch Panel Noise Rejection via Differential Excitation
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Solution Overview
Problem
Capacitive touch panels face noise interference issues, particularly 60-cycle noise, which can obscure touch events due to the small capacitance changes caused by finger contact, leading to inaccurate touch location detection.
Innovation Solution
The implementation of a touch panel controller with a differential excitation method, where adjacent electrodes receive alternating polarity excitation voltages during scan periods, and the use of an analog front end (AFE) to integrate and combine measurement signals, effectively reducing noise interference by operating in a differential arrangement and digitizing the signals for noise rejection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional single-ended excitation is used to simplify the circuit, then device complexity is reduced, but noise rejection capability deteriorates due to 60-cycle noise interference
Solution Approach 1:
The excitation signal is segmented into two complementary phases with opposite polarities applied to adjacent electrodes. This segmentation allows the system to differentiate between genuine touch events and noise by comparing the differential responses, thereby improving noise rejection without significantly increasing overall system complexity.
Solution Approach 2:
The excitation voltage polarity is changed between adjacent electrodes and time phases. By applying +Vexc to one electrode and -Vexc to its neighbor, then reversing the polarities in subsequent phases, the system creates parameter variations that help distinguish touch events from 60-cycle noise through differential measurement.
2Reliability
If differential excitation with alternating polarity is applied to improve noise rejection, then noise rejection capability is improved, but device complexity increases due to additional excitation phases and signal processing
Solution Approach 1:
The system employs periodic excitation phases where adjacent electrodes are alternately excited with opposite polarities in a repeating cycle. This periodic action creates a structured measurement pattern that simplifies the control logic compared to continuous complex modulation, as the controller only needs to execute the repeating phase sequence.
Solution Approach 2:
The measurement of multiple electrodes is merged into a differential comparison process. By combining the responses from adjacent electrodes excited with opposite polarities and processing them through integrated summing circuits, the system achieves noise rejection through signal combination rather than requiring separate complex filtering circuits for each electrode.
3Measurement precision
If integration time is increased to improve signal-to-noise ratio, then measurement precision is improved, but scan speed deteriorates due to longer integration periods
Solution Approach 1:
The system applies excitation voltages that are excessive in magnitude (higher than minimum required) to achieve sufficient signal levels during brief integration windows. This allows the use of shorter integration times while maintaining adequate signal-to-noise ratios, thereby preserving scan speed without sacrificing measurement precision.
Solution Approach 2:
The excitation signals are applied in advance during dedicated excitation phases before the measurement integration begins. This preliminary action ensures that the capacitive coupling is fully established and any transient effects have settled, allowing for shorter integration times while still achieving high measurement precision.
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 enhances noise rejection and improves the accuracy of touch event detection by generating a capacitance profile that effectively filters out noise, allowing for precise determination of touch locations despite noise interference.
Implementation Method 1
the AFE integrates the first and second measurement signal to generate first and second integrated signals
Implementation Method 2
projected capacitive sensing to determine touch locations... the capacitance between the two plates decreases
Implementation Method 3
the interface provides a first excitation voltage to a first terminal... and a second excitation voltage to a second terminal... wherein the second excitation voltage has the opposite polarity of the first excitation voltage
Data Source
AI summary
A method is provided. A first voltage is applied to a first set of column electrodes within a touch panel during a first interval, and a second voltage is applied to a second set of column electrodes within the touch panel during the first interval. The first and second sets of electrodes are adjacent to one another, and the second voltage has the opposite polarity of the first voltage. During the first interval, a first measurement signal is received from a set of row electrodes in the touch panel, and the first measurement signal is integrated to generate a first integrated signal. The first voltage is applied to the second set of column electrodes within the touch panel during a second interval, and the second voltage is applied to the first set of column electrodes within the touch panel during the second interval. During the second interval, a second measurement signal is received from the set of row electrodes in the touch panel, and the second measurement signal is integrated to generate a second integrated signal. Then, the first and second integrated signals are combined to generate an output signal.