Capacitive Touch Panel Sensing with Differential Noise Cancellation
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Solution Overview
Problem
Capacitive touch panels face challenges in measuring contact capacitance due to significant stray capacitance noise, especially when integrated with LCD screens, which complicates accurate capacitance measurement and requires numerous signal lines for electrode measurement.
Innovation Solution
A capacitance sensing apparatus with a differential comparator and multiplexer that uses a first and second discharging current source to generate a pulse width modulation signal, suppressing noise by comparing a modified reference voltage with the input voltage at the connection node, and reducing signal lines through multiplexer control signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If capacitance sensing is performed in a projected capacitive touch panel integrated with LCD screen, then multi-touch function is realized, but stray capacitance noise significantly degrades measurement accuracy
Solution Approach 1:
The patent converts the harmful stray capacitance noise into a beneficial reference signal. By intentionally generating a reference voltage that includes the same noise components as the input signal, the differential comparator can subtract the noise, leaving only the useful capacitance information. This transforms the noise from a detrimental factor into a useful reference for accurate measurement.
Solution Approach 2:
The patent introduces a reference voltage source as an intermediary element that mediates between the noisy measurement environment and the differential comparator. This reference voltage acts as a mediator that carries the noise signature to the comparator, enabling noise cancellation without requiring complex filtering that would affect the useful signal.
2Measurement precision
If multiple electrodes are measured individually, then capacitance measurement is achieved, but numerous signal lines are required increasing device complexity
Solution Approach 1:
The patent merges the measurement of multiple electrodes into a single measurement channel by using a multiplexer. The multiplexer sequentially connects different electrodes to the single capacitance sensing circuit, combining multiple measurement functions into one physical channel. This eliminates the need for separate signal lines for each electrode while maintaining full measurement capability.
Solution Approach 2:
The patent employs periodic switching of the multiplexer to sequentially measure different electrodes. By periodically connecting each electrode to the sensing circuit in turn, the system achieves time-multiplexed measurement of all electrodes using a single channel, thereby reducing the number of required signal lines while maintaining comprehensive measurement coverage.
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 enables precise capacitance measurement by suppressing noise and reducing the number of signal lines needed, enhancing measurement accuracy and operational efficiency in capacitive touch panels, particularly when integrated with LCD screens.
Implementation Method 1
the capacitance sensing apparatus measures a capacitance on the electrode
Data Source
AI summary
A capacitive touch panel and capacitance sensing apparatus and method for the same are disclosed. The capacitive touch panel includes a plurality of electrodes, a multiplexer and a capacitance sensing apparatus. The multiplexer selectively connects one of the electrodes to the capacitance sensing apparatus. The capacitance sensing apparatus includes a reference voltage source and a differential comparator, where the reference voltage source generates a modified reference voltage according to noise on stray capacitance of the capacitive touch panel. In a discharging process, the comparator compares an input voltage at the selected electrode and the modified reference voltage. The capacitance change in the electrode can be precisely detected because the noise is eliminated by differential comparison.