Mutual-Capacitive Touch Sensing Circuit Noise Suppression
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Solution Overview
Problem
Mutual-capacitive touch panels face interference from noise generated by reversing liquid crystals, making it difficult for the sensing circuit to distinguish signal from noise due to their proximity, leading to poor signal-to-noise ratio and increased manufacturing costs.
Innovation Solution
A mutual-capacitive touch sensing circuit that includes an operational amplifier, internal and external capacitors, and switches, which uses specific charging voltage phases to charge capacitors and differentiate between signal and noise frequencies, allowing for effective noise suppression by moving noise to a high-frequency band and filtering it out.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the mutual-capacitive touch sensing circuit is placed closer to the panel to reduce distance, then the manufacturing cost is reduced, but the circuit is more easily interfered by noise from reversing liquid crystals
Solution Approach 1:
The patent applies periodic action by using alternating charging voltages with different polarities (first charging voltage and second charging voltage) applied in sequential phases to the capacitor. This periodic switching creates distinct time windows where noise from liquid crystal reversal occurs versus where touch signal measurement occurs, enabling the system to operate closer to the panel while maintaining noise immunity through temporal separation of measurement and noise-generation phases.
2Object-affected harmful factors
If the mutual-capacitive touch sensing circuit is placed farther from the finger signal source to reduce noise interference, then the signal-to-noise ratio improves, but the manufacturing cost increases due to additional hardware
Solution Approach 1:
The patent changes parameters by using alternating charging voltages with different polarities (positive and negative) applied in sequential phases. This parameter variation creates time-dependent measurement conditions where noise from liquid crystal reversal occurs during voltage transitions, while touch signal measurement occurs during stable phases. This allows the circuit to be positioned closer to the panel without additional hardware while maintaining high signal-to-noise ratio through temporal and polaritic differentiation.
3Device complexity
If a simple low-pass filter is used to filter out noise, then the device complexity is reduced, but the noise cannot be effectively filtered because the noise frequency is too close to the signal frequency
Solution Approach 1:
The patent uses periodic action with alternating charging phases where the capacitor is charged with first charging voltage during a first phase, then with second charging voltage during a second phase. This creates periodic measurement cycles where noise from liquid crystal reversal occurs during voltage transitions, while touch signal measurement occurs during stable phases. The periodic nature enables simple filtering approaches to work effectively by exploiting temporal separation rather than relying on frequency separation.
Solution Approach 2:
The patent maintains continuous useful action by continuously alternating between charging phases and measurement phases without interruption. The sequential application of first charging voltage and second charging voltage creates an ongoing cycle of noise generation followed by measurement, ensuring that useful touch signal detection occurs continuously while noise is systematically excluded through the alternating polarity scheme.
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 improves the signal-to-noise ratio, reduces capacitance driving time and power consumption, and enhances touch sensing performance, making it suitable for high-noise environments while maintaining a better touch sensing effect.
Implementation Method 1
sensing a capacitance variation of a capacitor when the mutual-capacitive touch panel is touched
Implementation Method 2
the first switch, the second switch and the third switch are switched in a specific order to make the first external charging voltage and the second external charging voltage to charge the capacitor, the first external charging voltage and the third external charging voltage to charge the capacitor
Data Source
AI summary
A mutual-capacitive touch sensing circuit includes an operational amplifier, an internal capacitor, a first switch˜a third switch. A first input terminal and a second input terminal of operational amplifier are coupled to a capacitor and ground respectively and an output terminal of operational amplifier outputs an output voltage. The internal capacitor is coupled to output terminal and first input terminal. The first switch is coupled to a first external charging voltage, capacitor and first input terminal. The second switch is coupled to a second external charging voltage and the capacitor. The third switch is coupled to a third external charging voltage, the second switch and capacitor. The second external charging voltage and third external charging voltage have same magnitude but opposite polarities. The first switch, second switch and third switch are switched in a specific order to selectively charge the capacitor with different external charging voltages.