Capacitive Touch Sensing Circuit with Feedback Charge Accumulation
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Solution Overview
Problem
Capacitive touch panels face challenges in accurately determining the position of a touch point due to small changes in mutual capacitance values, leading to difficulties in distinguishing between touch and non-touch points.
Innovation Solution
The sensing circuit incorporates a feedback capacitor and multiple switches controlled by specific control signals, generating multiple pulses during driving cycles to accumulate charge and enhance voltage changes, allowing for more precise detection of touch points.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a simple sensing circuit is used, then the device complexity is low, but the measurement precision of touch point position is insufficient due to small mutual capacitance changes
Solution Approach 1:
The sensing circuit operates in periodic cycles with multiple sub-driving cycles (N cycles). Each cycle includes charging phases where driving signals are applied to driving electrodes, and discharge phases where receiving electrodes are connected to ground through switches. This periodic operation with multiple cycles accumulates the small mutual capacitance changes over time, transforming them into measurable voltage changes that enable accurate touch point detection.
Solution Approach 2:
The sensing circuit incorporates a feedback capacitor connected between the output terminal and the inverting input terminal of the operational amplifier. This feedback mechanism amplifies the small voltage changes caused by mutual capacitance variations. The feedback capacitor integrates the charge transferred during multiple driving cycles, providing continuous feedback that enhances the output voltage signal and improves measurement precision without requiring complex external circuitry.
2Measurement precision
If multiple sub-driving cycles are used to accumulate charge, then the signal-to-noise ratio improves, but the duration of action increases
Solution Approach 1:
The sensing circuit implements N sub-driving cycles within a single sensing period. Each sub-driving cycle consists of rapid charging and discharging phases that are completed within a short time frame. By repeating these periodic cycles N times, the circuit accumulates sufficient charge to achieve a high signal-to-noise ratio while maintaining a relatively short total sensing duration. The periodic operation allows parallel accumulation of signal without proportionally increasing the overall time.
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 increases the sensitivity of the touch panel, enabling the backend circuit to accurately determine the position of touch points by amplifying the output voltage, thereby improving the signal-to-noise ratio and overall sensing capability.
Implementation Method 1
a feedback capacitor Cf connected between a second end of the first switch and a negative input terminal of the operation amplifier
Implementation Method 2
generating multiple pulses during driving cycles to accumulate charge and enhance voltage changes
Implementation Method 3
an operation amplifier with a positive input terminal connected with a ground terminal, a negative input terminal connected with a first end of the fourth switch, and an output terminal connected with a second end of the second switch, a second end of the third switch and a second end of the fourth switch
Implementation Method 4
mutual capacitances Cs11 ̃Cs66 are existed between the driving electrodes d1 ̃d6 and respective receiving electrodes r1 ̃r6
Implementation Method 5
the coupling charge of the mutual capacitances Cs11 ̃Cs66 will be transmitted to the sensing circuits s1 ̃s6 through the receiving electrodes r1 ̃r6
Data Source
AI summary
A sensing circuit of a capacitive touch panel includes a first switch, a second switch, a third switch, a feedback capacitor, a fourth switch and an operation amplifier. The first switch and the second switch have respective first ends connected with a receiving electrode. The third switch has a first end connected with a second end of the first switch. The feedback capacitor has a first end connected with the second end of the first switch. The fourth switch has a first end connected with a second end of the feedback capacitor. The operation amplifier has a positive input terminal connected with a ground terminal, a negative input terminal connected with the fourth switch, and an output terminal connected with the second, third and fourth switches. These switches are controlled during a driving cycle of the driving signal, so that an output voltage is outputted from the operation amplifier.