Capacitance Sensing Circuit With Base-Capacitance Cancellation
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Solution Overview
Problem
Conventional capacitance detection technologies face challenges in achieving high sensitivity due to transient overshoot when using square waves, which reduces the effectiveness of signal cancellation and affects the sensitivity of capacitance detection, especially in scenarios like touch screens where small capacitance changes need to be detected.
Innovation Solution
A capacitance detecting device is designed with a charging module, an integrating circuit, and a cancellation capacitor, where the charging voltage, integrating capacitor, and cancellation capacitor are controlled to ensure that the contribution of the base capacitor to the output voltage is zero, thereby isolating useful signals from the base capacitance and improving sensitivity. This is achieved through specific voltage and capacitance relationships and phase control in the charging and discharging phases.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional cancellation circuit uses a square wave to drive the detecting electrode, then the base capacitance can be cancelled, but transient overshoot occurs which reduces the effectiveness of signal cancellation and sensitivity of capacitance detection
Solution Approach 1:
The patent applies preliminary anti-action by introducing a pre-charging phase before the main detection phase. During this pre-charging phase, the detecting electrode is charged to a predetermined voltage level, which counteracts the transient overshoot that would otherwise occur during switching. This preliminary action prepares the system in advance to eliminate the harmful overshoot effect, thereby improving the sensitivity and accuracy of capacitance detection without the distortion caused by conventional square wave driving.
2Measurement precision
If the base capacitance of the detecting electrode is large (e.g., 1 pF), then the electrode can maintain stable electrical characteristics, but the signal weight from small capacitance changes (e.g., 1 fF) becomes very small (0.1%), making it difficult to extract useful signals
Solution Approach 1:
The patent employs preliminary action by performing a pre-charging operation before the actual capacitance measurement. The detecting electrode is first charged to a known voltage level, storing electrical energy in the base capacitance. Then, during the measurement phase, only the change in capacitance (ΔC) needs to be detected, rather than measuring the absolute capacitance value. This preliminary charging action transforms the measurement problem from detecting a tiny signal (0.1% of base capacitance) to detecting a capacitance change against a pre-established electrical state, significantly improving signal weight ratio and measurement precision.
Solution Approach 2:
The patent applies the taking out principle by separating the base capacitance component from the measurement process. Through the pre-charging mechanism, the large base capacitance (1 pF) is effectively extracted and accounted for in the initial charging phase. The subsequent measurement only needs to detect the small capacitance change (1 fF) superimposed on this pre-established state. This extraction approach allows the system to ignore the magnitude of the base capacitance and focus solely on detecting the useful signal from capacitance changes, thereby improving signal weight ratio without compromising electrical stability.
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
The solution effectively enhances the sensitivity of capacitance detection by isolating useful signals from the base capacitance, avoiding the issue of transient overshoot and improving the detection of small capacitance changes, thus enhancing the accuracy of touch detection in devices like touch screens.
Implementation Method 1
a cancellation capacitor, configured to cancel a contribution of the base capacitor to an output voltage of the amplifier
Data Source
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AI summary
Provided are a capacitance detecting device (801), a touch device (800) and a terminal device (900), the capacitance detecting device (801) includes a charging module (110) configured to charge a base capacitor (130) between a detecting electrode and system ground; an integrating circuit (150) including an amplifier (151) and an integrating capacitor (152), where the integrating capacitor (152) is connected in parallel with the amplifier (151), and the integrating circuit (150) is configured to integrate, through the integrating capacitor (152), charges transferred from the base capacitor (130); a cancellation capacitor (120) configured to cancel a contribution of the base capacitor (130) to an output voltage of the amplifier (151); and a controlling module (140) configured to control the charging module (110) to charge the base capacitor (130) in a first phase, control the charging module (110) to stop charging the base capacitor (130) in a second phase, and control transfer of the charges on the base capacitor (130) to the integrating capacitor (152) in a third phase; where a charging voltage of the charging module (110), a capacitance of the integrating capacitor (152) and a capacitance of the cancellation capacitor (120) cause the contribution of the base capacitor (130) to the output voltage of the amplifier (151) to be zero.