Capacitance Sensing Circuit With Parasitic Current Cancellation
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Solution Overview
Problem
Capacitance detection devices face reduced detection sensitivity and accuracy due to the presence of parasitic capacitors, which are not completely eliminated by conventional active shields, and require precise setting of correction capacitors to cancel out parasitic effects.
Innovation Solution
A capacitance detection device that uses a first and second alternating current voltage output circuit to cancel out the alternating current through parasitic capacitors, with the second voltage output circuit attenuating the first voltage to ensure zero drive current in the absence of an object, and an operational amplifier to amplify the detection signal, reducing noise and improving accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a shield electrode is arranged around the detection electrode to reduce parasitic capacitor capacitance, then the detection sensitivity is improved, but the parasitic capacitor cannot be completely eliminated and residual effects remain
Solution Approach 1:
The invention applies preliminary anti-action by generating a cancellation signal that pre-compensates for the parasitic capacitor effects. The cancellation signal is generated based on the relationship between the drive signal and the parasitic capacitance, and is subtracted from the detection signal before processing, thereby eliminating the harmful effects of residual parasitic capacitors and improving measurement precision.
2Measurement precision
If correction capacitors are used to cancel out parasitic capacitor effects, then detection accuracy is improved, but the capacitance of correction capacitors must be precisely set to minute values which is difficult to achieve
Solution Approach 1:
The invention replaces the mechanical/physical approach of using precision correction capacitors with an electronic signal processing approach. Instead of physically setting minute capacitance values, the system uses a cancellation signal generated through operational amplifiers and capacitors in a circuit configuration, where the capacitance values can be much larger and easier to manufacture with standard tolerances. The key is the circuit topology and signal relationship rather than precise component values.
3Power
If the amplitude of output voltage is increased by parasitic capacitor effects, then the dynamic range of measured capacitance is reduced and detection sensitivity deteriorates
Solution Approach 1:
The invention extracts and separates the parasitic capacitor component from the total detection signal. By generating a cancellation signal that represents the parasitic capacitance contribution and subtracting it from the detection signal, the system isolates the true capacitance measurement from the harmful parasitic effects, thereby restoring the dynamic range and detection sensitivity.
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 suppresses the deterioration of detection sensitivity and accuracy caused by parasitic capacitors, allowing for precise detection of capacitance between an object and a detection electrode with improved noise reduction and dynamic range.
Implementation Method 1
The arrangement of the active shield reduces the capacitance of the parasitic capacitor because the detection electrode is less susceptible to electrostatic coupling with the surrounding conductor. Also, because the active shield is at the same potential as the detection electrode, the capacitance between the active shield and the detection electrode does not affect the detection result.
Implementation Method 2
a first voltage output circuit configured to output a first alternating current voltage supplied to a shield electrode disposed proximate to a detection electrode; a second voltage output circuit configured to output a second alternating current voltage having frequency and phase equal to those of the first alternating current voltage and amplitude less than that of the first alternating current voltage
Implementation Method 3
an operational amplifier to amplify the detection signal, reducing noise and improving accuracy
Data Source
AI summary
A capacitance detection device includes a first voltage output circuit configured to output a first alternating current voltage supplied to a shield electrode provided proximate to a detection electrode, a second voltage output circuit configured to output a second alternating current voltage whose frequency and phase are the same as that of the first alternating current voltage and whose amplitude is less than that of the first alternating current voltage, and a current output circuit configured to output a driving current Is to the detection electrode so that the difference between the voltage of the detection electrode and the second alternating current voltage becomes smaller, and output a detection signal corresponding to the driving current. The second voltage output circuit outputs a second alternating current voltage whose amplitude is adjusted so that the driving current in the absence of the object proximate to the detection electrode.