Capacitance Sensing Circuit With Parasitic Current Cancellation
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Solution Overview
Problem
Capacitance detection devices face reduced sensitivity and accuracy due to the presence of parasitic capacitors, which are not completely eliminated by conventional active shields, leading to noise and reduced dynamic range.
Innovation Solution
A capacitance detection device is designed with a configuration that includes a first and second alternating current voltage output circuit, a current output circuit, a subtraction circuit, and an A/D converter, where the second voltage output circuit cancels out the current through the parasitic capacitor by adjusting its amplitude, and the subtraction circuit removes the parasitic components from the detection signal, allowing for accurate detection of capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If self-capacitance detection type is used, then capacitance detection sensitivity is improved, but parasitic capacitor effect reduces detection sensitivity and dynamic range
Solution Approach 1:
The patent extracts and separates the parasitic capacitance component from the total measured capacitance. By using a test electrode to measure parasitic capacitance independently and then subtracting this value from the measurement electrode's total capacitance reading, the system isolates the actual object capacitance for accurate detection.
Solution Approach 2:
The patent introduces a test electrode as an intermediary element that specifically measures the parasitic capacitance between the measurement electrode and ground. This intermediary measurement enables the system to identify and compensate for the harmful parasitic effect without directly affecting the primary detection process.
2Object-affected harmful factors
If active shield is arranged around detection electrode, then parasitic capacitor capacitance is reduced, but residual parasitic capacitors still affect detection sensitivity
Solution Approach 1:
The patent implements a feedback mechanism where the measured parasitic capacitance value is used to compensate for its own effect in the final detection calculation. The system continuously measures parasitic capacitance and applies this information to correct the measurement electrode's readings, maintaining high detection sensitivity despite the presence of residual parasitic capacitors.
Solution Approach 2:
The patent changes the parameter measurement approach by separately measuring parasitic capacitance at different frequencies or conditions and using this data to adjust and correct the primary capacitance measurement parameters, thereby compensating for residual parasitic effects.
3Measurement precision
If correction circuit is used to cancel parasitic capacitor effect, then phase shift is corrected, but amplitude increase reduces dynamic range
Solution Approach 1:
The patent applies partial correction by only compensating for the parasitic capacitance effect to the extent necessary for accurate measurement, rather than fully amplifying the signal. This partial action maintains the dynamic range while still achieving the needed phase shift correction and parasitic effect compensation.
4Object-affected harmful factors
If correction capacitor with minute capacitance value is used, then parasitic capacitor effect is canceled, but setting accuracy becomes difficult
Solution Approach 1:
The patent creates a copy of the parasitic capacitance effect through the test electrode measurement and uses this copied information to compensate for the actual parasitic effect. Instead of requiring a physically minute correction capacitor, the system uses measured data from a test electrode to mathematically compensate for parasitic effects, avoiding the manufacturing precision issues of minute capacitors.
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 configuration effectively suppresses the deterioration of detection sensitivity and accuracy caused by parasitic capacitors, enabling precise and sensitive capacitance detection even in the presence of parasitic capacitors.
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
Implementation Method 2
Capacitance detection methods used in the input devices typically include a mutual capacitance detection type and a self-capacitance detection type
Data Source
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AI summary
The capacitance detection device 2 includes a first voltage output circuit configured to output a first alternating current voltage supplied to a shield electrode Ea provided proximate to a detection electrode Es, a second voltage output circuit configured to output a second alternating current voltage V2 whose frequency and phase are the same as that of the first alternating current voltage V1 and whose amplitude is less than that of the first alternating current voltage V1, and a current output circuit 23 configured to output a driving current Is to the detection electrode Es so that the difference between the voltage of the detection electrode Es and the second alternating current voltage V2 becomes smaller, and output a detection signal Vo corresponding to the driving current Is. The second voltage output circuit outputs a second alternating current voltage V2 whose amplitude is adjusted so that the driving current Is in the absence of the object 6 proximate to the detection electrode Es.