Capacitance Measurement Circuit With Parasitic Capacitance Neutralization
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Solution Overview
Problem
Existing capacitive sensors face challenges in accurately measuring small capacitances due to high parasitic semiconductor capacitances, which limit performance and detection area, especially in flush sensors where shield signals are not feasible.
Innovation Solution
An inverting amplifier with high open-circuit voltage gain is used, connected to a measurement capacitor and a negative feedback capacitor, with a rectifier circuit and holding capacitor to buffer the voltage supply, and a periodic control voltage to maintain a constant output voltage, effectively neutralizing parasitic input capacitances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If shield electrode or field bundle electrode is used to reduce parasitic capacitance, then measurement precision is improved, but device complexity increases due to additional electrodes and control circuits
Solution Approach 1:
The patent extracts and eliminates the shield electrode from the measurement system. Instead of using a shield electrode connected to ground or requiring active control, the invention uses a measurement electrode that is electrically isolated from the housing, thereby removing the source of parasitic capacitance rather than compensating for it
Solution Approach 2:
Instead of the conventional approach where the housing serves as ground and a shield electrode is added to compensate for parasitic effects, the patent inverts the approach by making the measurement electrode electrically isolated from the housing, thereby eliminating the parasitic capacitance problem at its source
2Volume of moving object
If measurement electrode is positioned close to housing for compact design, then device size is reduced, but parasitic capacitance increases due to proximity
Solution Approach 1:
The patent extracts the measurement electrode from the conventional configuration where it is connected to or near the grounded housing. By electrically isolating the measurement electrode from the housing through insulation, the design allows compact positioning without the harmful parasitic capacitance that would normally result from proximity to the housing
Solution Approach 2:
The patent introduces an insulating layer as an intermediary between the measurement electrode and the housing. This insulating layer acts as a mediator that allows the electrode to be positioned close to the housing for compact design while preventing direct electrical coupling and the associated parasitic capacitance
3Measurement precision
If buffer amplifier with high input impedance is used to control shield electrode, then parasitic capacitance influence is reduced, but device complexity and cost increase
Solution Approach 1:
The patent extracts and eliminates the need for buffer amplifiers and active shield electrode control circuits. By electrically isolating the measurement electrode from the housing through insulation, the system removes the parasitic capacitance problem at its source, making complex compensation circuits unnecessary
Solution Approach 2:
The insulating structure between the measurement electrode and housing provides self-service by automatically eliminating parasitic capacitance effects without requiring active control circuits, buffer amplifiers, or complex compensation mechanisms
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 allows for precise and robust detection of very small capacitances with minimal influence from parasitic capacitances, enabling larger detection areas and stable circuit operation.
Implementation Method 1
at the input of the amplifier, there is a so-called virtual ground, i.e. the potential changes with respect to ground are virtually zero
Implementation Method 2
an inverting amplifier with high open-circuit voltage gain, the input of which is connected to a connection of the at least one measurement capacitor and to a connection of a negative feedback capacitor
Implementation Method 3
a rectifier circuit which is fed the voltage at the output of the inverting amplifier and which generates at its output a voltage corresponding to the voltage at the output of the inverting amplifier
Implementation Method 4
a holding capacitor for buffering the voltage supply of the inverting amplifier and the rectifier circuit
Implementation Method 5
a supply voltage connection of the inverting amplifier is supplied with a periodic control voltage with respect to the reference potential
Data Source
AI summary
Circuit arrangement measuring at least one measurement capacitor having inverting amplifier with high open-circuit voltage gain, input connected to connection of at least one measurement capacitor and to connection of negative feedback capacitor, another connection of at least one measurement capacitor connected to a reference potential and another connection of negative feedback capacitor connected to output of inverting amplifier; rectifier circuit fed voltage at the output of inverting amplifier and generates at its output voltage corresponding to voltage at output of inverting amplifier; a holding capacitor buffering voltage supply of inverting amplifier and rectifier circuit, a supply voltage connection of inverting amplifier supplied with a periodic control voltage with respect to reference potential and other supply voltage connection of inverting amplifier supplied via a diode with external supply voltage, periodic control voltage generated by signal source having a control input via which level of periodic control voltage can be infinitely variably set and this control input connected to an output of a control device which compares voltage generated at output of rectifier circuit with a desired voltage and, depending on comparison, emits output signal fed to control input of signal source, until level of periodic control voltage fed to supply voltage connection of inverting amplifier reaches value at voltage generated at output of rectifier circuit matches desired voltage, if voltage present at output of rectifier circuit matches desired voltage, at least voltage present at output of control device is a measure for at least one measurement capacitor.