Capacitance Sensing Circuit With Complex Impedance for Small Signal Detection
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Solution Overview
Problem
Existing capacitance sensing assemblies are not sensitive enough to detect small changes in capacitance, particularly in grounded variable capacitors, which are essential for precision applications like accelerometry and gravimetry, and suffer from unwanted frequency-to-amplitude modulation.
Innovation Solution
Incorporating a complex impedance circuit, such as a resonant tank or negative capacitor circuit, to enhance the gradient of the phase-to-frequency response curve, and using a capacitor multiplier or negative capacitance to simulate larger capacitance variations with smaller capacitors, thereby increasing sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional capacitance sensing assemblies are used, then the device complexity is low, but the measurement precision is insufficient to detect small capacitance changes
Solution Approach 1:
The patent introduces an intermediary complex impedance circuit between the variable capacitor and ground. This intermediary circuit transforms small capacitance changes into larger, more detectable impedance variations, thereby enhancing measurement precision without requiring direct detection of minute capacitance changes. The complex impedance circuit acts as a signal amplifier that mediates between the capacitor and the detection system.
Solution Approach 2:
The patent changes the detection parameter from direct capacitance measurement to impedance measurement. By measuring the complex impedance (which includes both resistance and reactance components) of the circuit containing the variable capacitor, the system can detect capacitance changes more sensitively. This parameter transformation allows for enhanced detection capability while managing device complexity through standard impedance measurement techniques.
2Adaptability or versatility
If grounded variable capacitors are used, then the adaptability for precision applications is improved, but the sensitivity to detect small capacitance variations deteriorates
Solution Approach 1:
The complex impedance circuit serves as a mediator that enables grounded variable capacitors to achieve high sensitivity. The intermediary circuit translates the limited signal from grounded capacitors into enhanced detectable variations, allowing grounded capacitors to be used in precision applications without sacrificing detection sensitivity.
Solution Approach 2:
The patent moves the detection from a single-dimensional capacitance measurement to a two-dimensional complex impedance measurement (including both magnitude and phase). This dimensional expansion provides additional information about the capacitor's state and enhances the ability to detect small variations, thereby improving sensitivity while maintaining adaptability to precision applications.
3Measurement precision
If standard all-pass filter circuits are used, then the device complexity is low, but the gradient of phase-to-frequency response is insufficient for high sensitivity detection
Solution Approach 1:
The complex impedance circuit acts as an intermediary between the all-pass filter and the detection system, enhancing the phase-to-frequency response gradient. The intermediary circuit amplifies the phase variations produced by the all-pass filter, thereby increasing the detection sensitivity without requiring a complete redesign of the filter architecture.
Solution Approach 2:
The patent creates a composite circuit architecture by combining the all-pass filter with the complex impedance circuit. This composite structure leverages the frequency-selective properties of the all-pass filter and the signal-amplifying properties of the complex impedance circuit, achieving a high phase-to-frequency response gradient while maintaining reasonable device complexity through the synergistic combination of two established circuit types.
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 enhanced capacitance sensing assemblies can detect very small capacitance variations and physical perturbations, allowing for precise measurements of minute movements and electric fields, reducing unwanted frequency-to-amplitude modulation.
Implementation Method 1
Incorporating a complex impedance circuit, such as a resonant tank or negative capacitor circuit, to enhance the gradient of the phase-to-frequency response curve
Implementation Method 2
using a capacitor multiplier or negative capacitance to simulate larger capacitance variations with smaller capacitors
Data Source
AI summary
A capacitance sensing assembly is provided. The capacitance sensing assembly includes an op-amp arranged with four complex impedance circuits and a sensing capacitor connected to the fourth complex impedance circuit and ground; or to the non-inverting input of the op-amp, the fourth complex impedance circuit, ground and the second terminal of the fourth complex impedance circuit, wherein the fourth complex impedance circuit is connected in parallel to the sensing capacitor. The complex impedance circuits are configured to increase a gradient of a phase to frequency response curve of the capacitance sensing assembly relative to that of an all-pass filter with a second electrode of a capacitor connected to the non-inverting input of the op-amp and the ground without the fourth complex impedance circuit.