Common-Mode Interference Cancellation in Differential Signal Measurement
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Solution Overview
Problem
Current electrical and magnetic signal measurement technologies face challenges in effectively eliminating common-mode interference, which reduces the signal-to-noise ratio due to imperfect symmetry in measurement systems.
Innovation Solution
A general method is proposed that involves grounding the center symmetry point of the gradient antenna in magnetic measurements or using bipolar pickup electrodes in electrical measurements, followed by amplification and sampling of differential and common-mode signals. The sampled common-mode signal is used as an interference noise template to identify and remove common-mode interference components from the differential signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a differential amplifier with high CMRR is used to suppress common-mode interference, then electrical interference is reduced, but common-mode interference still remains in the measured signals
Solution Approach 1:
The patent segments the interference suppression process into two distinct stages: first using a differential amplifier to handle electrical interference, then using a notch filter to specifically target and remove remaining common-mode interference at the power line frequency. This segmentation allows each component to address specific types of interference without being limited by the shortcomings of a single approach.
Solution Approach 2:
The patent introduces a notch filter as an intermediary component between the differential amplifier and the final signal output. This intermediary specifically targets the frequency range where common-mode interference persists (power line frequency of 50Hz or 60Hz) and removes it, thereby bridging the gap between the differential amplifier's electrical interference suppression and the need for complete interference elimination.
2Measurement precision
If a gradient coil structure is used to detect magnetic signals, then magnetic signal detection is enabled, but common-mode magnetic interference cannot be avoided
Solution Approach 1:
The patent employs an asymmetric gradient coil structure where the two coils are positioned at different locations relative to the sample. This asymmetric configuration creates different induction conditions for external magnetic interference at the two coil positions, transforming common-mode magnetic interference into differential-mode signals that can be suppressed by the differential amplifier, while maintaining the ability to detect the desired magnetic resonance signals.
Solution Approach 2:
Instead of trying to make the gradient coil structure perfectly symmetric to reject common-mode interference (the conventional approach), the patent inverts the strategy by deliberately creating asymmetry. This inversion transforms the interference rejection mechanism from relying on geometric symmetry to relying on differential amplification of asymmetrically-induced interference signals.
3Object-affected harmful factors
If symmetric antenna structure is used for magnetic signal measurement, then common-mode interference suppression is improved, but manufacturing symmetry is difficult to achieve
Solution Approach 1:
The patent inverts the conventional approach by abandoning the pursuit of symmetric antenna structures and instead deliberately designing an asymmetric gradient coil configuration. This inversion shifts the interference suppression mechanism from geometric symmetry to electromagnetic field differentiation, where the asymmetric positions of the two coils create different induction conditions for external interference, achieving interference rejection without requiring precise manufacturing symmetry.
Solution Approach 2:
The patent replaces the mechanical/geometric symmetry requirement with an electromagnetic field-based interference rejection mechanism. Instead of relying on the physical symmetry of the antenna structure to cancel common-mode interference, the system uses the asymmetric positioning of coils combined with differential amplification to achieve interference suppression, substituting mechanical precision requirements with electromagnetic field manipulation.
4Object-affected harmful factors
If notch filter is added for further filtering, then common-mode interference is reduced, but device complexity increases
Solution Approach 1:
The patent uses a copy of the power line signal (either from a reference electrode or from the common-mode component of the measured signal) to create a reference waveform that matches the interference. This reference copy is then processed through the notch filter to generate a cancellation signal that is subtracted from the original measurement, effectively removing the common-mode interference without requiring complex adaptive filtering algorithms.
Solution Approach 2:
The patent implements a feedback mechanism where the output of the notch filter (or a reference signal representing the interference) is fed back into the signal processing chain to generate a cancellation signal. This feedback loop continuously adapts to the presence of common-mode interference and automatically generates the appropriate counter-signal, maintaining interference rejection without requiring manual adjustment or complex control systems.
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 method effectively eliminates common-mode interference, improving the signal-to-noise ratio and obtaining the original desired signal with reduced interference, thus addressing the technical bottleneck in existing measurement technologies.
Implementation Method 1
The detected signal is fed to a preamplifier circuit composed of a differential amplifier (or instrumentation amplifier) with a high common-mode rejection ratio (CMRR) for amplification
Implementation Method 2
The magnetic interference signal being measured generates equal and opposite currents in the coil, thus canceling out the common-mode interference
Data Source
AI summary
A novel general method for eliminating interference in both electrical and magnetic measurement systems is disclosed in this invention. The method applies to both electro and magnetic measurements using differential amplifiers, wherein the common-mode signal is extracted to serve as an interference template after being amplified and digitally sampled along with the differential signals. The method involves identifying matching template components within the signal and subtracting them to eliminate interference and improve the signal-to-noise ratio. For magnetic signal detection sensors using gradient coil structures, the symmetric point is grounded so that the mixed signal which contains the common-mode components representing interference and measurement signal are output at the two signal terminals. Similar to electrical measurements, the common-mode components_are_extracted from differential amplifier as an interference template, followed by template matching analysis as mentioned above and subtraction to remove interference components from the signal.