Common Mode Signal Removal in Body Potential Detection
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Solution Overview
Problem
In measuring devices for detecting body potentials, such as in electrocardiograms or electromyograms, common mode signals caused by capacitive coupling with the surrounding environment interfere with the detection of small useful signals, leading to signal overlay and difficulty in separation, especially when the device ground and patient potentials differ.
Innovation Solution
A subtracting unit is introduced downstream of a summing unit, which subtracts a reference signal adapted to compensate the common mode signal from the amplified output, ensuring that individual input signals are free from common mode components by using an amplification factor that corresponds to the reciprocal of the amplification factor used for the reference signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the device ground and patient are connected via an additional electrode to achieve the same potential, then the patient and device ground are at the same potential, but a common mode signal is amplified by the amplifiers which overlays and interferes with the useful signal
Solution Approach 1:
The patent extracts and removes the common mode signal from the amplified output signals by calculating the mean value of all amplified signals and subtracting this mean value from each individual amplified signal. This separation isolates the harmful common mode component from the useful differential signals, allowing the common mode signal to be eliminated without affecting the useful measurement signals.
Solution Approach 2:
The patent introduces a ground signal electrode that detects the common mode potential, and uses this as an intermediary to generate a ground signal. This ground signal serves as a mediator that is then subtracted from the amplified output signals, effectively using the detected common mode potential as a tool to remove itself and prevent interference with the useful signals.
2Adaptability or versatility
If high dynamics amplifiers are used to process both the useful signal and the higher common mode signal, then both signals can be processed, but the amplifier complexity and requirements increase
Solution Approach 1:
The patent extracts the common mode signal component from the amplified output signals by calculating their mean value and subtracting it from each signal. This separation removes the harmful common mode component before further processing, allowing standard amplifiers with lower dynamics requirements to suffice, rather than requiring high dynamics amplifiers capable of handling the large common mode signal swings.
3Measurement precision
If the mean value of the amplified signals is subtracted from the input signals, then the common mode signal is not amplified, but it is still output together with the useful signal at the amplifier output
Solution Approach 1:
The patent extracts and removes the common mode signal from the amplifier outputs by calculating the mean value of all amplified signals and subtracting this mean value from each individual amplified signal. This ensures that the common mode signal is completely eliminated from the output, preventing any overlay with the useful signals and enabling accurate detection of small useful signals even when the common mode signal level is high.
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 design effectively removes common mode signals from the output of amplifiers, optimizing the dynamics of amplifiers and reducing the requirements on downstream electronic analyzers, allowing for accurate detection of small useful signals by ensuring the output signals are free from common mode components.
Implementation Method 1
potentials form due to capacitive coupling on the skin of the patient. This effect can generally be described as the body being coupled capacitively, particularly to a 230V/50 Hz-alternating current voltage field
Data Source
AI summary
A device for detecting electric potentials of the body of a patient has measuring electrode inputs (Y1, . . . , Yn) connected with and a plurality of outputs (A1, . . . , An) via amplifiers (Op1, . . . , Opn). A summing unit (13) is connected with the outputs and outputs a mean value of the signals (E1, . . . , En) output by the amplifiers. Common mode signals are removed from the signals (E1, . . . , En) by a subtracting unit (19) which subtracts the output of the summing unit, amplified by an amplification factor (1/β), from at least a portion of the output of the subtracting unit. The output of the subtracting unit is connected with the inputs of the amplifiers. The subtracting unit amplification factor (1/β) and an amplification (β′) of the amplifiers for the output of the subtracting unit are adapted, such that the reciprocal value of the amplification factor (1/β) corresponds to the amplifiers amplification (β′).