Differential Voltage Measuring System Common-Mode Signal Rejection
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Solution Overview
Problem
Conventional differential voltage measuring systems for bioelectric signals, such as ECG, face challenges in effectively rejecting common-mode interference signals due to varying impedances and capacitances at measurement inputs, leading to signal attenuation and reduced signal/noise ratio.
Innovation Solution
A differential voltage measuring system with a separate interference-signal measuring circuit connected to a fixed reference potential, decoupled from the signal measuring circuit, uses an impedance and differential amplifier to measure common-mode currents, minimizing interference impact on the bioelectric signal measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a shunt resistor is integrated in the second measuring path to measure common-mode current, then the common-mode signal can be determined and rejected, but the shunt resistor influences the measurement signals by thermal noise
Solution Approach 1:
The patent introduces an intermediary capacitor connected between the second measuring path and the shunt resistor. This capacitor acts as a mediator that blocks the high-frequency thermal noise generated by the shunt resistor from reaching the measurement circuit, while allowing the low-frequency common-mode current signal to pass through for measurement. The capacitor thus separates the harmful thermal noise from the useful common-mode signal measurement function.
2Object-affected harmful factors
If adaptive filtering is used to reject common-mode components, then common-mode interference can be reduced, but the useful signal is also attenuated, reducing the overall gain of the signal/noise ratio
Solution Approach 1:
The patent extracts the common-mode current measurement function into a separate, independent measuring path that parallels the main signal measurement path. By measuring the common-mode current separately through the shunt resistor and capacitor combination, the system can reject common-mode interference without requiring adaptive filtering of the main measurement path, thereby preserving the useful signal and maintaining the signal/noise ratio.
3Object-affected harmful factors
If the shunt resistor is placed in an additional measuring path separate from the second measuring path, then thermal noise influence is reduced, but there is still a direct electrical connection between the shunt resistor and the second measuring path causing interfering interaction
Solution Approach 1:
The patent uses a capacitor as an intermediary element connected between the additional measuring path containing the shunt resistor and the second measuring path. This capacitor provides the necessary electrical connection for common-mode current measurement while blocking direct low-impedance paths that would cause interfering interactions between the measuring paths. The capacitor thus enables path separation while maintaining measurement functionality.
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 achieves optimal separation of measurement signals from common-mode interference, maintaining a high signal/noise ratio and preventing interference from impairing bioelectric signal measurement.
Implementation Method 1
The current flowing between the potential and the fixed reference potential can be measured, for example, by an impedance being connected between the potential and the fixed reference potential and having a differential amplifier circuit connected in parallel with it as a voltage measuring unit
Implementation Method 2
having a differential amplifier circuit connected in parallel with it as a voltage measuring unit
Data Source
AI summary
A differential voltage measuring system is described. The differential voltage measuring system has a signal measuring circuit for measuring bioelectric signals, and an interference-signal measuring circuit coupled to the potential of the differential voltage measuring system and electrically connected to a fixed reference potential. In this case, the interference-signal measuring circuit is designed for measuring a current flowing from the potential of the differential voltage measuring system to the fixed reference potential. A differential voltage measuring system with an additional path is also described. Furthermore, a differential voltage measuring system with an averaging potential measuring method is described.