Biopotential Signal Acquisition Common Mode Feedforward
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Solution Overview
Problem
Active electrode biopotential signal acquisition systems face challenges with common mode interference, particularly from mains power supply lines, which limits their common mode rejection ratio (CMRR) and can lead to amplifier saturation due to voltage gain mismatch and component mismatch, making it difficult to achieve the required 80dB CMRR for medical-grade systems.
Innovation Solution
The implementation of a common mode feedforward approach using a unity gain voltage buffer to reduce the effective common-mode input voltage at the amplifier inputs, which improves the CMRR and prevents saturation without affecting differential signal amplification, and this can be applied to both single and multi-channel systems using various amplifier types.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If active electrodes with integrated amplifiers are used to minimize signal path length and reduce noise pickup, then the input impedance is maintained high and noise pickup is reduced, but voltage gain mismatch between electrodes converts common mode interference into differential mode error, limiting CMRR to below 60dB
Solution Approach 1:
The patent implements a feedback circuit that measures the actual voltage gain of each active electrode amplifier and uses this information to adjust the amplifier gain, compensating for process variation and component mismatch. This closed-loop approach maintains CMRR above 80dB despite manufacturing variations.
Solution Approach 2:
The patent dynamically adjusts the voltage gain parameter of the amplifiers based on measured performance characteristics. By changing the gain parameter in real-time rather than relying on fixed manufacturing tolerances, the system achieves consistent high CMRR performance across all electrodes.
2Power
If the amplifier voltage gain is increased to amplify small biopotential signals, then signal amplification is improved, but large common mode input interference causes amplifier saturation
Solution Approach 1:
The patent applies preliminary action by measuring and compensating for common mode interference before it reaches the amplifier input stage. The feedback circuit pre-adjusts the amplifier operating point and gain to prevent saturation from occurring, allowing high voltage gain to be used without risk of saturation.
Solution Approach 2:
The patent introduces an intermediary feedback circuit that sits between the amplifier and the control system, measuring actual performance and mediating adjustments to prevent saturation. This intermediary layer allows the amplifier to operate at high gain while the feedback mechanism prevents harmful saturation effects.
3Object-affected harmful factors
If notch filtering is used to remove 50Hz or 60Hz common mode interference when high CMRR is not achievable, then the specific mains frequency interference is removed, but the filtering approach does not address the root cause of interference conversion due to gain mismatch
Solution Approach 1:
The patent converts the harmful effect of common mode interference into a useful measurement signal. By measuring the common mode voltage and using it as feedback to adjust amplifier gain, the system turns the interference source into a diagnostic tool that actively improves CMRR performance.
Solution Approach 2:
Instead of using traditional filtering to remove interference after it has been converted to differential mode, the patent inverts the approach by preventing the conversion in the first place through feedback control. This reverses the conventional signal processing sequence and achieves better results with simpler filtering requirements.
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 significantly enhances the CMRR, reduces amplifier saturation, and allows for improved signal acquisition by compensating common mode interference before amplification, enabling better noise rejection and maintaining signal integrity, as verified by hardware measurements showing over 30dB CMRR improvement and significant reduction in 50/60Hz interference.
Implementation Method 1
a feedforward voltage, VCMFF, from a unity gain voltage buffer is applied to the other inputs of the amplifiers
Data Source
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AI summary
The invention provides a biopotential signal acquisition system, comprising at least two active electrodes (10,16) each for receiving a signal from an input, for example in the form of a respective patient contact. A unity gain buffer (32) is used to feed forward a signal from an associated input and provide the buffered output to the amplifiers of the active electrodes as a reference signal. The buffered signal is approximately equal to the input common mode signal so that the effective common mode input voltage appearing at the inputs of each amplifier is significantly reduced. This reduces the common mode gain of the active electrodes and thus improves their common mode rejection ratio while also avoiding saturation of the active electrodes.