AC-Coupled ECG Front End for Higher Common-Mode Rejection
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Solution Overview
Problem
ECG signal acquisition systems face challenges in accurately recording heart signals due to low amplitudes being masked by common-mode signals from AC power lines, requiring high common-mode rejection ratio (CMRR) to differentiate and amplify the signals effectively.
Innovation Solution
The system employs a first amplifier with biasing resistors and an average estimation circuit, a driver amplifier with a low-pass filter, and capacitors to enhance input impedance and suppress common-mode signals, using a right leg drive signal to improve CMRR and reduce interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If instrumentation amplifiers are used to amplify ECG signals, then the differential ECG signals can be amplified selectively, but common-mode signals from AC power lines can still mask the ECG signals due to low input impedance
Solution Approach 1:
The patent implements a feedback mechanism where the output of the instrumentation amplifier is fed back through a feedback resistor to the inverting input, creating a virtual ground that maintains high input impedance. This feedback loop actively compensates for common-mode signals by continuously adjusting the inverting input voltage to match the non-inverting input voltage, thereby rejecting common-mode interference while maintaining high input impedance for accurate ECG signal detection.
Solution Approach 2:
The patent creates an equipotential condition at the input terminals by using the feedback mechanism to maintain equal voltages at both inputs of the instrumentation amplifier. This equipotential state ensures that common-mode signals appear equally at both inputs and are therefore rejected by the differential amplifier, while the high input impedance prevents loading effects on the ECG signal sources.
2Reliability
If the input impedance of instrumentation amplifiers is increased to improve CMRR, then common-mode rejection is enhanced, but the circuit complexity increases
Solution Approach 1:
The feedback resistor serves multiple functions simultaneously: it establishes the gain of the amplifier, maintains the virtual ground condition for high input impedance, and enables common-mode rejection. By making the feedback resistor perform multiple roles, the patent achieves high CMRR without adding excessive circuit complexity, as a single component accomplishes what would otherwise require multiple dedicated elements.
Solution Approach 2:
The patent optimizes the feedback resistor value to achieve the desired balance between input impedance, gain, and common-mode rejection. By carefully selecting the feedback resistor parameter, the circuit achieves high CMRR while maintaining practical input impedance levels that do not require overly complex impedance matching networks or additional active components.
3Measurement precision
If AC coupling capacitors are used to block DC offsets, then ECG signals can be transmitted, but the input impedance seen by the ECG sources is reduced
Solution Approach 1:
The feedback mechanism acts as an intermediary that decouples the AC coupling capacitors from the input impedance seen by the ECG sources. The virtual ground created by the feedback loop presents a high impedance barrier to the input, while the AC coupling capacitors can still perform their function of blocking DC offsets in the signal path. The feedback mechanism compensates for the impedance effect of the capacitors, maintaining high overall input impedance.
Solution Approach 2:
The feedback loop continuously monitors the voltage at the inverting input and adjusts it to match the non-inverting input, effectively canceling out the impedance-reducing effect of the AC coupling capacitors. This feedback action maintains the virtual ground condition, ensuring that the input impedance remains high despite the presence of coupling capacitors in the signal path.
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 configuration effectively increases the CMRR, allowing for accurate detection and recording of ECG signals by suppressing common-mode interference and maintaining high input impedance, thereby enhancing the quality of ECG signal acquisition.
Implementation Method 1
The low-pass filter allows DC current from the output of the driver amplifier to pass through, and it applies a DC bias voltage across the non-inverting and inverting inputs of the first amplifier
Implementation Method 2
ECG signals are produced by contractions of the human heart which drives electrical currents and create different potentials throughout the body. By placing electrodes on the skin, ECG signals are detected and recorded
Implementation Method 3
The instrumentation amplifiers apply amplification selective to the frequency components of the ECG signals and attenuate or reject common-mode signals. The ratio of the amplification of the differential signal to the amplification of the common-mode signal is termed the common-mode rejection ratio (CMRR)
Data Source
AI summary
An ECG signal acquisition system includes a first amplifier which has a non-inverting input adapted to be coupled to a first differential input, an inverting input adapted to be coupled to a second differential input, and an output. The system includes first and second biasing resistors coupled between the non-inverting and inverting inputs of the first amplifier. The system includes an average estimation circuit which has a first input coupled to the non-inverting input of the first amplifier and a second input coupled to the inverting input of the first amplifier. The system includes a driver amplifier which has an inverting input coupled to the output of the average estimation circuit, a non-inverting input coupled to receive a reference common-mode voltage, and an output. The system includes a low-pass filter coupled between the output of the driver amplifier and the biasing resistors.