Active Electrode Shielding With Chopper Modulation for High CMRR
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Solution Overview
Problem
Existing bio-potential recording technologies face challenges in effectively rejecting common mode noise and achieving high noise immunity due to finite input impedance and parasitic capacitances, particularly in dry-contact and capacitive electrodes, which affects the quality of signals like EEG and ECG.
Innovation Solution
An active electrode design with a shield and integrated amplifier, utilizing chopper modulation and a buffered path with frequency shifting, enhances input impedance and provides improved Common Mode Rejection Ratio (CMRR) and Power Supply Rejection Ratio (PSRR, thereby reducing noise interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional passive electrodes are used, then device complexity is low, but noise immunity and CMRR are insufficient due to finite input impedance and parasitic capacitances
Solution Approach 1:
An active shield electrode is introduced as an intermediary element between the sensing electrode and the environment. This shield is driven by a buffer amplifier to create an equipotential surface that actively cancels parasitic capacitance effects and common-mode noise, thereby improving noise immunity without requiring changes to the core sensing mechanism
Solution Approach 2:
The system uses its own output signal to drive the shield electrode, creating a self-regulating active shielding mechanism. The buffer amplifier monitors the electrode potential and automatically adjusts the shield potential to maintain optimal noise rejection, enabling the system to service itself without external intervention
2Reliability
If active shielding is implemented, then common mode rejection is improved, but parasitic capacitances and input impedance issues persist
Solution Approach 1:
The active shield electrode is driven to maintain the same potential as the sensing electrode by using the electrode's own signal as feedback. This creates an equipotential region that eliminates voltage differences across parasitic capacitances, effectively nullifying their impact on measurement precision while maintaining high common-mode rejection
Solution Approach 2:
A buffer amplifier is configured with feedback from the sensing electrode to the shield electrode, continuously monitoring and adjusting the shield potential to match the electrode potential. This feedback mechanism dynamically compensates for parasitic capacitance variations and maintains optimal input impedance characteristics
3Reliability
If chopper modulation is used, then noise immunity and CMRR are enhanced, but device complexity and circuit design difficulty increase
Solution Approach 1:
A chopper modulation scheme is implemented where the buffer amplifier operates in periodic switching mode, alternating between sampling the electrode potential and driving the shield. This periodic action converts low-frequency 1/f noise to higher frequencies where it can be more easily filtered, enhancing noise immunity while using simple switching circuitry rather than complex continuous control
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
The active electrode design achieves ultra-high input impedance, significantly diminishing common mode interference and enhancing noise immunity, making it suitable for high-quality bio-recording systems, including wearable devices like ear-EEG.
Implementation Method 1
The shield being connected to said output of the integrated amplifier to actively drive the electrical potential of said shield, thereby providing an active shielding of said electrode
Implementation Method 2
a first mixer in front of the integrated amplifier for frequency shifting the input signal from a basic frequency range to a higher frequency range
Implementation Method 3
a second mixer on the output of the integrated amplifier for frequency shifting the amplified signal from said higher frequency range back to said basic frequency range
Data Source
Figure 1~2
Figure 3~4
Figure 5~7
AI summary
An active electrode has an electrode for sensing an electric potential and generating an input signal, and a shield placed near said electrode but being electric insulated from the electrode. An integrated amplifier (10) has an input connected to the at least one electrode for receiving the input signal, and providing a buffered path outputting a buffered output signal. The shield being connected to said output of the integrated amplifier to actively drive the electrical potential of said shield, thereby providing an active shielding of said electrode. The buffered path includes a first mixer (11) in front of the integrated amplifier for frequency shifting the input signal from a basic frequency range to a higher frequency range, and a second mixer (12) on the output of the integrated amplifier for frequency shifting the amplified signal from said higher frequency range back to said basic frequency range. The active electrode may be used for recording EEG signals.