Chopper-Stabilized Active Electrode Shielding for Low-Noise EEG
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Solution Overview
Problem
Existing bio-potential recording technologies face challenges in achieving high noise immunity against capacitive coupled interferences due to finite input impedance and parasitic capacitances, particularly in dry-contact and capacitive electrodes, which affect the Common Mode Rejection Ratio (CMRR) and Power Supply Rejection Ratio (PSRR).
Innovation Solution
An active electrode design incorporating a shield with chopper modulation and a unity-gain buffer amplifier, which actively drives the shield to provide improved shielding, frequency shifting the input signal to reduce flicker noise and enhance CMRR and PSRR, while maintaining low power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional amplifier is used in active electrode, then the device complexity is low, but the input impedance is finite and parasitic capacitances affect CMRR and PSRR
Solution Approach 1:
The amplifier is segmented into multiple functional blocks: input stage with chopper modulation, buffer stage, and output stage. This segmentation allows each stage to be optimized independently - the input stage handles noise immunity through chopper modulation while the buffer stage manages impedance, resolving the contradiction between reliability and complexity.
Solution Approach 2:
A buffer amplifier is introduced as an intermediary between the chopper-modulated input stage and the output stage. This buffer acts as a mediator that isolates the high-impedance input from the output load, preventing parasitic capacitances from degrading CMRR and PSRR while maintaining overall system functionality.
2Reliability
If chopper modulation is applied to frequency shift the input signal, then flicker noise is reduced and CMRR/PSRR are enhanced, but the device complexity increases
Solution Approach 1:
The chopper modulation functionality is merged directly into the input amplifier stage rather than being a separate preprocessing circuit. The modulation switches are integrated with the input transistors, allowing frequency shifting to occur at the earliest possible point in the signal chain, which reduces flicker noise before amplification without requiring additional discrete modulation circuits.
3Object-affected harmful factors
If the shield is actively driven by the amplifier output, then capacitive coupled noise is minimized, but the amplifier must drive capacitive load which affects stability
Solution Approach 1:
A buffer amplifier is introduced as an intermediary between the main amplifier output and the shield drive. This buffer acts as a mediator that can drive the capacitive shield load without affecting the stability of the main amplifier, as the buffer is specifically designed to handle capacitive loads while the main amplifier operates in its stable region.
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 solution significantly increases input impedance, improves CMRR and PSRR, and reduces noise immunity against interferences, making it suitable for high-quality bio-recording systems like EEG and ear-EEG, particularly for daily use in monitoring conditions such as detecting Hypoglycemia in diabetes patients.
Implementation Method 1
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 2
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
Implementation Method 3
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
Data Source
AI summary
An active electrode has an electrode for sensing an electric potential and generating an input signal, and a shield placed near the 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 the output of the integrated amplifier to actively drive the electrical potential of the shield, thereby providing an active shielding of the 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 the higher frequency range back to the basic frequency range. The active electrode may be used for recording EEG signals.