Abstract
Dry electrodes are a promising solution for prolonged EEG signal acquisition, whereas wet electrodes may lose their signal quality in the same situation and require skin preparation for set-up. Here, we review the impedance and noise of passive and active dry EEG electrodes. In addition, we compare noise and input impedance of the EEG amplifiers. As there are multiple definitions of impedance in each EEG system, they are all first defined. Electrodes must be compatible with amplifiers to accurately record EEG signals. This implies that their impedance plays a significant role in amplifier compatibility and affects total input-referred noise. Therefore, we review the impedance and noise of state-of-the-art amplifiers and electrodes. Furthermore, we compare the various structures and materials used and their final impedance to that of wet electrodes. Finally, we compare state-of-the-art electrodes and amplifiers to the standards of the IFCN and IEC80601-2-26. We investigate bottlenecks and propose a guideline for future work on passive and active dry electrodes, as well as EEG amplifiers.
Highlights
C URRENTLY, there are numerous techniques for monitoring brain activity, such as computer tomography (CT) [1], magnetic resonance imaging (MRI) [2], functional magnetic resonance imaging [3], positron emission tomography (PET) [4], magnetoencephalography (MEG) [5], and electroencephalogram (EEG) [6]
We have focused more on electrodes than amplifiers, the noise and input impedance of amplifiers play a significant role in the design of dry electrodes, while their input impedance can limit the performance of the electrode and their high noise may lead to loss of brain signal
We examined the compatibility of dry electrodes and amplifiers as an EEG system
Summary
C URRENTLY, there are numerous techniques for monitoring brain activity, such as computer tomography (CT) [1], magnetic resonance imaging (MRI) [2], functional magnetic resonance imaging (fMRI) [3], positron emission tomography (PET) [4], magnetoencephalography (MEG) [5], and electroencephalogram (EEG) [6]. The most conventional clinical electrodes are wet electrodes They utilize a saline or gel environment to increase the contact area, decrease impedance, and record high-quality signals. The current goes from body to electrode via contact area whereas non-contact electrodes work based on the electric field between body and electrode which are shown in figure 2. Dry contact electrodes have a higher signal quality, but dry non-contact and insulated electrodes provide greater user comfort and easier set-up for applications that use one or two channels, such as those required in BCI. Dry contact electrodes have a high offset, they exhibit lower impedance than insulated and non-contact electrodes, and record EEG signals more accurately [21]. We analyze the relationship between noise and impedance of the amplifier to identify bottlenecks in an EEG system
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