An active electrode for biopotential recording from small localized =
bio-sources
Emil S Valchinov and Nicolas E Pallikarakis=20
Department of Medical Physics, University of Patras, Patras 26500, =
Greece
BioMedical Engineering OnLine 2004, 3:25 doi:10.1186/1475-925X-3-25
Published 22 July 2004=20
Abstract=20
Background
Laser bio-stimulation is a well-established procedure in Medical =
Acupuncture. Nevertheless there is still a confusion as to whether it =
works or the effect is just placebo. Although a plethora of scientific =
papers published, showing positive clinical results, there is still a =
lack of objective scientific proofs about the bio-stimulation effect of =
lasers used in Acupuncture. The objective of this work was to design and =
build a body surface electrode and an amplifier for biopotential =
recording from acupuncture points, considered here as small localized =
bio-sources (SLB). The design is aimed for studying SLB potentials =
provoked by laser stimulus, in search for objective proofs of the =
bio-stimulation effect of lasers used in Medical Acupuncture.
Methods
The active electrode presented features a new adjustable anchoring =
system and fractionation of the biopotential amplifier between the =
electrode and the cabinet's location. The new adjustable electrode =
anchoring system is designed to reduce the electrode-skin contact =
impedance, its variation and motion artifacts. That is achieved by =
increasing the electrode-skin tension and decreasing its relative =
movement. Additionally the sensing element provides local constant skin =
stretching thus eliminating the contribution of the skin potential =
artifact. The electrode is attached to the skin by a double-sided =
adhesive pad, where the sensing element is a stainless steel, 4 mm in =
diameter. The fractionation of the biopotential amplifier is done by =
incorporating the amplifier's front-end op-amps at the electrodes, thus =
avoiding the use of extra buffers. The biopotential amplifier features =
two selectable modes of operation: semi-AC-mode with a -3 dB bandwidth =
of 0.32=E2=80=931000 Hz and AC-mode with a bandwidth of =
0.16=E2=80=931000 Hz.
Results
The average measured DC electrode-skin contact impedance of the proposed =
electrode was 450 k=CE=A9, with electrode tension of 0.3 kg/cm2 on an =
unprepared skin of the inner forearm. The peak-to-peak noise voltage =
measured at the amplifier output, with input terminals connected to =
common, was 10 mVp-p, or 2 =CE=BCVp-p referred to the input. The =
common-mode rejection ratio of the amplifier was 96 dB at 50 Hz, =
measured with imbalanced electrodes' impedances. The prototype was also =
tested practically and sample records were obtained after a low =
intensity SLB laser stimulation. All measurements showed almost a =
complete absence of 50 Hz interference, although no electrolyte gel or =
skin preparation was applied.
Conclusion
The results showed that the new active electrode presented significantly =
reduced the electrode-skin impedance, its variation and motion artifact =
influences. This allowed SLB signals with relatively high quality to be =
recorded without skin preparation. The design offers low noise and major =
reduction in parts, size and power consumption. The active electrode =
specifications were found to be better or at least comparable to those =
of other existing designs.
----- Original Message -----=20
From: Nagi Hatoum=20
To: 'Discussion of EEG hardware, software, training protocols, NFB =
theory - for developers and users.'=20
Sent: Tuesday, August 28, 2007 1:47 AM
Subject: Re: [Openeeg-list] Problems using active electrodes
Active electrodes are based on the theory that the differential =
amplifier is near the source and does not pick up noise from the eeg =
wires. But to have a differential amplier you need an eeg wire going to =
another reference point so it's self defeating. Add to that the power =
wires that are going to the active electrodes and you have a bulkier =
sensor than a simple eeg wire.
It is best to concentrate on making an ultra high input impedance =
amplifier with ultra high CMRR.
There is a paper by a greek researcher who outlines his version of =
active electrodes in details. He uses it to detetct any effect from =
laser acupuncture. There is a lot of details, explainations and pictures =
in his paper and it's also SMD. Google it.
-------------------------------------------------------------------------=
-----
From: openeeg-list-bounces@... =
[mailto:openeeg-list-bounces@...] On Behalf Of Armagan =
Amcalar
Sent: Monday, August 27, 2007 6:47 AM
To: Discussion of EEG hardware, software, training protocols, NFB =
theory - for developers and users.
Subject: Re: [Openeeg-list] Problems using active electrodes
Hi, I have been designing / testing active electrodes for approx. 6 =
months now, for my grad project. I have built my own active electrode =
designs with more filters and implemented on SMD technology. But, I was =
unable to proceed with them, as I am constantly facing your problem...=20
Everything works perfectly when connected to calibration signals, even =
on 6 different channels on ModEEG. The problem occurs whenever you take =
off the calibration signals and connect the electrodes to your head. =
Nothing happens, and either you see a hell lot of a noise, or you see =
saturation, or nothing.=20
The problem may rise from unbalanced handling of noise sources, =
surely, the calibration signal is near perfect, with the least bit of =
noise, but even in open air, the electrodes pick a lot of noise. Add to =
these the noise your body picks up from your surroundings and its =
unbearable. Although I have tried numerous ways to handle noise, that =
range from complex feedback paths to using ear / wrist electrodes in =
various styles, I was unable to come up with a firm, solid solution that =
always works.=20
Eye blinks are thought to be good tests for electrodes, since they are =
to produce peaks in acquired signals. While the electrodes always work =
with calibration signals, I was frustrated to keep them working with EEG =
signals, only 1/5 of the time the electrodes responded to my eye blinks. =
4/5 of the time there was noise, or nothing.=20
Static electricity might be another problem that drives the active =
electrodes to saturation, too. The best way to keep from static =
electricity is to either short yourself good to ground, or use a good =
shielding for the electrodes. I was unable to shield the electrodes, =
though, so I do not know if that's the solution.=20
Hope I could give a little bit insight.
Would be delighted if somebody knew an answer :)
By the way, I was unable to make my active electrodes work from =
signals from a normal function generator, too - of course, voltage =
divided to a few uV's. I was very desperate :)=20
On 7/26/07, Bastian Holtermann <openeeg@...> wrote:=20
Hello,
I build active electrodes with the schematic from Joerg Hansmann.=20
Now I have the following Problem:
When I connect the calibration signal to the electrodes =E2=80=93 =
everything is fine.
But when I try to use the electrodes for eeg =E2=80=93 there is no =
way to get a signal that responses to my acting (increasing alpha by =
relaxing).
I have tried passive electrodes with the same modularEEG device and =
there is no problem. The Signal is stable and resonses to my acting =
(increasing alpha by relaxing)
=
=
=20
Does anybody have an idea what the problem could be?
I tried the schematic from Jarek Foltynski, too. But the same =
problem: calibration wonderfull =E2=80=93 signal worse
Regards
Bastian Holtermann
=
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