Re: [Openeeg-list] Problems using active electrodes

[Georges O. <geo...@pa...>]

Grounding is absolutely necessary in order to avoid all the massive =
environmental noise a body picks up. Without it all ampli's get directly =
saturated. However one cannot directly connect a human being to the =
earth as this would be very hazardous. Shielding the electrodecables is =
OK but a good earth is best.
Active electrodes (look also at Biosemi: r'eal pro's !)

Georges
  ----- Original Message -----=20
  From: Armagan Amcalar=20
  To: Discussion of EEG hardware, software, training protocols, NFB =
theory - for developers and users.=20
  Sent: Monday, August 27, 2007 12:47 PM
  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 <op...@ba...> wrote:
    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 =96 =
everything is fine.



    But when I try to use the electrodes for eeg =96 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 =96 signal worse



    Regards



    Bastian Holtermann




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