What impedances are to expect using the standard dry electrodes?

jeyemjeyem
edited February 2018 in Cyton
I mounted my cyton + daisy headset. After all electrodes had proper contact to the skin, I made an impedance test. The pictures show the signals (1) and the impedance measurement (2). Hardware settings are set to default (3). Chewing and closing the eyes yielded the expected signals (EMG articfacts (4), and 10Hz alpha on all channels (5)). After some minutes of recording, an error occured andthe GUI crashed (6).

1: https://drive.google.com/open?id=1_4vMbeStS66RHSeQrj7nuKSSTX-ENV9u
2: https://drive.google.com/open?id=1GGJA6dBZNiXp9HJySawfEWbI4UpgAVpW
3: https://drive.google.com/open?id=1aB0MvQPbJLjJ8Lt6EZBkTfi9x3dlFzQe
4: https://drive.google.com/open?id=1VBDIDHcOb1riOxoC2nlCJJY1cwq-YWue
5: https://drive.google.com/open?id=1t-HCq6GvyMeK7OWNLd2moLrnTeaL9l7m
6: https://drive.google.com/open?id=1pFoGuxuMMAD8DDBNnMi07sZeQ5eHbGKR

My questions:
1.) What impedance levels are to expect / do we aim for with the dry electrodes to get a proper signal? Could anyone share his experiences?

2.) Do different impedance levels (badly) affect the signal quality?

3.) Is it normal, that when I click the Ohm buttons, the signal amplitude goes up and there's much high-frequncy noise an the respective channels?

4.) What might have caused the error?


Any help would be highly appreciated,

Johannes

Comments

  • wjcroftwjcroft Mount Shasta, CA
    edited June 2019
    Johannes, hi.

    There are lots of previous posts regarding Cyton impedance measurements. For example:


    Cyton amplifier normal impedance is 1 gigaohm. During impedance check that drops to 500 megohms. Since the amp impedance is so high, the electrode impedances are much less an issue. See this paper,


    Your second image of the impedance measurement looks very odd. No, it should not look like that. Are you sure you have good contact at the left and right ear clips? Try swapping the two cables, you may have a bad or intermittent connection.

    William

  • jeyemjeyem
    Thanks ,William.

    I took a screen video of the impedance check procedure. As soon as I press the Ohm Button there will be massive 30 Hz interference in all channels. The noise seems to be modulated by the impedance of the respective electrode, that is it is lower when impedance is small.

    https://drive.google.com/open?id=1EGmqL8AsBQM_yr8Mx7dQsCQiZ8YHvBXi

    I don't think that the ear clips are a problem. I also tried it with other electrodes as reference and ground - same picture.
  • wjcroftwjcroft Mount Shasta, CA
    By connecting 3 electrodes together: channel, reference, and ground (Bias), you should see close to zero microvolts on that channel. As well as low impedance. This is a general check used with EEG for cable continuity.
  • jeyemjeyem
    Do you mean electrodes or PINs of the cyton board? Probably the latter. I connected SRB, BIAS and CHAN1 what led to roughly 0,13uV. When I clicked the Ohm Button I got about 0,65 Ohm, the voltage jumps to 12uV and it shows a 30 Hz peak in the FFT, see this picture: https://drive.google.com/open?id=1VC178PzJsvGE1WDyo7iYzWKXnK6dsXWU

    Is this expected behavior?
  • wjcroftwjcroft Mount Shasta, CA
    The impedance check feature DOES inject a test signal, yes.

    It's actually better to short the electrodes at the electrode end of the cables, not at the pins. This confirms you have good cable and electrode connections.
  • jeyemjeyem
    edited February 2018
    I made the test again with the electrode end of the cables that are connected to the pins. All channels looked the same as in the picture in my previous post.
    The small white cables of the electrodes and the electrodes themselves cannot be tested that way, can they?

    It's good to know that the 30 Hz peak comes from the test signal and is expected behavior. The large scale of the previous image then maybe was due to bad signal quality of the respective electrodes or the REF.

    Actually, I do not care too much about this impedance measurement - I am more concerned about getting good and reliable signal quality in a reasonable amount of time and just thought that large impedances were to blame. I wonder whether there are any tricks to achieve good signal quality with the standard dry electrodes.... maybe moisten the scalp, or getting rid of hair... I am even worried if this is ever going to work with even longer and more dense hair than mine, esp. women's hair.

    Any tips?
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