Impedance and active/passive electrodes.

SkalbeSkalbe Latvia
in Electrodes
Hi!
I am using Cyton with the AgCl electrodes and remember reading something that I didn`t process as relevant right away about active/passive electrodes.
What is the difference between passive and active electrodes? Which ones is the Cyton board using and why?
Also can I determine the skin resistance from my dry scalp electrodes?

Thank you.

Comments

  • wjcroftwjcroft Mount Shasta, CA
    Skalbe, hi.

    (1) active electrodes contain a small circuit chip or board behind each electrode. This is a pre-amplifier that can reduce noise levels. This type of electrode is found in high end research and medical EEG systems. MOST EEG systems are using 'passive' electrodes, no electronics. This is the case with OpenBCI Cyton and Ganglion.


    If you do a search on active electrodes here on the forum you can see some related posts.


    (2) The OpenBCI GUI can show you the calculated impedance of each electrode. This is not the same as skin resistance. If you want to measure skin resistance between pairs of electrodes, you could try using a digital multimeter. Values in ohms will vary widely depending on skin pressure, skin moisture, etc.

    Regards,

    William

  • SkalbeSkalbe Latvia
    Thank you for the prompt response, Mr, Croft!
    Just to clarify `shielded` means that its protection is just the wire?
    Also are you affiliated with the OpenBCI in some way? I`ve been trying to contact them with no avail. I`m asking just because you seem the most active knowledge representative.
  • wjcroftwjcroft Mount Shasta, CA
    edited January 2019
    Shielded wire is a type of wire containing an outer coaxial braid or foil. The inner wire or wires are somewhat protected from electromagnetic interference by this shield. The inner wires can be any number, 1, 2, 3, etc. 

    https://en.wikipedia.org/wiki/Shielded_cable

    I am a staff member at OpenBCI and moderate the forum here.

    The best way to contact the lab in Brooklyn is to email to contact at openbci.com, which will open up a ticket.

  • SkalbeSkalbe Latvia
    Thank you, but then from what I`ve seen, OpenBCI cables are NOT shielded?
    Yeah I did that. No response.

    As to the resistance, that is what I will attempt then. I want to use the actual skin resistance as a factor multiplier in emotion recognition.
  • wjcroftwjcroft Mount Shasta, CA
    In general passive EEG cables are not shielded, and do not need to be shielded.

    If you measure skin resistance, it is best to do it at locations that have more sweat glands. Scalp is NOT a typical location.

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