Ganglion impedance measurement details? [resolved]

musfequrmusfequr europe
edited March 2022 in Ganglion

Can anyone explain about how ganglion measure the electrode impedance? And is ganglion measure impedance for a different frequency or for a certain frequency?
Thank you and have a happy Christmas and new year...

Comments

  • wjcroftwjcroft Mount Shasta, CA

    Musfequr, hi.

    See the multi-page schematic here,

    https://raw.githubusercontent.com/OpenBCI/Ganglion_Hardware_Design_Files/master/Ganglion_SCH.pdf

    Impedance injection DAC is located on the 2nd page. And then see the impedance code section here:

    https://github.com/OpenBCI/OpenBCI_Ganglion_Library/blob/master/OpenBCI_Ganglion_Library.cpp#L404

    A fixed frequency AC signal is injected, then resistance measured via ohm's law: R = voltage / current.

    William

  • wjcroftwjcroft Mount Shasta, CA

    If you examine the code section linked above, you can see that the injected waveform is NOT a pure sine wave, which is a single frequency. But instead a triangle wave is injected, which carries a range of frequencies. But actually EEG waveforms are not sine waves either, and frequently have 'spiky' / jagged waveforms.

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

  • Thank you @wjcroft for your reply. I will be helpful for my work.

  • Hello,
    I have another question. For my work, I need impedance values during the measurement. If I record the impedance value from the software interface will it be accurate or do I need further calculation to get the impedance across the electrode? Some of the publications they use this formula: Actual Average Impedance(Ω) = Average Impedance(Ω) − 5000 to calculate cyton electrode impedance.

  • retiututretiutut Louisiana, USA

    By injecting current into a channel to measure impedance, it interferes with the actual signals that can be picked up from the electrode. Also, this can only be done on one electrode at a time, since there is signal bleed from injecting the current on a given channel into other nearby channels. For Cyton, you could check impedances first, then use a "Railed Percentage" calculation to roughly gauge the signal quality from all electrodes at once.

  • Hello,
    Thank you for the reply. I am recording the skin impedance from GUI for the Ganglion device. Is the GUI impedance data is raw data of that particular channel or do I need further calculation to get the skin impedance? As I got an answer from the code that it uses a 4 kHz single frequency during impedance measurement. And is this impedance value is reference value-dependent of the Ganglion?

  • wjcroftwjcroft Mount Shasta, CA
    edited February 2022

    @musfequr said:
    Thank you for the reply. I am recording the skin impedance from GUI for the Ganglion device. Is the GUI impedance data is raw data of that particular channel or do I need further calculation to get the skin impedance?

    The GUI widget is showing skin impedance. Check the widget source to see if it is applying more calculations, beyond what the firmware is doing.

    https://github.com/OpenBCI/OpenBCI_GUI/blob/master/OpenBCI_GUI/W_GanglionImpedance.pde

    As I got an answer from the code that it uses a 4 kHz single frequency during impedance measurement. And is this impedance value is reference value-dependent of the Ganglion?

    I don't believe this code section (previous link repeated below) is injecting 4 kHz. It is a much lower frequency than that. Frequency is in the brainwave region, no higher than 40 Hz. As I mentioned before it is a triangle wave, so not a single frequency.

    https://github.com/OpenBCI/OpenBCI_Ganglion_Library/blob/master/OpenBCI_Ganglion_Library.cpp#L404

    William

  • Thank you william

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