Impedance value changes for no reason

giladgilad Israel

Hi,
I coded my Cyton board to run continuous impedance measurements and connected resistors to check the performance. For a few hours everything run smoothly and the impedance measurements were constant. But suddenly I started getting very high impedance values, more than 50X the original values. Then at some point I got close to zero values and the issue never resolved until I restarted the Cyton again. I've seen this in two different Cyton boards I have.
Do you know what is the reason for that? Why does the measured voltage is so big suddenly? I understand the injected current is 6na so I hope this is constant. Why does the voltage then drops almost to zero.
How can I prevent this? I need to do long measurements and I can't physically reboot the Cyton every time it happens.
Thanks for any help.
Gilad

Comments

  • wjcroftwjcroft Mount Shasta, CA

    Gilad, hi.

    Have you seen this Github issue?

    https://github.com/OpenBCI/OpenBCI_GUI/issues/1204

    If you mimicked (in Brainflow) the impedance code (hardware setting command) sent by the GUI, you may be seeing the bug. It is expected a new GUI version (reverting to previous impedance mode setting command), will fix this issue. Galea engineering priorities have postponed getting this fix implemented. Hoping that it will resolve early in 2024.

    William

  • giladgilad Israel

    Hi William,
    I'm not using the GUI for my measurements, I calculate the impedance from the voltage measurements. The voltage itself is changing for some reason, and it changes suddenly after being ok for a few hours.
    Gilad

  • wjcroftwjcroft Mount Shasta, CA

    @gilad said:
    ...
    I'm not using the GUI for my measurements, I calculate the impedance from the voltage measurements.

    Yes, I knew that. What I am suggesting is that the "hardware settings" that you send to the Cyton periodically, to put the ADS1299 into impedance measurement mode, may be distorting the impedance values measured.

    https://docs.openbci.com/Cyton/CytonSDK/#leadoff-impedance-commands
    https://docs.openbci.com/Cyton/CytonSDK/#channel-setting-commands

    Did you read the Github issue link I gave? One of the combinations 'drifts' in reported values.

    One hypothesis is that in the somewhat distant past (before the creation of the 'impedance widget'), the proper impedance and channel setting commands were being sent and impedance measurement worked fine. Then at some point the commands changed for the widget, and this issue began. By examining an older GUI before the widget happened, you may be able to deduce what those original settings commands were, that did not create artifacts. That's just my personal guess.

    Regards, William

  • giladgilad Israel

    Thanks William,
    As a workaround, I noticed that if I turn the Cyton board off and on again it solves the issue. Is there a command that I can send from the dongle to do the same? Sending the letter 'v' doesn't do the job, I understand this is only a soft reset. I think I need a hard reset and I wonder if there is a command for that.
    Thank you,
    Gilad

  • wjcroftwjcroft Mount Shasta, CA

    The only SDK serial port commands are listed here:

    https://docs.openbci.com/Cyton/CytonSDK/

    There is no 'hard reset' command. But you might experiment with say turning the channel off for a second, then on. (The first commands listed.) Or using the POWER_DOWN (then up again) setting in the 'Channel Setting Commands' section.

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