External trigger interference with EEG signals ?

plutopluto mars
edited February 2022 in Cyton

@wjcroft and @retiutut Thank you so much for your constant support.
In our recent experiment, we noticed that when Analog/Digital read is enabled, EEG channels were getting interfered by the external trigger signal. Any idea why is this happening and how to resolve this?

Comments

  • wjcroftwjcroft Mount Shasta, CA
    edited February 2022

    All cables carrying electricity can 'interfere' with each other, by means of inductive or capacitive coupling. If your EEG cables are bundled with the trigger cable, or even near the trigger cable, such coupling can happen. So first thing to try is to get some cable separation.

    The PIC32 ADC 'trigger input' / analog input, is a 10 bit ADC with 0V being 0 and 3.3V being 1023 (2^10 - 1). Reducing the voltage OR current in the trigger line, will result in less coupling. Voltage can be reduced via appropriate voltage divider network. So to utilize this tip, you need to read in 'analog' mode, not 'digital' (which only has two levels, 0V and 3.3V).

    https://www.allaboutcircuits.com/tools/voltage-divider-calculator/

    William

  • retiututretiutut Louisiana, USA

    I either used a resistor(via voltage divider circuit) or an Optoisolator to resolve this. Thanks for asking!

  • wjcroftwjcroft Mount Shasta, CA

    Yes, I assumed @pluto was already aware of the optoisolator document,

    https://docs.openbci.com/Cyton/CytonExternal/#optoisolation

    If you are not using this, you should. If you ARE using it, and seeing coupling, try the voltage divider tip.

Sign In or Register to comment.