Functions of BIAS(ground) electrode

edited August 2023 in Cyton

I was browsing the forum and on this link: https://openbci.com/forum/index.php?p=/discussion/136/50hz-power-filter-rather-than-60hz-power-filter
It is mentioned that:
"* The BIAS ("ground") signal which is injected back into the client, which is 180 degrees out of phase with the line noise, hence adding to the reduction."
I have a few questions:
What are the functions of BIAS(ground) electrode in EEG?
What does is meant by "signal is injected back into the client"? How much current? Is this common in EEG systems?

Comments

  • wjcroftwjcroft Mount Shasta, CA

    @Silver said:
    ...
    I have a few questions:
    What are the functions of BIAS(ground) electrode in EEG?

    All EEG has a Ground lead. Some EEG has the 'Bias' function included in the Ground connection. This is the case with the ADS1299 chip in the Cyton. Ground has the function of 'centering' the differential amplifiers used on each channel. Without it the amps could get out of range with large signals.

    What does is meant by "signal is injected back into the client"? How much current? Is this common in EEG systems?

    Because the mains EMF electromagnetic field artifact / noise is so strong at 50 or 60 Hz, it helps to have some hardware assist to 'counter' this influence. That is what the (very small) counter-mains injection does. This feature is also used in ECG medical systems where it is called DRL Driven Right Leg.

    https://www.google.com/search?q=driven+right+leg+circuit

  • edited August 2023

    I am not sure if I am understanding the function of the EEG Ground electrode correctly.

    Can you share any references or links so I can read more?

    Is it correct that the Ground electrode values are subtracted from the Reference electrode values and these new values of the Reference electrode are subtracted from the "Signal electrode" values to give the output signal values?

  • wjcroftwjcroft Mount Shasta, CA

    @Silver said:
    ...
    Is it correct that the Ground electrode values are subtracted from the Reference electrode values and these new values of the Reference electrode are subtracted from the "Signal electrode" values to give the output signal values?

    NO. ALL EEG / ECG systems have a 'Ground' electrode. As previously mentioned this only functions to keep the differential amps in a good range of their op amp inputs.

    NO such subtractions occur as you surmised. Not sure how you came to that conclusion.

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