what is the relationship between high impedance on EEG channels and 50/60hz noise?

khofstadterkhofstadter Cambridge, U.K.
Hello! 

What is the relationship between high impedance on EEG channels and 50/60hz noise?

For instance, can a high impedance on one EEG channel e.g. on PZ add to the amplitude of the 50/60hz noise?

Or, can a not well 'grounded' GND channel add to the impedance of a EEG channel e.g. PZ? 

Thank you! k


Comments

  • wjcroftwjcroft Mount Shasta, CA
    Kris, hi.

    Did you read the paper by EGI scientist Tom Ferree?


    EGI makes clinical and research grade saline electrode "geodesic mesh" caps.


    The Greentek saline cap you are using has many similarities to what EGI is doing. Hence if your measured impedance (with Cyton) is 40K ohms or less, you should be fine. Could probably be even somewhat higher, since the ADS1299 chip has a 1 gigaohm input impedance. The higher the amp input impedance, the less important to comply with the (previous generation EEG equipment) requirement for 5K ohm skin impedance. At one point EEG equipment had input impedances in the low megohms range (say 1 to 10 megohms). This is no longer the case with amps that have input impedance in the hundreds or thousands of megohms.

    Yes, if your skin impedances are way out of range, they will increase noise. But also be sure to use a notch filter at the mains frequency.

    The Bias/Ground lead on the ADS1299 is not intended to be connected to a mains wall ground pin. But it does help to center the differential amps, and in the case of 'Bias', inject some inverse signal to counteract common mode noise. Your scalp ground connection is usually best positioned on the midline. Electro-cap uses AFz position. 

    Regards,

    William

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