Best connection for 1 channel EEG: differential or SRB2?

compujohnnycompujohnny
edited June 2018 in Cyton
If I want to record 1 channel of EEG say T4 - P4, Is it better to connect Bias to A2, SRB2 to T4 and Bottom 1N pin to P4, or use top 1P pin for T4 and bottom 1N pin for P4? and is there any difference in the resulting signal for either approach?

I am planning on doing multiple positions per session but only one at a time (ex: T4 - P4 for 10 min, then T4 - FP2 for 10 min, then T3 - T4 for 10 min) and I don't want to replace the T4 electrode as it's common in all 3 positions, so I was thinking that I will put T4 in SRB2 and put P4, FP2 and T3 in 1N, 2N, 3N, so I can connect 4 electrodes plus BIAS and switch the active channel mid session by software

Will that wiring work for the purpose I explained, or does anyone have better suggestions?

Comments

  • wjcroftwjcroft Mount Shasta, CA
    John, hi.

    Using SRB2 as your common reference bus is fine, and is the default firmware configuration. No advantage to treating channels as P/N differential. The tutorial only uses differential ECG as an example, so the scale factor can be changed per channel allowing simultaneous EEG and ECG.

    Regards,

    William

  • compujohnnycompujohnny
    Thanks a lot, can you check the impedance of SRB2?
  • wjcroftwjcroft Mount Shasta, CA
    edited June 2019
    Because the input impedance of the ADS1299 is so high, (1 gigaohm), electrode impedance is rarely an issue. SRB2 is not measured separately.


  • compujohnnycompujohnny
    Does that mean that I don't need to worry about electrode impedance with the ADS1299? As long as I am not getting 50/60 Hz noise then I am fine?

    In the Othmer method course they stress upon doing an impedance check before starting the session, does that mean that their $5000 device has less input impedance than the Cyton?

    Or is that just a legacy concept that don't apply in more advanced electronic chips like the ADS1299?
  • wjcroftwjcroft Mount Shasta, CA
    Did you read the Ferree paper linked above? Which concludes,

    Results: There was no significant amplitude change in any EEG frequency bands as scalp-electrode impedance increased from less than 10 Kohm (abraded skin) to 40 Kohm (intact skin). 60 Hz was nearly independent of impedance mismatch, suggesting that capacitively coupled noise appearing differentially across mismatched electrode impedances did not contribute substantially to the observed 60 Hz noise levels.

    And that was with an amp that had 200 Mohm input impedance. ADS1299 is FIVE times that at 1 gigaohm.

    4. Discussion
    We have reviewed theoretically and tested experimentally the dependence of EEG signal attenuation and 60 Hz noise on scalp-electrode impedance. We have shown that if the amplifier input-impedance is high enough (~ 200 Mohm), there is negligible signal attenuation when using electrodes without abrasion. This remains true even for older amplifier systems with modest input impedances (~ 10 Mohm). In our experiments, using an amplifier with an input-impedance of 200 Mohm and scalp-electrode impedances up to 40 Kohm, there was no significant attentuation in any of the standard EEG frequency bands. Circuit analysis suggests that, for this input impedance, scalp-electrode impedances up to 200 Kohm still allow for accurate (~ 0.1% error) signal acquisition.

    It's worth checking impedance occasionally, just so you have confidence in your wires and connectors. The skin impedance appears to be not much of an issue.

  • wjcroftwjcroft Mount Shasta, CA
    I assume you are using Ag-AgCl electrodes. Those are essential for ILF.

  • compujohnnycompujohnny
    Thanks, Yes I am using Ag-AgCl electrodes
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