MyoWare and EMG, need to adjust SRB, Bias?

benevans101benevans101 France
edited July 2018 in Hardware
Apologies for having to ask such a basic question, but all of this is very new to me. My question has to do with understanding the relation between BIAS, SRB, and the desired measure. 

So, in the first "setup" tutorial for the Cyton,  we connected one electrode to a juicy muscle (the one we wanted to measure), and another to our wrist (which hasn't much muscle). The arm muscle electrode was connected to the bottom 4 pin, and the wrist to the top 4 pin. These were then taken OFF SRB and BIAS in the console, and there was a clear explanation for this. So far so good. 

Now, in setting up the MyoWare, two wires go to the same top and bottom pins on a given channel, as above. But a third wire (the R (likely reference) wire) goes to the top BIAS pin. However, in this case I'm not given any instruction to turn off SRB and BIAS on the hardware console. Why is that? In the first case the explanation was:

"The reason we do this is because the uV values for EMG and EKG are much bigger (and easier to pick up) than the EEG signals on channels 2 and 7. As a result, we want to prevent channel 4 from influencing the common mode noise rejection of the BIAS, as well as remove it from the EEG reference channel (SRB2)"

So isn't that a good idea to do here as well? If it isn't necessary, why not? I'm just trying to understand what is going on with these connections...

Thanks for your help!

Comments

  • wjcroftwjcroft Mount Shasta, CA
    Ben, hi.

    Are you using this MyoWare tutorial?


    The Cyton tutorial advises adjusting SRB and Bias settings, because that tutorial is doing simultaneous EEG and EMG. And the EMG signals would leak into the EEG channel and distort it. The MyoWare tutorial assumes you are doing only EMG, so no need to adjust settings.

    If your own experiments are only doing EMG or ECG, and no EEG, then the same applies -- no need to adjust settings.

    Regards,

    William

  • benevans101benevans101 France
    Yep, that's the tutorial I followed. 

    I'm trying to do simultaneous EMG, EEG and ECG so that I can do basic polsomnography. So I'm assuming that using the MyoWare in the configuration recommended in the tutorial will then require turning it off at least the SRB. But since it is connected to the TOP bias pin, I'm assuming I shouldn't turn it off of BIAS? 

    I suppose what I should be asking is for some clarity on precisely what BIAS and SRB are and how they function so I can figure this out myself. My impression is that the two SRB pins act as reference points - so when we measure an EEG signal what we are doing is comparing that signal with some more "neutral" point, like an earlobe, mastoid point or Cz location. OK so far, but then what does the BIAS do? In the Cyton Tutorial it is also connected to an earlobe, so I assume it is some sort of reference too? But what is it referencing? 

    Thanks,
    Ben

  • wjcroftwjcroft Mount Shasta, CA
    Ben, hi.

    All the Bias pins are the same. Bias is the same as 'Ground' in other EEG / EMG systems. This helps to center the differential amplifiers. And for the EEG especially, Bias injects a reverse phase mains tiny voltage, to cancel some of the mains noise. Since you already have other Bias connections with your EEG, you don't need one with the MyoWare / EMG / ECG.

    SRB2 (the pin closer to the board), is the default reference pin for all the channels, unless changed in the GUI settings.

    William

  • skiria8skiria8
    Does Bias always need to be connected somewhere? 

    When recording only EMG data (eg: detect muscle activation), could you deactivate the bias and use only 2 electrodes connected at N1 (bottom) +SRB2 (reference, bottom)? 
  • wjcroftwjcroft Mount Shasta, CA
    Skiria, hi.

    All biosensing applications should have the Bias electrode SOMEWHERE on the body. Bias (same as Ground on some equipment), centers the differential amplifiers in the analog to digital converter.

    In the tutorial examples we are doing simultaneous EEG and EMG, so the ear clip used for EEG Bias, doubles for the EMG channel, even though the EMG channel is disconnected from SRB and Bias buses. In the tutorial EMG is measured differentially between top and bottom pins on one channel, not using the SRB2 bus; nor does it use the Bias, because Bias (out of phase injection of mains noise), is intended for EEG primarily.

    The Bias line on the ADS1299 has combined functions: out of phase injection PLUS acting to center the differential amps.

    The short answer is, all biosensing needs to have a Ground / Bias somewhere on the body. If the Bias is not located near the head (for EEG), then it can be disconnected from the Bias in the GUI settings, so that other location does not contribute to the out of phase injection.

    Regards,

    William

  • skiria8skiria8
    Hi William, 

    That makes sense. So for EMG only I always need a min of 3 electrode ( signal electrode + reference + bias), right? 

    What is the diffrence between the top and bottom pins of the bias in the cyton board? 

    If I want to collect EMG + EOG + EEG, does this look like a good configuration?  
    - Facial EMG: N1n + N1p (acting as reference), No bias, No SRB2
    - EOG: N2p+N2n (acting as reference), No bias, No SRB2
    - EEG: N3n-N8n + SRB2 + Bias

  • wjcroftwjcroft Mount Shasta, CA
    The Cyton board labeling is slightly abbreviated. The actual TI ADS1299 pins are labeled IN1P, IN1N, IN2P, IN2N, etc.

    So yes, your pin mapping and settings look great.

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