Why we need reference electrodes in bi-polar configuration

ls870825ls870825 San Diego
edited June 2017 in Electrodes
Sorry if this question has been asked before, I am new to the EMG signal capture. I have a fundamental question that why we need reference electrode in bipolar configuration for EMG signal capture. 

I understand the reason of having reference electrode for monopolar configuration to used as 'common mode rejection'. However in bipolar configuration, we are capturing the difference between two senor point (V_a - V_b), the difference itself should already take care of the 'common mode rejection', Why in practice, people use (V_a - V_ref) - (V_b - V_ref)?

Thanks in advance!
Sen

Comments

  • wjcroftwjcroft Mount Shasta, CA
    Sen, hi.

    The short answer is "you don't use reference" for the EMG tutorial examples shown on (for example),


    These tutorials for the EMG case show the + and - leads connected to opposite arms. The tutorial still does show SRB2 (reference) connected for the EEG. The CHAN SET tab is used to disconnect the EMG electrodes from the SRB bus.


    William

  • ls870825ls870825 San Diego
    Hi William.

    Thanks for your comments. In tutorial case, two electrodes are connected on different arms, which can capture a signal of some kind - I assume we are capture the potential difference between two arms which the signal amplitude would be relatively large. but it is not specific to a certain (maybe small) muscle of interest. For example, if I am interested in only taking EMG measurement of vocal cord muscle, which can be noisy due to the low signal amplitude, how reliable it would be without using a reference electrode? I am not sure you used MyoWare EMG sensor for your application before, it is a bipolar configuration and it is recommended to have reference port connected as well, do you see why is that?

    I also noticed that in the tutorial, people are using gold cup electrode. I guess it is not a active/pre-amplified dry electrode, right? I have learned that many applications are using wet/gel Ag-Agcl electrode, is there a reason we choose one other the other?

    Best,
    Sen
  • wjcroftwjcroft Mount Shasta, CA
    edited June 2017
    Sen, hi. To clarify my previous post, the two (channel #4) electrodes are used for both EMG and ECG. Green (+) to the right forearm and blue (-) to the wrist. Both of these are disconnected from the Reference and Bias (Ground) buses by the CHAN SET steps in the tutorial.

    In this case the wrist (with little muscle activity) is acting more as a neutral place. The forearm is what has much more muscle EMG available to see. You ask about vocal cord muscle. That could be difficult because it is so interior to the larynx. And I think multiple small muscles, ligaments are involved.


    ANY area not directly over muscle can be used as your reference, I guess you should just experiment around and see what you can pickup. Perhaps seeing how various research papers do this.


    I havent used MyoWare myself, so I'm just referencing the OpenBCI tutorial. It could be in the MyoWare that "reference" is in fact "ground". It's always a good idea to have a Ground (or sometimes called Bias) lead connected in a biosignal application. This Ground lead allows centering the differential amplifiers, and also can be used for some degree of active noise cancellation by feeding back a inverse version of the common mode noise. (In the tutorial example this was disconnected for the EMG ECG leads so as not to interfere with the simultaneous EEG).

    So for your question about vocal cord measurement with OpenBCI, alone without simultaneous other biosignals, you could just use the default configuration of reference and bias pins. So you would use the reference pin (SRB2) as your no-muscle neutral area, then attach various IN*N (closest to board) pins to parts of the throat. Finally your Bias lead could go to any convenient area on the neck.

    Pre-gelled snap electrodes are handy and a good way to go. In your case perhaps get the small kind that are not too large in diameter. The example just uses gold cups because they are in the OpenBCI kits. Advantage of the pre-gelled is that there is no paste application or mess. AgCl signal is also a bit cleaner than gold. They are more expensive over the long run because they are disposable. 

  • ajebulonajebulon South Korea
    Dear @wjcroft,

    I'm sorry to bring up old topic. I have similar question as OP. I noticed that there are 2 kind of electrode placement for EMG measurement, monopolar and bipolar, as shown in this link. 

    I've already read the Cyton Getting Started tutorial about how the electrode placement to measure EMG + ECG. That is an example for monopolar recording, isn't it? Please correct me if I'm wrong.

    My questions are:
    1. Is it possible to do bipolar recording using OpenBCI? Where to connect to?
    2. If I connect 1P 1N close with each other, what pin should I connect as reference (for bipolar recording)? Is it SRB2 or BIAS? 
    3. And same question as OP, if reference is not used in bipolar recording as shown in this link, will it affect the result much?

    Thank you.
  • wjcroftwjcroft Mount Shasta, CA
    edited October 2017
    Hi Ajebulon,

    The Cyton Getting Started Guide is showing one EMG electrode at the wrist ("Electrically Neutral Tissue" as shown at your link). And one electrode at the forearm "Muscle / Detection Surface". So it corresponds to the monopolar diagram. However the connection at the Cyton is using + and - "differential" pins, so as not to interfere with the simultaneous EEG. The Bias (same as Ground) lead is on the ear lobe.

    (1) the bipolar (according to your link) is really setup the same way, except that both + and - are on the muscle being measured.

    (2) your link calls the 3rd lead "reference", but it is really "ground", as shown by the schematic symbol used, so it is really the same wiring as the monopolar. The first diagram at your link just omitted the Ground lead for some reason. The Bias/Ground lead can be anywhere on the body. In EEG it is typically near or on the head.

    (3) You always want to have the Ground/Bias lead somewhere on the body, as it helps to center the differential amplifiers.

    William

  • ajebulonajebulon South Korea
    Dear @wjcroft,

    Thank you for your response. Great explanation as always. I want to make sure whether my setup is correct.

    My setup for bipolar EMG recording:
    1. Connect 1P to the muscle
    2. Connect 1N to the muscle next to #1
    3. Connect BIAS pin somewhere neutral
    4. Turn off SRB2/SRB1 in the OpenBCI GUI
    5. Turn on BIAS in the OpenBCI GUI

    Is that correct?

    And another question, if I change the configuration as in step #4 and #5, is the recorded trace file also affected? Or only the GUI is affected? I mean, is the recorded value in the trace file already excludes SRB2/SRB1 and include the BIAS? I ask this because when I need to apply filtering (notch or band-pass) to the recorded value in the trace file, even if I have selected the kind of filter I want to use in the OpenBCI GUI. So, I wonder if the hardware setting also behaves the same as filter.

    Thank you.
  • wjcroftwjcroft Mount Shasta, CA
    Yes, your setup looks right. Bias should already be turned on for all channels.

    The default power up (and GUI program load state) is to use Bias and use SRB2 as the common reference. With the channels using the IN*N pins. So you should just have to disable the SRB2 connection of those channels you want to operate differentially. 

    Yes the on screen and recorded file reflects your setting choices. However remember that the recorded file DOES NOT include any notch or bandpass filtering. You need to do that yourself.
  • ajebulonajebulon South Korea
    Dear @wjcroft,

    Thank you! I'll try it. Hope this could be good for others too
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