Driven Ground not connected?

edited February 2017 in Ganglion
Based on the open source schematics for the ganglion board, it looks like driven ground is not being used in any functional way for EEG/EMG acquisition (it's connected to input protection, and there's a voltage follower but it's not connected to anything else). What's going on here?


  • wjcroftwjcroft Mount Shasta, CA
    I believe I heard Joel mention that they did testing with DRL both connected and disconnected, but found no significant improvement in CMRR. (Common Mode Rejection Ratio.) So vestigial circuit elements may be present but not wired. Joel @biomurph may comment further.

  • edited February 2017
    Interesting... so the ground electrode is not being used at all? To be clear, I'm talking about the label D_G/driven ground, which is supposed to be connected to the ground reference electrode. Where would it be connected if it wasn't vestigial?

    Thanks :)
  • wjcroftwjcroft Mount Shasta, CA
    Ground is not the same as Driven Ground. DRL, "driven right leg" is a derived injected signal, that is 180 degrees out of phase with the common mode noise, typically mains noise. The injected low level signal cancels some of the mains noise. Some amps do this injection, some do not.

    So in short, ground exists and is used in all EEG. Ganglion may not have driven ground, if it was tested and found to not improve the CMRR.

    Does that make sense?
  • We're on the same page I believe. For context, I have personally implemented an EMG acquisition system based around the ADS1298. When I say 'ground electrode', I am referring to the same DRL/RLD (right leg drive) electrode as you, not electrical ground. This is the electrode that should normally be placed on the elbow in a conventional EMG system. 

    I do see that there is an implementation of the right leg drive in the Ganglion schematics, but it never actually gets connected to the electrode itself, meaning that anyone using the Ganglion would get the exact same performance with nothing connected to the pin labeled D_G that they would with an electrode there. I found this confusing because the getting started guide indicates that you should connect an electrode to D_G, but based on the schematic, it wasn't being used anywhere. The main reason I asked about it is because based on my experience, I find it surprising that you got the same CMRR without using a DLR/RLD circuit; I know that on my system there's a significant difference in signal noise when the elbow/ground electrode  is connected vs. when it is not. However, my system doesn't have instrumentation amps before the inputs to the ADS1298 (the device has them built in); perhaps that's the reason your CMRR is so good? 

    Thanks for taking the time to answer my questions! 
  • wjcroftwjcroft Mount Shasta, CA
    I'm imagining Joel @biomurph may have some comments here. But I understand the lab is pretty busy just getting gear out the door.
  • biomurphbiomurph Brooklyn, NY
    The D_G pin on the Ganglion is connected to the output of a follower circuit that outputs 1.5V. This is the '0V' in our circuit. In the design phase we never saw any better CMRR between the D_G or a DRL/RLD type circuit with inverted common mode. 
    It is recommended that you connect an electrode to your skin that goes to the D_G output to ensure that your body and the Ganglion both agree on what '0V' is.
  • edited July 2017
    I didn't notice your reply! Looking back it appears I misunderstood the role of your driven ground circuit. What you're saying makes perfect sense. 
    Thanks for helping me out :)

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