Ganglion test with square wave input, droops somewhat

mefox6mefox6 Dallas, TX
edited June 2019 in Ganglion
Hi! I'm trying to get a square wave to come out of the Ganglion board still square. I have the BP filter off, but I'm getting some drooping. Does anyone know what the capacitance of the board is? Or any info on capacitive compensation for the board and connections? 

Thanks!

Comments

  • wjcroftwjcroft Mount Shasta, CA
    Mefox, hi.

    Are you wiring some type of signal generator or clock to the input pins? Are you using a voltage divider, to get the output into the say 50 microvolt range?

    Typical EEG skin impedance is nominally in the few Kohms range; or at most some tens (or maybe hundred with dry sensors) of Kohms. Whereas the input impedance of the Ganglion was originally thought to be in the 100 Mohm range. But later calculations by Bruce Pritchard @bpwork, more realistically set it at around 660 Kohms. 


    To contrast, the input impedance of the Cyton (ADS1299) is nominally 1 Gohm. Higher input impedances are especially important with dry electrodes, which can have high skin impedance. 

    The other factor which may distort the square waves you are seeing, is that the Ganglion A/D output stream is compressed. And sudden changes of large magnitude may not delta compress as expected. Try changing your divider so that changes are not so abrupt.


    Regards,

    William

  • wjcroftwjcroft Mount Shasta, CA
    Or instead of a square wave, use a sine or triangle wave. These have more normal waveforms that change gradually over time. EEG, ECG waveforms are not square in nature.
  • mefox6mefox6 Dallas, TX
    Yes; I'm using a function generator and a 60DB attenuator to get an input of about 500uV that goes directly into one of the channel inputs.

    Is 500uV too high even though the scaling of the time series screen can go much higher? I couldn't find anything about maximum input for the device itself, just information on EEG and ECG which is fine, but I'm trying to see what else the Ganglion can be used for. 

    I'm not having an issue with it reading the sudden changes; the problem is that a constant DC input won't stay constant, it falls off, suggesting the need for capacitive compensation at the input? I added a capacitor of 100uF in parallel with the input and it helped a little bit, but I'm hoping someone else has tried to do this already and could give me pointers!

    Thank you for those links! They were helpful. And per your second comment... I specifically need it to be able to retain a square wave form though! I'm sure it can work, just not totally sure how to get there.

    Best,
    Mary
  • wjcroftwjcroft Mount Shasta, CA
    Please try 50 uV. The raw samples are 24 bits, but the low order bits are fractions of a microvolt. EEG does not jump 500 uV at once.

    If the delta compression cannot keep up, there will be a lag.
  • mefox6mefox6 Dallas, TX
    I'll try. But if it was just lagging, would my voltage level stay the same if it's constant already?
  • wjcroftwjcroft Mount Shasta, CA
    edited June 2019
    If your function generator can produce triangle and sine waves, do THEY look ok?
  • wjcroftwjcroft Mount Shasta, CA
    https://www.google.com/search?q=eeg+phantom+head
  • mefox6mefox6 Dallas, TX
    I didn't check! I'll try tomorrow and let you know. Thanks for your help so far!
  • wjcroftwjcroft Mount Shasta, CA
    https://www.google.com/search?q=impedance+matching

    EEG amp inputs are expecting impedances in the range of less than 5 Kohms, to a few ten K ohms. Your source might not be meeting that requirement. But there are ways to impedance match. Phantom heads are designed for testing EEG amps and reproduce typical skin impedances.
  • mefox6mefox6 Dallas, TX
    My impedance check before I tried the square waves was less than 5kohms... I'm not trying to do EEGs. I just want to know if I can get a square wave to come out square :)
  • wjcroftwjcroft Mount Shasta, CA
    Cyton would have no problem with your square waves. It's stream is uncompressed. Ganglion uses BLE, which is highly compressed.

    Unclear if the Ganglion impedance check system works the same on skin vs. electronic outputs such as your signal generator. Skin impedance combines resistive and capacitive elements.

    If you used a WiFi shield with the Ganglion, then I believe the delta compression is no longer used. Your sine wave / triangle wave test will tell if this is a compression failure issue. Another test you could run is just with a DC source and a potentiometer to see if you can reach both voltages.
  • mefox6mefox6 Dallas, TX
    edited June 2019
    Oh okay! I'll try these tests and let you know how it all goes! Thank you so much!

  • mefox6mefox6 Dallas, TX
    Everything seems fine with square and sine waves at inputs up to 500uV. Maybe I'll just work with triangles. Thanks for your help!
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