not detecting a 100uV DC signal generator

giuseppetx1giuseppetx1 Italy
edited October 2017 in Cyton
Hi, and pleased to meet you all. I have just registered on the forum...
I am using Cyton board and trying to amplify a low DC signal (about 100 microVolts) generated directly from a signal generator.
I have connected the output signal to one of the SNP pin of the OpenBCI board (for example, the n. 2).
Then I have grounded the DC signal generator and the board connecting the ground with the GND pin of the OpenBCI.
I am not seeing any signals on the OpenGUI.
Surely I am getting many thins wrongs...Could you help me in some way?
Thank you to youall in advanced....
G.

Comments

  • wjcroftwjcroft Mount Shasta, CA
    Giuseppe, hi.

    If you are trying to simulate an EEG signal with your generator, then connect the same as you would with the scalp: channel, reference (SRB2) and Bias (Ground). Your + generator lead would go the the channel pin (closest to board), and the - lead connect to both SRB2 and Bias.

    Try first generating a 100 uV AC signal and see if that shows up.

    William

  • wjcroftwjcroft Mount Shasta, CA
    edited October 2017
    You might be confusing the *GND pins (which are not normally used) with the scalp pins. The ADS1299 only measures differentially between channel and reference or + and - channel if configured as disconnected from the SRB bus.

  • Dear William, thank you for your reply.
    I have not yet an AC generator, only DC.
    Do you think can it work?
    I have likely got something wrong because I cannot retrive any signals,
    only noise (1uV Vrms).

    Thank you, Giuseppe.
  • wjcroftwjcroft Mount Shasta, CA
    Did you follow the previous instructions I left? You cannot use the GND pins on the Cyton. Instead connect the same as you would connect to the head: channel, reference (SRB2), and Bias.

    If you have a DC signal available, you can surely simulate AC or square wave, by turning it on and off??

    The DC capability of ADS1299 is known to work.

  • Hi again William, yes I followed your instructions and it looks work fine right now! thanks.
    Just another task....sending a 100 uV AC to the N2P pin (for example) of the Open BCI board
    I see almost the whole signal (just below 100 uV) with the PGA Gain setted at x24.
    While setting it at x1 o others the signal acquired drops down to a few tens of uVrms (20 or 30)...
    What does the PGA Gain mean at the end???

    Thank you for your support !
    G.
  • wjcroftwjcroft Mount Shasta, CA
    The Gain adjust is just a way of reducing amplitude on strong signals such as ECG. See the Cyton tutorial. In that example a simultaneous measure of EEG and ECG and EMG is done. Each on a different channel. With the gain reduced for the non-EEG channels.
  • OK. Many Thanks. Then Looking for datasheet i would need to know what is the max voltage amplitude that a SNP pin can receive as input signal? Thank you. G.
  • wjcroftwjcroft Mount Shasta, CA
    You could go up into the millivolts, even 1 volt if you adjust the scale. Wouldn't go higher than that.


    See page 7.

  • wjcroft , I want to understand electronically what happens if I use the GND of the OpenBCI shorthed with reference (SRB pin or the negative input of PGA)?
    Because in the datasheet pag 69 and figure 74 a sine generator is connected to the PGA using the GND...
    http://www.ti.com/lit/ds/symlink/ads1299.pdf

    Thanks a lot.

    Geraldo Gomes.
  • wjcroftwjcroft Mount Shasta, CA
    Geraldo, hi.

    Normally in EEG applications, many times the Ground / Bias lead is placed on the 10-20 map at the midline between hemispheres. So that the 180 degree out of phase common mode injection (from Bias), is distributed more evenly across the skull. With Electro-cap, that position is AFz. (10-10 map).

    Generally Reference (SRB2) is at another location than Bias / Ground. These are two different functions in all EEG amplifiers I'm aware of. Another function of Ground is to center the differential amplifiers.

    Please see the previous post above for test connections of a signal generator: Never use the GND pin, simulate a human connection with channel, SRB2 and Bias.

    ----

    Giuseppe, hi.

    If you are trying to simulate an EEG signal with your generator, then connect the same as you would with the scalp: channel, reference (SRB2) and Bias (Ground). Your + generator lead would go the the channel pin (closest to board), and the - lead connect to both SRB2 and Bias.

    Try first generating a 100 uV AC signal and see if that shows up.

  • Hey wjcrof, thanks for your reply!
    I was not clear, I'm sorry. I understand the functions of bias and the reference ;)

    I just want to understand what happens electronically if I connect the GND pin of the board to
    the reference (SRB pin). I know that you are saying that this is a wrong connection, but I want to understand why, because
    the datasheet seems to use the analog ground (not the bias) to inject signal as you can see in the pag 69 and figure 74.

    Thanks a lot
  • wjcroftwjcroft Mount Shasta, CA
    edited May 2019
    My interpretation of Figure 74 on page 69, is that it shows the standard symbol for "Ground" attached to one lead of the signal generator. "Ground" and "Bias" on the ADS1299 is the same function, labeled as "Bias" on the Cyton header.

    The AGND pin on the header should in general, not be used for external connection. Because this is the overall "board ground" bus, also shown as GND in the schematic. 


    Page 69 also mentions that their test setup had quite a bit of mains noise and needed to be digitally filtered. It seems logical that they would want to take advantage of the inverse common mode injection feature of the ADS Bias / Ground to eliminate some of this mains noise.

    So DON'T connect the header AGND pin to any other header input pins.

    Regards,

  • OK.

    Thanks a lot for you help!
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