DIY Cyton Board: ADS1299 Clamping Signal

GEngGEng
in Build-it-yourself
Hello,
I'm developing my own board and using the cyton firmware.
I'm injecting the signal but visualization on the OpenBCI GUI shows the
signal clamped as you can see below:


I have measured some connections in my new board and I realized that voltage
between VREFP (pin 24) is negative regarding to the VREFN (pin 25) what is strange...

I have build here one OpenBCI board, and I have measured in the same pins and
the voltage on VREFP is positive regarding to the VREFN. So my board is probabily
clamping the signal because this error, but I cannot find where is the error that is
causing this to happen. Someone that knows depper the ADS1299 can help me?

Here you can see my schematic:

schematic

Thanks in advance.

Comments

  • GEngGEng
    edited August 2019
    I have found here that I was measuring the pins incorrectly :D
    The voltage on VREFP is right, but I dont know what is making
    the signal get digitilized without the negative part
  • wjcroftwjcroft Mount Shasta, CA
    Gerald, hi.

    Perhaps you have an issue with your voltage source or the way you are connecting it to the ADS1299 / DIY Cyton.

    What happens when you try to measure alpha waves? Use the tutorial instructions for hookup: ear lobe reference to SRB2, other ear lobe to Bias, electrode on O1 or O2.

    Did you change the + and - analog voltage supplies to the ADS?

    William

    PS your schematic is not an image, but an .sch file. Suggest you post an image file.
  • GEngGEng
    William, hi.

    I did not measure alpha waves because I dont trust in the eeg signal...
    A did not change only the DVDD supply to one that has a wide input range but the same
    output voltage value. I need only six N channels in reference to SRB2, so connected all the
    other channels to AVDD, is that a problem?
    I measured the voltage of all the power supplies and the values are correct 2.5V, -2.5V, 3.3V.
    I had read the firmware of one board that works well with the pickit 3, and I got this .hex file and
    flashed it into my Custom DIY Cyton, is that a problem?
    Here bellow you can see .pdf of my schematic:

    schematic.pdf

    Greetings
  • wjcroftwjcroft Mount Shasta, CA
    "so connected all the other channels to AVDD..."

    NOTHING should be connected to AVDD. Your only scalp connections are to the IN*N pins, SRB2, and Bias.
  • GEngGEng
    Hi William,

    I followed what the datasheet says in the page 6:

    "(2) Connect unused analog inputs directly to AVDD."

    I connected all the unused all the analog inputs to AVDD, is that a problem?
    For example, I need the 1N in channel in respect to SRB2, so I connected the
    1P to AVDD, is that a problem?

    Please, to help me, take a quick look at the schematic pdf attached in the previous post.

    Greetings.

  • wjcroftwjcroft Mount Shasta, CA
    The GUI and firmware default state, is to use the IN*N pins for scalp electrodes, SRB2 as common reference, and Bias as ground. NOTHING should be connected to AVDD. SRB2 is bussing all the IN*P channels as common reference. If you have channels you are not using, turn off those channels using the GUI buttons. Have you done the Cyton tutorial? When you turn off a channel in the GUI, it sends SDK commands to the firmware to turn off the corresponding ADS1299 channel. NO HARDWARE connections to AVDD should be made.
  • GEngGEng
    OK Thanks for your explanation ;)
    Theese connections to AVDD can cause the signal is being clamped?
    Greetings
  • wjcroftwjcroft Mount Shasta, CA
    WIth the default firmware and GUI settings, SRB2 is acting as a bus, interconnecting all the IN*P pins. If you are connecting IN*P pins to AVDD, then yes, that AVDD signal is getting connected to EVERY channel.

    PLEASE perform all the steps of the Cyton tutorial to familiarize yourself with the operation of the GUI and settings.

Sign In or Register to comment.

© Vanilla Keystone Theme 2026