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ADS1299-4: periodical problem

Artem Zagoskin
Prodigy 20 points
Part Number: ADS1299-4

Hello,

I have a problem which lasts 10 seconds randomly. No matter internal or external signals. I see no reason why it appears.  

Here are some figures with problem.

1. No zoom.

2. The first problem case.

3. The second problem case.

4. Connection.

Thank you,

Artem.

  • 0
    TI__Guru 55152 points

    Hi Artem,

    To me, this looks like a combination of low sample rate and low resolution. 

    If you have not done so already, try increasing the gain of the input stage to be as large as possible, such that the full-scale input range of the device is maximized. 

    Additionally, try increasing the sampling rate to be as high as your system allows. 

    Is there any room for improvement with these two parameters? 

  • 0 in reply to Alexander Smith
    Prodigy 20 points

    Hi, Alexander!

    I'm in one team with Artem.
    Could you comment if this is a correct shape for the internal test signal?  I expected it to be a meander.

    Thanks for your support,

    Dmitry.

  • 0 in reply to Dmitry Mochalin
    TI__Guru 55152 points

    Hi Dmitry,

    The internal test signal should be a square wave. I'm not sure what you mean by a meander. 

    To me, it looks like not enough points are being taken per signal period, resulting in distorted waveforms. 

    If the register settings can be posted, that would help as well. 

  • 0
    Prodigy 20 points

    I pulled up "start" pin. It solved my problem, thanks for help.

  • 0 in reply to Alexander Smith
    Prodigy 20 points

    Thank you, It helped a lot!

    One more question:

    Our signal, after every relatively big artifact, shifts the baseline for a certain value for exactly 10 seconds. After 10 seconds it goes back. If the artifact was positive the shift is negative and vice-versa.

    On the screenshot you can see two shifts, both for 10s the first is small the second is bigger. 

    All electrodes shortened together, artifacts were simulated by disturbing contacts.  But on real EEG we had the same picture after coagulator artifacts.

    Could you advise what could be a reason for this? I expect this could be because of some baseline algorithms, but I found no description in the datasheet.

    Regards, 

    Dmitry.

  • 0 in reply to Dmitry Mochalin
    TI__Guru 55152 points

    Hi Dmitry, Hi Artem,

    Happy to hear that the initial problem is resolved! 

    The fact that is it always 10s is interesting. 

    What other electrical circuits share the same ground? Is there something turning on or off for 10s that could cause the ground to shift? 

    Depending on the magnitude of these shifts, you may consider digital filtering to remove this shift, otherwise I would recommend identifying and trying to fix the issue in the analog domain, somewhere on the PCB. 

    I often have questions related to a wandering DC baseline that occurs as the RLD loop balances out the circuit after deriving its amplitude from the input electrodes. Is the RLD circuit active during this test? If the RLD loop is deriving it's output signal from the input electrodes, it may simply be the result of this derivation. I do not expect this to be the reason though as it always lasts exactly 10s, and there is no variation to the baseline for the duration of the 10s.