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ADS1299: 1000SPS in Cascade mode

Sanskar Kumar
Prodigy 30 points
Part Number: ADS1299

Tool/software:

Hi,

Has anyone tried 1000SPS data rate in cascade mode with 2 or 3 ADS1299.
We have 3 ADS setup in cascade mode all the settings are done successfully and we are able to receive data as well. The issue is we are not getting 1000 samples as expected.

We have also analyzed DRDY generation for all the three ADS1299s, we observed that when we are not reading data DRDY is generated as expected. But as soon as we start reading (SPI enabled) DRDY is frequently missed.

In the attached image you can see DRDY generally goes LOW and immediately goes HIGH indicating that SCK is received. But sometimes it goes LOW and stays LOW all the time and goes HIGH for 4tclk before going LOW again. Hence I am calling it missed.

Is there something that I am missing or not considering.


Thanks for the help

  • 0
    TI__Guru 66386 points

    Hello Sanskar,

    Thank you for your post.

    The image you attached does not show nDRDY. Also, I do not see SCLK toggling in your logic analyzer capture. Instead it appears to remain high the whole time. Maybe you attached the wrong image by mistake?

    Also, note that the ADS1299 SPI expects mode 01, meaning CPOL = 0 and CPHA = 1. SCLK should idle low between frames.

    In Cascade Mode, it is only necessary to monitor the nDRDY pin of one device after a START pulse synchronizes their respective conversions. Each nCS pin should be brought low one at a time to read all 216 bits in each frame. At 1 kSPS, you'll need an SCLK frequency greater than 648 kHz. Can you confirm your SCLK frequency?

    It would help to see just one or two frames showing the cascaded data read from all three ADS1299 devices following an nDRDY interrupt to review the timing and sequence are as expected.

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 30 points

    Hi Ryan,

    Apologies for the confusion that I created. Although the image attached is the correct one, the labels are not. The 3 signals shown in the image are all DRDY signals of three different ADS1299.

    We identified the issue and resolved it but we have more observations related to DRDY which are confusing us. I will create a separate post about it since it is unrelated to this one.

    Thank you for your quick response.