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ADS1158: Extremely weak MISO pin voltage relative to MOSI and SCK

Nikhil Ayyala
Prodigy 20 points
Part Number: ADS1158
The SPI pin we are observing a low voltage level on is the MISO pin. I have attached a MATLAB graph displaying the discrepancy that is observed between the SCK, MOSI and MISO voltage swings.
 
We have already looked through the data sheet and are observing a voltage maximum far below the minimum level.
 
I have provided to you separate compilations of screenshots that depict the pertinent sections of our layout and schematic.
  • 0
    TI__Mastermind 45641 points

    Hi Nikhil Ayyala,

    It is hard to tell on your schematic and layout, but it seems like you have separate grounds for AGND and DGND. You also show 4 ground symbols on your schematic (GND and QGND). Is it possible there is a voltage difference between these grounds such that DOUT is being offset? The fact that the signal you show reaches a steady level instead of just being all over the place suggests to me that the actual signal is okay, it is the way you are measuring it that could be the issue. Can you show on your schematic where you are probing to that this data?

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 20 points

    Hi Bryan,

    I did not provide a full view of the graph that displays the issue at hand. In the screenshot attached, it is clear what we are observing: we see full voltage swing in some SCK cycles and not in others.

     

    Furthermore, we analyzed our ground signals, and they are all grounded correctly. No issues on that end.

  • 0 in reply to Nikhil Ayyala
    TI__Mastermind 45641 points

    Hi Nikhil Ayyala,

    Can you provide these signals on a scope instead? It is easier to interpret the information when the signals are not overlapped. Please make sure to label each signal in the scope images so it is clear what we are looking at

    Are you seeing this behavior on all of your boards, or just one?

    Are you sure that "weak voltage" is not just a glitch, and is not actually intended to be a 1 (logic high)? Since I don't know what you are doing it's hard to know what you expect to happen versus what is actually happening

    -Bryan