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ADS131A04: PCB layout query

Karol Dalupang
Prodigy 75 points
Part Number: ADS131A04


Good Day

I've been developing a 4 layer board for the ADS131A04, I've used the schematics from its evaluation module to serve as basis. I've completed assembling the board circuitry and was able to initialize the chip just fine. However, when taking samples, I'm only able to get 2.5V for channels 1 and 2, then -2.5V for channels 3 and 4, regardless of the input. I would like to get feedback on whether I screwed something up with the layout for the differential signals. I've attached the board schematic and the board layout (layer stackup is top-GND-PWR-bottom). Please note that colored traces mean they are power traces (3.3V for yellow, 2.5V for violet, -2.5V for cyan, 5V for green)

I wanted to think it was an assembly issue since I soldered the chip myself but the chip was able to initialize so that must mean it's working. I thought that maybe it's a layout issue (ie. some of the power rails interfering with the inputs) but I measured the voltages across capacitors C28, C32, and C36. They were all consistent with the DC signal I was putting in which means the signal was not altered on its way to the adc's pin.

I would also like to rule out the possibility that the conversion in my code is erroneous because I've developed this code and tested it using the ADS131A04 evaluation module itself, the conversion was correct.

Any insight on why this is happening would be very much appreciated. Thanks!

Edit: Can't seem to attach pdf files, the design files are available here https://drive.google.com/open?id=0B-E2XwTBQeNOclliNmp2Q244WkU

  • 0
    TI__Guru 66706 points
    HI Karol,

    Thanks for your question!

    Have you probed the reference voltage as well? To me, it sounds like the ADC output is clipping at either positive or negative full-scale. From the schematic, it appears you intended to use the internal reference; however, you have tied REFN to the positive analog supply, 2.5V. REFN needs to be tied to AVSS, which is -2.5V in your system. I understand this may be a typo, but please confirm that the reference voltage is correct by measuring the differential reference voltage (REFP - REFN). Also, double-check that you have selected the external reference option (INT_REFEN = 0).

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 75 points

    Thanks for the reply Ryan.

    You're right, the REFN pin was connected to AVDD (2.5V) when it should have been connected AVSS (-2.5V). Would this explain the readings I've been having? I'll fix this in the  design right away.

    Ryan Andrews said:
    From the schematic, it appears you intended to use the internal reference;

    I was trying to use -2.5V as an external reference source, may I ask why it appeared otherwise?

    Ryan Andrews said:
    Also, double-check that you have selected the external reference option (INT_REFEN = 0). 

    Yes, I've checked that INT_REFEN was set to 0 so that it uses an external reference.

    Again, thank you very much for catching my error.

  • 0 in reply to Karol Dalupang
    TI__Guru 66706 points

    Hi Karol,

    In my first post, I should have said "...double-check that you have selected the internal reference option (INT_REFEN = 1)." My initial assumption was that you were using the internal reference because you do not have an external reference voltage source on the schematic.

    If you are trying to use an external reference (INT_REFEN = 0), then you need to connect an external reference voltage to the REFEXT pin. Currently, you only have a bypass cap between REFEXT and AVSS. The voltage that can be applied to the REFEXT pin must be between VREFN + 2V and AVDD - 0.5V. Please see the Recommended Operating Conditions on page 7.

    Without a proper reference voltage, the ADC cannot convert the input voltage, so I would expect to see a positive or negative full-scale reading as you have described.

    Best Regards,