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ADS124S08: Noisy Readings

Andrew5821
Genius 4555 points
Part Number: ADS124S08
Other Parts Discussed in Thread: LM4132,

Hello,

We are getting noisy ADC readings beyond what we would expect.  If I ground the input, I get readings that match Table 7, for example, but as soon as we apply voltages higher than that the noise goes up.  For example, at 200SPS we still get 0.8mV of peak to peak noise at ~2.5V.  At 2000sps we get 1.4mV.  To ensure our sensors weren't the issue, I also fed 2.5V from just a E3620A bench supply and got similar results.  I've attached a schematic for review.  I've reviewed the PCB layout looks very good per the layout guidelines and we are usually pretty good about laying out the data converters properly.  The reference is an LM4132 providing an external 3V reference.

  • 0
    TI__Guru** 112895 points

    Hi Andrew,

    One thing schematically is you are missing the required 330nF cap between AVDD and AVSS.  If you are going to use the internal reference, REFCOM should be connected to AVSS.  It would be helpful to know the configuration settings you are using.  Have you noticed if the data is showing power line-cycle noise?  Or possibly some other noise pattern?  Can you provide a set of raw contiguous data for me to review?

    Aside from poor PCB layout, noise can come from the input voltage (which may include EMI/RFI) as well as the supplies and reference.  It appears that in most cases you are using PGA_Bypass according to the schematic notes.

    When troubleshooting it is good to monitor the supplies and reference to ensure that there is no oscillations taking place.  As you are missing the required 330nF cap there may be some instability within the ADS124S08 as well.  I would try monitoring your AVSS supply.  There may be some instability in this supply configuration.  There may be some issues with the ability of the op amp to sink/source the required current.  I would try connecting AVSS direct to GND, and then see if this clears up the noise.

    Once the source of the noise is determined, then we can work on ways to resolve the issue.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Genius 4555 points

    Bob,

    I disconnected the AVSS supply and tied it to ground.  It did not improve things.  Also, adding a 1uF capacitor between AVDD and AVSS it also did nothing to help.  As far as I can tell, the supplies look clean at least to the degree of what an oscilloscope can tell you.  I'm ordering an EVAL board to hack for comparison.  AVDD (3.1V) also looks clean.

  • 0 in reply to Andrew5821
    TI__Guru** 112895 points

    Hi Andrew, 

    There are a couple of other things you can try.  First check the external reference by comparing the results to the same configuration but using the internal reference.

    Also, to eliminate power line-cycle noise, try converting at 20sps to see if there is improvement.

    It would be helpful for me to know the register settings and to see the data results you are getting.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Genius 4555 points

    Hi Bob,

    I'll try the internal reference.  I'm a little concerned we have REFCOM tied to ground instead of Avcc which concerns me a bit since the datasheet suggests connecting REFCOM to Avcc

  • 0 in reply to Andrew5821
    TI__Guru** 112895 points

    Hi Andrew,

    At this point we are still trying to find the source of the noise you are seeing.  Using the internal reference, even though it is not optimally connected helps to limit the potential issues.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Genius 4555 points

    Hi Bob,

    I have some different references coming today.  I'll update you on how those perform.

    Andrew