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ADC 1248 Getting random values for conversion results

Chris Ouellette
Intellectual 290 points

Other Parts Discussed in Thread: ADS1248

My hardware is configured like figure 85 in http://www.ti.com/lit/ds/symlink/ads1248.pdf

I'm using +5v for AVDD and DVDD as well for the other logic signals, CS, START, RESET.

I am able to write to the registers to configure the device and when I read the registers back the results are what I wrote.


When I read the conversion results I get random values.

To narrow down the problem, I'm reading the internal temperature diode and still get the random values.

The device is setup to use the internal reference and internal clock.

As in Figure 85 I do not have anything connected to VREFCOM and VREFOUT.

Page 29 of the data sheet mentions connecting a capacitor between VREFCOM and VREFOUT and grounding VREFCOM. Do I need to do that? Could that be causing the random values?

Chris Ouellette

  • 0
    TI__Guru** 100955 points

    Chris,

    Sorry I missed this post. I didn't know it was out there.

    Yes, the VREFOUT needs to have a capacitor to VREFCOM of a value between 1uF to 47uF. This ensures the stability of the reference. Additionally, the VREFCOM needs to be connected to some AC ground node with less than 10Ohms. Both of these comments are in the datasheet on page 29.

    Either one may cause the reference to be unstable and cause bad data? Are you still having problems with getting errant readings out of the device?

    Joseph Wu

  • 0 in reply to Joseph Wu
    Intellectual 290 points

    Adding the capacitor and the AC ground to VREFCOM fixed the problem. It would have been nice to have that included in the 3 wire RTD example in the data sheet. After all you did show all the other necessary ground connections AND the Rbias resister needed in the example. Just a thought

  • 0 in reply to Chris Ouellette
    TI__Guru** 100955 points

    Thanks Chris, we'll consider that when we have to go back and revise the datasheet.

    Joseph Wu