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ADS1248: The sample result gradually increases from a smaller value to the actual value

Wayne Wang20
Expert 4360 points
Part Number: ADS1248
Other Parts Discussed in Thread: MSP430F5529

Hi team,

My customer is using ADS1248 to measure a DC voltage. But we met a strange phenomenon. Please see the details as below.

When the device samples the input, the sample result gradually increased from a smaller value to the actual value. For example, when the input is 1.5V, customer will firstly get a 1.4V sample result. Then the following sample result gradually increases from 1.4V to 1.5V in 5 or 6 seconds. This issue always happens after powering up. When the device operates for a while, the issue is gone. Do you have any comment on this issue?

What’s more, do you have the ADS1248 reference software code so I could ask customer check if this is caused by software. Thanks

Best regards,

Wayne

  • 0
    TI__Guru* 92655 points
    Wayne,


    It's likely that this is caused by some sort of settling within the circuit, however 5 or 6 seconds is a rather long time. The most common cause would be a reference voltage that hasn't settled. Often, especially with large capacitive loads, the reference voltage requires longer to turn on and settle to the final value.

    However, in your description, you say that the measurement starts smaller then recovers to the larger, correct value. This is opposite of what would normally happen with reference settling. If the reference value is smaller than expected the input measurement will be larger than expected.

    It's also possible that there is settling elsewhere in the circuit. If IDACs are used to measure an RTD, and there are large input filtering capacitors, then enabling the IDAC current may require time to settle the circuit. Again this doesn't necessarily fit your description, because with most common values used in filters, the settling takes a time that is in milliseconds.

    Because this takes seconds, I would guess that the customer could measure this phenomenon with a multimeter and observe this settling. I would measure both the input, the ADC reference, and the ADC internal reference (if it is enabled). All should come up to final values quicky, before the data is taken.

    Are you able to show a schematic? It might help to see what possible problems are in the circuit. Another thing that may help is a configuration sequence showing how the ADC is powered up.

    One other quick point. Are the boards cleaned after soldering? Leaving the flux on the boards may also cause problems similar to this.


    Joseph Wu
  • 0
    TI__Guru* 92655 points
    Wayne,


    I missed that you had asked for example code for the ADS1248. Just in case you still need it, the code is listed in the product folder under the Tools and Software (under Software). You can find a zipped file here:

    www.ti.com/.../sbac144

    This example code project is MSP430 based for the MSP430F5529.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Guru* 92655 points
    This post is currently being handle offline. I'll update it when there's more information.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Guru* 92655 points
    Wayne,


    I haven't heard from you in a week (either through an E2E post or through email. After reviewing your schematic I had suggested to short the inductors in the circuit, remove any potentially leaky protection diodes, and check to see if the large capacitances were slow to settle for the reference. Hopefully one of these suggestions did help clear up the gradual change in reading.

    I'll close this post for now, but if your customer continues to have problems, post back or email me again.


    Joseph Wu