• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 46 subscribers
  • Views 573 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1248 RTD 3 wire interface???

Sandeep NS
Intellectual 400 points
Other Parts Discussed in Thread: ADS1248

Hi,

When we are using ADS1248 in 3 wire RTD, say in AIN1 uses IEXC1 current and AIN2 uses IEXC2 current. In which mode i need to configure the analaog inputs.Unipolar or Bipolar mode???

  • 0
    TI__Guru* 97585 points
    Sandeep,


    I'm not sure what you mean by bipolar or unipolar, but let me answer this with an example.

    There is an application note on using RTDs with the ADS1248. You can get it from this link:

    www.ti.com/.../sbaa201.pdf

    Figure 10 has an example with a 3-wire RTD.

    To get this measurement, you would Select AIN0 as the positive input and AIN1 as the negative input. The you would set the IEXC1 and IEXC2 currents to the proper value and outputs. For the IEXC currents, you will need to internal reference to be turned on (even if it is not used for measurements). Lastly you'll need to select the REF0 reference for the ADC.

    There isn't a bipolar/unipolar selection for the ADC. The ADC will report the data in a two's complement notation. In this setup, you won't have any negative inputs to measure, so you'll miss half of the input range.


    Joseph Wu
  • 0
    TI__Guru* 97585 points
    Sandeep,


    Here's another design with a 3 wire RTD. It has some additional calculations for the RTD setup that you can review.

    I've mentioned this other posts, but we generally don't recommend hardware compensation. Using an Rcomp may be problematic. It may give you more resolution by using the full bipolar range, but it may also be less accurate. You need to ensure that the currents are completely matched in the IDAC. The part has a typical IDAC mismatch of 0.15%, but is not guaranteed for a max. Also you need to consider the drift of any resistances in the analog input path. That would include Rcomp and any filter resistances in the inputs. Any filter resistances need to match exactly as their mismatch and drift will get gained up by the IDAC.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Guru* 97585 points
    Sorry - I forgot the link:

    http://www.ti.com/tool/TIPD152


    Joseph Wu