• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 46 subscribers
  • Views 893 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.

4-wire RTD with Rcomp

Beat
Prodigy 40 points
Other Parts Discussed in Thread: ADS1248

Hi

Forgive me if this is a trivial question: I would like to hook up several RTDs (4, if feasible) to an ADS1248. The design proposal for a 3-wire RTD (Fig. 82 of the manual) features a compensating resistor to zero the measured voltage for a certain temperature, allowing the PGA gain to be set to high values. I can't figure out how to achieve the same for a 4-wire setup (without loosing the advantage of independence of individual line resistances). Can anybody help me out, if this is possible at all without using additional active components.

TIA,

Beat

  • 0
    TI__Guru* 97795 points

    Beat,


    I've attached a couple of slides to this post on some 4-wire setup possibilities. The first is a setup that's was derived from a colleague of mine. It shows 3 four-wire setups using the ADS1248.

    The second slide shows a second, alternate setup that you might consider. If you put two four-wire RTDs in series, you might change the input common mode, but you should be able to get it to work without loosing the benefit of a four-wire measurement.

    Let me know what you think, or if you have any problems with it.


    Joseph Wu

    ADS1248_4wire.pdf
  • 0 in reply to Joseph Wu
    Prodigy 40 points

    Dear Joseph

    Thanks a lot for your schematics. However, I'm not sure whether you answered my question regarding the possibility to integrate a compensating resistance that would allow me to use high PGA gains in order to achieve the highest resolution. If you did, can you maybe draw me an example?

    Best wishes,

    Beat

     

  • 0 in reply to Beat
    TI__Guru* 97795 points

    Beat,


    Sorry, I didn't catch the reference to the Rcomp (I should have - it's in the title).

    The setup in Figure 82 with Rcomp is best suited for a 3-wire RTD setup. In a 4-wire setup, the idea is that the current follows a path in and out, and the measurement path doesn't see the current/parasitic resistive path provided that the measurement path doesn't require current.

    In using hardware compensation, you need to inject current into one of the measurement paths (with Rcomp) so that a 4-wire measurement doesn't make sense anymore. In this case, you should use a 3-wire measurement so that the parasitic resistance along the measurement paths match, as does the current through both paths. This way, the voltage drop across the parasitic resistance cancels and you are left with the RTD voltage and the Rcomp voltage that presumably cancel out at some reference temperature.

    Regardless, here's a version of the 3-wire RTD setup with compensation. You should be able to work with four 3-wire RTDs with the ADS1248.


    Joseph Wu

    ADS1248_3wire_wcomp.pdf
  • 0 in reply to Joseph Wu
    Prodigy 40 points

    Hi Joseph

    Thanks again for your help. In that case, I'll stay with the 4-wire setup without compensation resistance.

    Best regards,

    Beat