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Expected error in XTR108 circuit

Juliana Campos Silva
Prodigy 180 points

Other Parts Discussed in Thread: XTR108

Hello, 

 

I would like to know which is the expected error when the MOSFET is burned in XTR108 circuit. When the output current signal is up then 70mA, what problem might be happening? Might be the MOSFET?

The implemented circuit is exactly the same as in datasheet.

 

Thanks in advanced.

 

  • 0
    TI__Mastermind 27715 points

    Hello Juliana,

    When I have MOSFET failures on XTR108 boards, usually I observe the output current reaching the current limit of my power supply. Have you tried replacing the FET, and if so what was the result?

    Which FET are you using? I recommend the Supertex DN2540.

    Best regards,

    Ian Williams

  • 0 in reply to Ian Williams
    Prodigy 90 points

    Hi Ian,

    I am having a similar problem only when I try and set the temperature span at 0-100F.  Is the xtr108 capable of this range?  It seemed unstable and eventually ran away and failed.  The circuit has been working great as 0-100C and 0-200C.  A customer really wants a 0-100F.  Is there anything I can do to improve this?

    Bill

  • 0 in reply to Bill Wilson
    TI__Mastermind 20160 points
    Hi Bill,
    XTR108 should be capable of handling a Pt100 or Pt1000 sensor over the sensor's full temperature range, so 0-100°F should be possible. The XTR108 itself should work in environments ranging from -40° to 85°C (-40 to 185°F).

    Is the runaway you're experiencing during the calibration process, preventing you from calibrating correctly for a 0-100°F span, or does the system calibrate okay, then fail later?
  • 0 in reply to Alex Davis
    Prodigy 90 points

    It works fine over larger temperature ranges.  It is the narrow 0-100F that is a problem.  The unit drifts during the calibration process.  Is there too much gain for this narrow range?  

    Edit: for clarification I am trying to set this 0 degrees F = 4mA

                                                                       100 degrees F = 20mA

    Bill

  • 0 in reply to Bill Wilson
    TI__Mastermind 20160 points
    Hi Bill,
    What are the values of the Rlin, Rset, and Rz resistors in your application? Running through the datasheet calculations, it looks like Rlin=15.8k, Rset=12.1k, Rz=93.1 should work for this application. I haven't tested this on an EVM, however. I'd guess that one of the resistor values chosen is preventing the calibration from working at the low temperature end, since 0F is ~-18C. It's possible too that too high of a gain is being used, but the calibration process should choose an appropriate gain if possible.
  • 0 in reply to Alex Davis
    Prodigy 90 points

    Using the xtr108EVM find resistors tab Rz = 931, Rset = 12.1k, Rlin = 357k and Rcm = 536.  I am setting these up for a PT1000.  That Rlin number you calculated is much different.  I wonder if that is the issue.

  • 0 in reply to Bill Wilson
    TI__Mastermind 20160 points
    Hi Bill,
    Redoing the calculations for a Pt1000 sensor, I'm coming up with very similar values to yours, with Rlin=357k. Could you provide the values of Iref, Izero, PGA gain, and G.Lin coeff from the XTR108 EVM software? Register contents after a calibration could be useful too. Out of curiosity, have you tested this with another XTR108? We have seen cases where damage to an XTR108 causes it to fail to calibrate, but otherwise appear functional.
  • 0 in reply to Alex Davis
    Prodigy 90 points

    Hi Alex,

    I think I may have been damaging the XTR with the loop power supply current limit too high.  If I limit the supply to 30mA I can calibrate the unit as normal.  I think this range may be "off" enough after loading the model file that it draws excessive current before I get a chance to calibrate it perhaps damaging the XTR.

    Bill