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LMP90100 Reference Input Current makes RTDs unuseable??

Graham Igglesden
Prodigy 10 points
Other Parts Discussed in Thread: LMP90100

Hello

The LMP90100 Data Sheet states the REFERENCE INPUT  IREF Current is Typ. +/-2uA.

Is this correct?

If so then using a bias resistor for a 4 wire RTD application of say 2K (2V into REFP2 @ 1mA through RTD) would give a typical error of 4mV due to IREF?

Thanks

Graham

  • 0
    Prodigy 40 points

    Hi,

    I have the same question as Graham. While testing our product where we use LMP90100 as 4 wire PT100 interface we found out that with 10k reference resistor and IB=100uA the readings with precise 100 ohm resistors (0.01%, 5ppm/deg) connected as RTD were 0.5 to 0.9 deg C instead of 0 we expected.

    Another question is - if this is a typical value then what is a maximum?

    Regards,

    Mariusz

  • 0 in reply to Mariusz Krzyzanski
    TI__Genius 9655 points

    Hi Mariusz,

    the IREF parameter is a typical, we do not have the max.

    Is there any chance for you to change the Rref from 10K to 5K and double the IB ? In this way you can cut the weight of the uncertainty of IREF of about 50%.

    This change should not impact too much on the self-heating of the RTD.

    regards,

    Domenico

  • 0 in reply to domenyko
    Prodigy 40 points

    Hi Domenico,

    We decided to increase the measurement current from 100uA to 800uA and change reference resistor to 1k. The error observed decreased by a factor of 3 so was +0.1 to 0.2 deg C for one unit and 0.3 deg C for the second.

    So both Graham and me understand the datasheet correctly and this current incorporates an error of 0.25% if 800uA IREF is set and may explain the effect observed. Am I correct?

    Best regards,

    Mariusz

  • 0 in reply to Mariusz Krzyzanski
    TI__Genius 9655 points

    Hi Mariusz,

    your assumption is correct. In few words higher is the bias current lower is the contribution of the IREF. 

    The voltage across the RTD is  VRTD=  RTD*VREF/RREF +/-2uA*RTD,

    Ibias=VREF/RREF, so VRTD=RTD * (Ibias +/- 2uA)

    If Ibias >> +/-2uA the contribution of IREF becomes negligible.

    regards,

    Doemenico

  • 0 in reply to domenyko
    Prodigy 40 points

    Hi Domenico,

    Thanks for confirmation.

    Best regards,

    Mariusz