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XTR112: The output current of the circuit can not keep constant with different RL

user5222239
Prodigy 80 points
Part Number: XTR112

Hello
I have a problem with XTR112, my circuit is according to datasheet XTR112 / FIGURE 1. RTD(PT1000) Input, Current Excitation,input resistant is adjusted in range 700Ω - 1200Ω.
When the input resistant is constant the output current is changing with the load resistance ：(R2+R3).

Thanks for help

  • 0
    TI__Expert 6250 points
    Hi user5222239,
    could you provide your loop supply voltage and R2+R3 values? I'm wondering if increasing R2 and R3 is causing the XTR112 to fall below its minimum rated supply voltage, which could cause this behavior.
  • 0 in reply to Alexander Davis
    Prodigy 80 points

    the loop supply voltage is 24V DC.the R2+R3 value is between 250Ω～600Ω.
    The position of an ammeter in the circuit influnce the output current.
    for example:
    in the circuit (fig 1) ,
    when the RTD (between RA and RB) =1000Ω,(R2+R3)<250Ω, the value of the ammeter is 9.3mA;
    when the RTD (between RA and RB) =1000Ω,(R2+R3)=400Ω, the value of the ammeter is 10.9mA;
    when the RTD (between RA and RB) =1000Ω,(R2+R3)=600Ω, the value of the ammeter is12.1mA.
    the output current is not correct !

    but in the circuit (fig 2) ,
    when the RTD (between RA and RB) =1000Ω,(R2+R3)<250Ω, the value of the ammeter is 9.3mA;
    when the RTD (between RA and RB) =1000Ω,(R2+R3)=400Ω, the value of the ammeter is 9.3mA;
    when the RTD (between RA and RB) =1000Ω,(R2+R3)=600Ω, the value of the ammeter is9.3mA.
    the output current is correct !

  • 0 in reply to user5222239
    TI__Mastermind 23050 points
    Hello,

    Unless there are elements in your loop that are not shown here, I don't think this is a compliance issue on a 24V supply. The XTR112 with D1 in the circuit needs at least 8.2V to regulate properly. With a 600 Ohm load and 20mA of output current you drop 12V, leaving 12V for the XTR.

    I believe there is an issue with the connections you are making to measure current. You must break the loop with the current meter in series to properly measure current. It is possible though that the resistance of your meter is large enough in the milliamp measurement range to act as a load resistance that pushes you past the compliance limit. To eliminate this variable, you may try indirectly measuring the current by measuring the voltage across the load instead.