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xtr105 construction

boehm
Prodigy 50 points
Other Parts Discussed in Thread: XTR105

Hello,

 

I have a problem concerning the construction of the XTR105. For my system I need to measure the temperature on 8 different surfaces. Therefore I like to implement 8 XTR105 to my system connected to a multiplexer which is connected to my ADC via resistor to get the chance of the RTD in voltage.

At the moment I have a problem to adapt the XTR105.

I used the temperature range from 0°C-90°C because the resistors Rg and Rlin1 are more suitable for orders from distributors.

At the moment I have these values:

Rz = 100
R1 = 115,54 -> RTD (90-0°C)/2
R2 = 130,9 -> RTD 90°C

Rg = 73,014 -> 73,2 (1%)
Rlin1 = 37478 -> 37,4k (1%)

Rcm = 1000 (1%)
Q1 = TIP31C
C = 10nF (film capacitor) for Rcm and input voltage

My problem is to set up the xtr105 finally.
1) Do I need the Q1 transistor because in the datasheet there is a statement that says "transistor needed for extended temperature range". But what is the extended range? Is the range from 0-90°C below this range and is just the 3.3k resistor required?

2) If I apply the Rg to the xtr105 the gain of the output is different. With the equation [IO = 4mA + VIN • (40/RG)] for the output current I will receive ~13mA (with 24V supply voltage). What does this mean? New output current from 13mA to 20mA for the temperature range? Or is it possible to neglect the Rg for an output of 4 to 20mA?

I am looking forward to receive any help for my problem.

 

Regards Dennis

 

 

  • 0
    TI__Mastermind 27715 points

    Hello Dennis,

    Thank you for your interest in the XTR105, and welcome to the TI E2E forums!

    Firstly, I calculated slightly different resistor values for your input range of 0-90C. I have attached my calculations page in MathCAD to this post. My values are as follows:

    1. Rz = 100
    2. Rg = 70.346 --> 69.8 (1%)
    3. Rlin = 37.478k --> 37.4k (1%)

    Now to address your other questions:

    1. Q1 is not required for your temperature span, but it is recommended in general as it offloads the majority of the power dissipation from the XTR105. This is beneficial to overall accuracy, since the power dissipation being moved off-chip allows the XTR105 to stay cool and therefore the precision input/current source circuits do not experience thermal drift.
    2. The equation [Io = 4mA + Vin*(40/Rg)] does in fact describe a 4-20mA output. Vin corresponds to the differential voltage across the Vin+ and Vin- pins of the XTR105, which is just the voltage across the RTD (Vin+) compared to the voltage across Rz (Vin-). Let's analyze the min and max conditions of your circuit as an example.
    • At 0 degrees C, both the RTD and Rz equal 100 ohms, so the 800uA reference currents cause an 80mV drop across RTD and Rz, resulting in a Vin of 0V. If Vin is 0V then Io is simply 4mA by the output equation.
    • At 90 degrees C, the RTD now has a resistance of 137.41 ohms while Rz is still 100 ohms. The voltage drop across the RTD now becomes (800uA*137.41ohms) = 107.77mV, while the voltage drop across Rz is still 80mV. We now have a differential Vin of 27.77mV, so if we plug that into our Io equation we get [Io = 4mA + 27.77mV*(40/69.8ohms)] = 19.91mA.

    Finally, let me make an observation about your intended application circuit. You said that you plan to multiplex the outputs of 8 XTR105s to a load resistor whose voltage you will then read with an ADC. Keep in mind that the XTR105 is a 2-wire transmitter, and that means it must float with respect to its power supply. I recommend using 8 load resistors (one at the output of each XTR105), and then multiplexing the resulting voltages to your ADC. Your circuit would look exactly like Figure 1 on the XTR105 data sheet, except that the Vo node across R_L would connect to one of the inputs on your Mux.

    Please let me know if any of this is unclear.

    Best regards,

    Ian Williams
    Linear Applications Engineer
    High Performance Linear

    Mathcad - Calculations.pdf
  • 0 in reply to Ian Williams
    Prodigy 50 points

    Hi Ian,

    thanks for your detailed reply.

    I have to check my resistor calculation again. Maybe I used a wrong RTD table or something else. Now a new problem occured. The distributor who is deliviering our ordered electronic parts just have resistors with an accuracy of 0.1% for the 69.8 type. 

    Again thanks for the improvment regarding the multiplexer circuit. After I studied the datasheet, especially figure 1, it was clear to provide a load resistor for each output.

     

    Regards
    Dennis

  • 0 in reply to Ian Williams
    Prodigy 50 points

    Hi,

    I found the mismatach between our values.

    Ian Williams said:

    Firstly, I calculated slightly different resistor values for your input range of 0-90C. I have attached my calculations page in MathCAD to this post. My values are as follows:

    1. Rz = 100
    2. Rg = 70.346 --> 69.8 (1%)
    3. Rlin = 37.478k --> 37.4k (1%)

    In your MathCad calculations you used equation for the 3-wire construction. I used the the equation for a 2-wire construction. The calculation of Rg is slightly different.

     

     

     

  • 0 in reply to boehm
    TI__Mastermind 27715 points

    Dennis,

    You are correct - nice catch! Sorry for the confusion.

    Regards,
    Ian Williams