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PT100 3 wire RTD using XTR105

Other Parts Discussed in Thread: XTR105

Respected sir,

I am using xtr105p (DIP package). The input to the same is PT 100 3 wire RTD, and the temp range is 0-150 degree Celsius. We are facing a problem with the IC's lead resistance compensation for 3 wire PT100 RTD.

With no lead resistance i get a proper output as follows:

for 0 degree = 4mA

for 150 degree=20mA

But when a "LEAD RESISTANCE" of  "4.7ohms"  is connected it gives me following error in output:

for 0 degree = 6.64mA

for 150 degree=22.45mA

In the datasheet it is mentioned that the xtr 105 provides the compensation for the lead wire resistance. The circuit diagram borrowed from the datasheet is FIGURE No.3 on PAGE 10. We have verified the resistances of the 3 lead wires, they are as (4.78, 4.79, 4.95 ohms).

so why this problem is occurring and what can be the possible solution for the same?  

  • Hello,

    Thank you for using the XTR105, it is a very good solution for a single chip all-analog RTD to 4-20mA converter.

    If the circuit is connected as shown in Figure 3 in the product datasheet then the lead resistance effects will be greatly reduced based on:  a.) the matching of the two current sources (0.02% typical) and b.) the matching of the 3 lead resistances which you've measured to be up to 0.2Ohms apart.  

    Even with the small imbalance you've measured in your lead resistances I would not expect the results to deteriorate by as much as you've indicated.  Because the output current increases by ~2.5 mA during your testing it would indicate that your testing caused an increase in voltage on the positive input of the XTR105 which therefore caused the output current to increase.  Is it possible that you unbalanced the inputs by placing your test lead resistances in the wrong place?  For instance if you placed two of the resistors in series with the RTD and nothing in series with the Rzero resistor then you've created a ~10 Ohm offset between the two inputs which will create the increase in output current as you're experiencing.  For a 0 - 150C application the PT100 will change from 100 Ohms to roughly 157 Ohms so a 5 or 10 Ohm imbalance accounts for a large percentage of your full-scale differential resistance input     

    Are you sure you have things connected exactly as shown in Figure 3 and is it possible for you to share a schematic of your solution so I can look for other possible issues?  I suspect this is likely a basic connection issue because I've used the XTR105 in many applications and find that the lead resistance cancellation works pretty well and should certainly be able to overcome ~5 Ohms of resistance.  

    Let us know what you find.

  • respected sir thanks for your guidance.

    i am attaching the schematic of my ckt. i checked for the offset between  the two inputs but there no offset observed 

    thanks and regards,

    Pramodkumar yadav

  • Hello,

    Your schematic appears to be correct, thanks for sending it.  Could you please measure the voltage on pin2 (VIN-) and pin13 (VIN+) in reference to pin 6 (IRET) before and after the lead resistances were added?  I'd like to see how much if any the differential voltage is changing with the added line resistances.  Also, can you try removing the lead-off (broken wire) transistor to see if it is conducting current unexpectedly which would explain the issue?

  • respected sir,
    As you suggested i measured the vtg on pin no- 2 and pin 13 with respect to pin no. 6
    the values are as follows:
    4.7 ohms lead resistance
    at pin no.-2- 1.68V(for zero degree)
    --                  
                           - 1.72(for 150 degree)
                   
    at pin no.13- 1.68V(for zero degree)
                           
                         - 1.76(for 150 degree)
    vtg without lead resistance
    at pin no.-2- 1.68V(for zero degree)
    --              
                         - 1.71(for 150 degree)
                   
    at pin no.13- 1.68V(for zero degree)
                     
                          - 1.76(for 150 degree)
    I also  removed Q2  but there is no  change.
    thanks and regards
    pramodkumar yadav
  • One more thought on this.  Have you looked at signals with an Oscilloscope.  Also it would be good for us to see a complete schematic with output transitor and all bypass capacitors.  Is this built on a PCB ro just wired together?