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XTR111 - undefined behaviour and high output current drift

Torsten Rehwald
Prodigy 50 points
Other Parts Discussed in Thread: XTR111

Hi all,

i use the XTR111 for getting a 4 to 20 mA output current in a product, which i am developing. When i measure at Vin and the output current over temperature i get sometimes an unclear behaviour of xtr111 and a very high output current drift. 

Sometimes means, that not all XTR111 have the same issues.

For example

When i measure at 85°c it is possible, that Vin become higher and the Output current decrease. 

When i measure the output current of an other test PCB with XTR111 on it, which was created by a colleague, the output current drift can be 100µA by increase the enviroment temperature to 85°C.

I was not able to recognize the problem with the help of the datasheet and the calculations in that forum.

Have anybody some hints for me.

Kind regards

Torsten Rehwald

  • 0
    TI__Guru 64165 points

    Hi Torsten,

    We should be able to determine what is causing the output current deviations at higher temperatures.  First, would you please share a schematic of your system showing all connections to the XTR111?  Please include the manufacturer's part number for the resistor you've used as the "RSET" resistor in your design. 

  • 0 in reply to Collin Wells
    Prodigy 50 points
    Hi Collin,

    please find attached a picture of device circuitry and additional information about the measurements. On the one hand we made measurements with original circuit of the device and on the other hand measurements with a reference diode outside the climate chamber for supplying a voltage to Vin of XTR111.

    Kind regards
    Torsten Rehwald
  • 0 in reply to Torsten Rehwald
    Prodigy 50 points
    Hi Collin,

    how i can add a picture to the reply?

    Kind regards
    Torsten Rehwald
  • 0 in reply to Torsten Rehwald
    TI__Guru 64165 points

    Hello Torsten,

    See the image below for instructions to attach files and images.

  • 0 in reply to Torsten Rehwald
    TI__Guru 64165 points

    Hello Torsten,

    Thanks for sharing your schematic.  Can you share your measurement results as well so we can see the behavior of the device output current vs. temperature? 

    Could you also share the manufacturer part # for the R10 2.2k resistor?  We'd like to see the temperature drift specification of this component.  Initial tolerance and temperature drift of this resistor will directly impact the accuracy of this design.

    The BAV99 protection diodes on the output will have significant leakage currents at higher temperatures as shown in the BAV99 product datasheet, this may be contributing to some of the output error.  Could you test the design without the output diodes to see if the error is removed?

  • 0 in reply to Collin Wells
    Prodigy 50 points

    Hello Collin, 

    for measurement of the current flow I use a Keithley 2700.

    In table number 1 are presented the measurement results for the measurement with the reference diode outside the climate chamber and the assembled protection double diode on the PCB.

    PCB Number 25°C 65°C 85°C
    1 18,635mA 18,6337mA 18,624mA
    2 18,6084mA 18,5464mA 18,4994mA
    3 18,5523mA 18,5486mA 18,538mA

    In table number 2 are shown the measurement results for the measurement with the reference diode outside the climate chamber and the removed protection double diode.

    PCB Number 25°C 65°C 85°C
    1 18,638mA 18,633mA 18,629mA
    2 18,611mA 18,545mA 18,511mA
    3 18,536mA 18,54mA 18,542mA

    In another measurement i tested only the circuit from and including the MOSFET and the protection circuit of the output current. The drift of the test circuits was everytime zero.

    I made measurements with different R10 2K2 resistors. The results was everytime the almost equal. 

    For the measurements mentioned  above i used the resistor RC0603FR-072K2L from Yageo.

    Kind regards

    Torsten Rehwald

     

  • 0 in reply to Torsten Rehwald
    TI__Guru 64165 points

    Hello Torsten,

    Thanks for sharing your data.  I looked through it and ran some calculations that you'll find attached and below. 

    First, PCB2 performed significantly worse than the other two and should be looked at to determine if there is a difference between it and the other two PCBs.

    That said, the drift of the gain set resistor that you've selected is 100ppm/C.  Therefore, a system drift less than or equal to 100ppm/C is within tolerance of the resistor you've selected.  To reduce the drift select a resistor with a lower temperature coefficient.  The XTR111 has a typical span drift of 5ppm/C for reference so you may consider selecting a resistor with characteristics closer to that.

    While it didn't make a tremendous difference, you will notice that the drift of the system without the output protection diodes was less than the original system, especially in the 65 - 85C range shown in the 2nd image.

    Error_Calculations.xlsx