This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

XTR111 - Output current inaccurate and load dependant

Other Parts Discussed in Thread: XTR111

Dear Sir,

I would like to use XTR111 to obtain 0 to 20mA/4 to 20mA current output for 0 to 5V input. Attached herewith is the schematic along with PMOS (CPH3351) and BJT(BC557b) datasheets for reference.

The problem is, when I connect different load resistances in the range of 10 ohm to 1.1K ohm for the same input voltage of 5V, the output current changes from 3.2mA for 10Ohm to 19.3mA for 1.1KOhm. For a load of 250 Ohms current is 17.6mA at 5V. When I remove R load and directly connect ammeter at the output I get 19.8mA at 5V.

 I tried simulating the circuit on TINA but couldn’t find the PMOS part number. Can you guide me how to add a particular part in its library? While selecting the PMOS I took following parameters into consideration: Id=1.8A, Vdss=-60V, Vgss=+/-20V, Cinput=262pF. Do you think there is problem with PMOS or BJT part number selection(Attached datasheets)?

 I have also tried the following experiments to troubleshoot :

 1)      I changed the transistor base resistance(R3) from 15 ohm to 2.2 Ohms; but there was no change in the result.

2)      I changed the Rset resistor from 2.5KOhm to 2KOhm to check if this problem persists for other configuration i.e 0 to 25mA output for 0 to 5V input.

For 5V input, 250Ohm load, output current was 22.6mA.

For 5V input, No load(Only Ammeter), output current was 24.96mA.

Please guide me as to how I could resolve this problem.

Thank you,

Regards,

Komal Marathe

Niyantran systems

PMOS-CPH3351.pdf

Transistor- BC557.pdf

  • Hi Komal,
    For debugging purposes, try removing the external current limiting BJT (Q2) and replacing resistor R3 with 0 ohms and do a basic measurement of the output current vs. input voltage. I would also recommend lowering your power supply voltage temporarily for the measurement to less than 20V to guarantee you don't damage the PMOS, and ensure that the PMOS is connected with proper polarity (source to IS pin of XTR111).
  • Dear John,

    Thank you for your prompt reply.

    I modified the circuit for debugging as per your suggestions; but there appears to be no change in the result.
    I am still getting 17.6mA output instead of 20mA for 5V input at 250 Ohm load resistor. The power supply voltage was lowered to 18V in order to protect PMOS.

    Please help me with more suggestions.

    Thank you,
    Regards,

    Komal Marathe
    Niyantran Systems.
  • Try replacing the 10k resistor on the output disable pin with a 0 ohm (short) resistor to eliminate the possibility of the device being "partially" disabled. Also would you please measure the voltage directly at the input pin (pin 6) and the VG pin (pin 3)?
  • Dear John,

    I have replaced the 10K Ohm resistor with 0 Ohm on the OD pin.

    The voltage measurements are as under:

    1) Circuit schematic same as attached in my first post (10K OD resistor replaced by 0 ohm).

    • Vin (pin 6) wrt Gnd (pin 10) = 5.01V
    • Vg (pin3) wrt Gnd (pin 10)= 20.53V
    • Vin (pin 6) wrt Vg (pin 3) = 15.5V
    • Iout = 17.6mA

    2) Circuit schematic as attached in my first post with 10K OD resistor replaced by 0 ohm, current limiting BJT Q2 removed and R3 (15 Ohm) replaced by 0 Ohm, Supply voltage 18V .

    • Vin (pin 6) wrt Gnd (pin 10) = 5V
    • Vg (pin3) wrt Gnd (pin 10)= 14,78V
    • Vin (pin 6) wrt Vg (pin 3) = 9.77V
    • Iout = 17.6mA

    Kindly guide me.

    Thank you,

    Regards,

    Komal Marathe

    Niyantran Systems.

  • Hello Komal,

    I've supported the XTR111 for a number of years and the product does not behave in the manner you're describing if the circuit is connected properly. 

    We noticed that the schematic you posted is a modified version of the EVM schematic with a few values/components changed using an image processor.  Are you making measurements on a printed circuit board (PCB) or on a breadboard / prototyping board?  If you're working on a PCB, could you please provide the actual schematic and PCB layout files for us to review? 

    Is it possible for you to post the results of a linear input sweep while measuring the output current?  We'd like to see if the output current is becoming non-linear with respect to the input signal or if something is causing a gain error in the circuit as the load resistor changes.  Please show results with the a 0Ohm load as well as the 250 Ohm load you're having trouble with.

    Finally, please be sure you're measuring the current in series with the load resistor and that there aren't any other paths for the output current to return to GND through that wouldn't go through the ammeter.

     

     

  • Dear Collin and John,

    We are making measurements on a prototyping board; and as Collin rightly pointed out, the current was taking another path to return to ground. Therefore this issue was resolved.

    We regret the trouble caused to you and truly appreciate your support.

    Thankyou for helping us.

    Regards,

    Komal Marathe

    Niyantran Systems.