• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Subscribers 49 subscribers
  • Views 1633 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

Calibrating span and zero for XTR115 PWM to 4-20mA Industrial Application

Jayant Deshpande
Expert 1495 points
Other Parts Discussed in Thread: XTR115, TLV313, TLV333, TLV314

Please suggest a span-zero calibration for the typical PWM to 4-20mA application of the XTR115. Schematic attached for reference.

Thanks,

Jayant

  • 0
    TI__Guru 62095 points

    Hi Jayant,

    Your schematic did not come through as an attachment.  Could you try attaching it again? 

    The span can be adjusted by trimming the resistance in series with the filtered/buffered PWM output.  The offset can be adjusted by trimming the resistance from the reference output to the "Iin" pin. 

    Let's take a look at your schematic and we'll go from there.

  • 0 in reply to Jayant Deshpande
    Expert 1495 points
    Hi Collin,

    Hopefully the schematic is loaded now..
    Intend to replace an expensive 4-20mA DAC and ISO72xx with a 6N137 and XTR115 combo. PWM resolution from the microcontroller is 12bits at a frequency of 3.9kHz.
    When the optocoupler LED is OFF, current through R1 will add some error, which needs to be compensated somehow.

    Please suggest better alternatives, if any.. This product is expected to fetch good volumes, so keeping it low cost is really important.

    Thanks,
    Jayant
  • 0 in reply to Jayant Deshpande
    TI__Guru 62095 points
    Hi Jayant,

    First, have you considered an opto-coupler with a push-pull output stage? This would eliminate the need for the R1 pull-up resistor which will impact the circuit behavior as you've suggested.

    Also, we've built and tested a few PWM to 4-20mA designs and generally find the best performance when buffering the filtered PWM output with a low-power, cost optimized op amp, like the TLV313, TLV314, or TLV333, before driving the input resistor to the XTR. This eliminates issues with the output impedance of the optocoupler, negatively affecting the circuit performance.

    In your circuit, the buffer would go after the C1 capacitor and would provide a low-impedance output to R3 resistor and input to the XTR.

    Also, it may have been left out for simplicity, but the design should include a 10nF - 100nF between V+ and Iout pins of the XTR115. Another 10nF - 100nF on the VREG line will likely also help performance.
  • 0 in reply to Collin Wells
    Expert 1495 points
    Hi Collin,

    Thanks, I will proceed with the TLV333 as suggested.

    Regards,
    Jayant