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XTR117: USAGE PROBLEM

FIRAT ATAS
Prodigy 10 points
Part Number: XTR117
Other Parts Discussed in Thread: XTR116

Hi,

I used the XTR117 integration in 4-20mA 2-wire transmitter circuit. I used the regulator output of the XTR117 to supply the microcontroller. According to the source current from XTR117, there are voltage differences at the output of the regulator, and this causes a problem for the loop current values.(because of the MCU PWM voltage value). 

How can i fix this problem? Can we use the XTR116 and switch(PWM) the Vref output with the transistor, and connect to the current input of the XTR116?

Thanks.

  • 0
    TI__Guru 63905 points

    Hello,

    Take a look at the forum thread at the bottom of this message. We posted a circuit in that thread that accepts a PWM input through a digital isolator to create a low-cost input-isolated 2-wire sensor transmitter.  The circuit achieves modest accuracy, limited mostly by the gain and offset errors caused from the inaccuracy of the VREG output as you've noted. 

    An upgrade to this circuit would be to use the XTR116 and use the VREF output to offset the input and control the PWM voltage.  You'll need to buffer the VREF output with an op amp because I doubt the VREF output will act well with the transient current load from the PWM generator attached directly to it. 

    Depending on your circuit accuracy targets the circuit changes above may still not get you into the range you're trying to achieve.  One of the largest errors is going to be from the VOH and VOL levels associated with the PWM generator which will change with temperature and other factors.  

    For best system performance consider using a voltage output DAC to drive the XTR opposed to a PWM signal.