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XTR110 span and zero adjust

David Prince92104
Prodigy 250 points
Other Parts Discussed in Thread: XTR111, XTR110

I am using two XTR110s in my product. The span adjust is barely large enough, and has a known interaction with the zero adjust. The zero adjust does not have enough range to make up for some voltage offsets that are created by the amplifers earlier in the circuit. I am using 100K ohm pot on the zero and 100k ohm pot and a 10k ohm resister on the span adjust.

Is there any way to increase the zero adjust range?

Thank you.

  • 0
    TI__Guru 62935 points

    Hi David,

    Just so we're on the same page, would you please include a basic schematic of your circuit as well as your desired Iout vs. Vin characteristics?  The span adjust on the XTR111 should be very configurable to a wide range of different desired output spans.  What is your desired output span?.    What is your desired final output range (4-20mA, 0-20mA, 5-25mA), and what is your system's input voltage that needs to produce this range?  With this information we can work towards a solution.

    Regards,
    Collin Wells
    Precision Linear Applications 

  • 0 in reply to Collin Wells
    Prodigy 250 points

    Sorry about that. I have it set to the 0-5v input and 4-20mA output.

    R46 is a 10K ohm. The input, Pin 5, has a DC offset between 0.010 and 0.1 volts, this is making the zero adjust difficult. The highest voltage on pin 5 is about 4.7 volts, which is 19.6mA or so, that is fine. My PLC thinks 19.7mA is max value.

    The issue is, the DC offset is stressing the zero adjust and then the span adjust is pulling the zero up as well. So in most cases the zero adjust is turned to minimum and the device still wont put out 4mA, but if i turn the span adjust down it will go closer to 4mA. Can I make the zero adjust stronger?

  • 0 in reply to David Prince92104
    TI__Guru 62935 points

    Hi David,

    Thank you for the additional information.  Although your goals seem like they should be pretty reasonable, I will need to go into the lab and test this out to be sure.  With my current project loading I think it will take until middle of the day tomorrow before I'm able to report back with any results.  In the mean time, please confirm this is correct.

    XTR110 Input: 0.1V - 4.7V

    Desired XTR110 Output:  4mA - 20mA.

    Regards,
    Collin Wells
    Precision Linear Applications

  • 0 in reply to Collin Wells
    Prodigy 250 points

    yes that is correct.

    thank you,

    David Prince

  • 0 in reply to David Prince92104
    TI__Guru 62935 points

    Hi David,

    I tested the circuit today and came to the same conclusion as you that the offset trim was simply not enough to get to 4mA with a 0.1V input while using the SPAN trim to get near 19.7mA with a 4.7V input.  I could only get the output down to 4.1mA or so. 

    Luckily I had an idea.  Since you're using the 4-20mA mode and we're already offsetting the input to 4mA using the +10V VREF voltage and the internal resistors, why not decrease the effect of the 10V reference and thereby reduce the zero-scale output level?  This was achieved by adding a 2.2 kOhm resistor between the VREF_F (pin 15) output and the VREF_IN (pin 3) input.  It took a little bit of an iterative process to determine that 2.2 kOhms was the right resistor value because the SPAN and OFFSET trims interfere with each other.  The result was that the output was pushed down to 3.827mA when the input was 0.1V.  By adjusting the SPAN trim and the OFFSET trim potentiometers from there I was able to achieve a very accurate 4 - 20mA output with a 0.1V - 4.7V input. 

    Let me know how it goes and if you're able to repeat my results.  I used the same values as you did, using 100k potentiometers, a 33 Ohm in series with the 16mA SPAN input, and a 10kOhm in series with the SPAN_ADJ pin.

    Best Regards,
    Collin Wells
    Precision Linear Applications