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XTR110 span and zero resistance

David Prince92104
Prodigy 250 points

The data sheet does not specify a recommended value for the span and zero pots, it does use a 100K ohm pot in an example. I have been using 100K ohm pots in the production boards just fine. With a new revision I want to change the pots out for digital pots. So can I use a 25K  or less preferable a 250K pot? 

note: I am using a 10K ohm resister in series with the span pot, like the data sheet says.

  • 0
    TI__Guru 62945 points

    Hi David,

    This should be fine.  The 100k pots give a little finer resolution in the trim, but with the control of a digital potentiometer this may not be needed. 

    The two offset potentiometers simply direct a little more current down one of the two legs of the input differential pair and therefore changes the offset voltage of the input stage.  To stay more within the range of the datasheet values for this resistance, you may consider placing 25k resistors in series with each leg of the 25k pot.  If this limits the trim range by too much then try reducing the value of the series resistors. 

    The 25k potentiometer will work fine for the SPAN adjust.  I do believe the datasheet specifies a 50k resistor in series with the SPAN trim, so I would use that instead of 10k.  The resistance of the SPAN pin simply provides an impedance in parallel with the internal gain setting resistors reducing the value and increasing the gain.  Therefore the 25k will have a greater affect than the 100k pot, so consider changing to the larger series resistance of 49.9k.

  • 0 in reply to Collin Wells
    Prodigy 250 points

    I will set up my bread board circuit based on your insight, and try it out, thank you.

  • 0 in reply to Collin Wells
    Prodigy 250 points

    So I tried the circuit with  20K ohm pots. I set up the Zero using pin 5 to ground with the span pot at minimum, then brought pin 5 to 5 volts dc. At 5 volts i record the min and max span range sweeping though the pot range. All pots are mechanical.

    100k pot with a 10k series resister 19.895-22.756mA

    20k pot with 50k series resister 20.034-20.311mA

    20k pot with 10k series resister 20.260-22.355mA

    I know I can fix this by changing the R4 resister, but changing resisters is not easy, as the board is surface mount.