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Design and work around

Discussion in 'Electronic Basics' started by Richard, Nov 20, 2004.

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  1. Richard

    Richard Guest

    I had a company design a custom terminal board for me. It does numerous
    things, but what I'm fighting with is the analog section. Standard 0-20ma
    inputs and , Thru a Precision 250 ohm resistor to convert to 0-5 volts for
    a ADC Card.
    ADC Card is a 16 channels single ended 12 bit analog.

    Problem is, that no matter what the source is, the voltage always seems to
    be approx. 50mv lower than actual. I have an Omega MA Simulator in order to
    test this. If I send 4 ma then the ADC Card reads 0.950 volts and 20 ma =
    4.950 volts.
    It appears to be pretty linear. Not sure what is causing it, because I have
    not seen this on my other boards designed with 250 ohm resistors.

    Anyway, since it's linear, and we built abunch of these boards, I need a
    correction factor in my scaling in order to make the engineering units
    correct. I would like to scale it on the voltage side since we know the
    offset is the same on all the channels and not offset the scaled value.

    I have the following for each channel:
    Factor and Offset which works normally (When I don't need an offset)

    Vin * Factor + Offset

    I can't add the millivolt difference to Offset because offset is adding to
    the scaled value. But I could add it to Vin like this....
    CrFactor = 0.50
    ((Vin + CrFactor) * Factor) + Offset

    The above works for Sensors 4-20ma, because we are actually scaling 1 - 5
    volts or 0.950 to 4.950 and by adding the correction factor it brings it
    to 1.00 to 5.00

    Follow?

    Okay, But now, we actually have a sensor that is 0-20ma or 0-5volts, the
    above formula and correction factor does not work because Zero volts is
    already zero volts (no more voltage drop) so the actual voltage shown is
    0 to 4.950

    Using the above correction factor, we end up with 0.050 to 5.000 which is
    incorrect and 0 to 4.950 is also incorrect.
    Would it be 0 - 4.900 ?

    Not sure, Anyone have an idea how I can handle this voltage drop for all my
    channels?
    My software "could" look at MinVoltage and MaxVoltage and determine if the
    scalling starts at Zero or Negative and use a different formula for each
    case?

    Richard
     
  2. John Larkin

    John Larkin Guest

    Maybe you gave a ground loop problem, with the single-ended ADC. Do
    you have a spare ADC channel? If so, run it to the common point on the
    termination board, digitize (and maybe smooth) that, and subtract it
    from all the other ADC channel readings.

    John
     
  3. Rich Grise

    Rich Grise Guest

    The problem is, there is no such thing as "Standard 0-20 ma". The standard
    for current-loop control or sensing is 4-20 mA. This way 0 mA can be used
    to indicate a fault condition.

    Please incorporate this into your spec, and come back if you're still
    having problems. Let's eliminate as many variables as possible! :)

    Good Luck!
    Rich
     
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