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Accuracy of division using MPY634

Discussion in 'Electronic Basics' started by [email protected], Mar 15, 2005.

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  1. Guest

    Hi all,

    I'm doing a simple project which will do the voltage division,
    Vo = 10(V1/V2). I do this using MPY634.

    My connections are as below:

    _____________________
    V2 --- | X1(1) (14)+Vs|--- +15V
    GND--- | X2(2) (12)Out| -----------
    |SF(4) (11)Z1 |--- GND |
    GND--- | Y1(6) (10)Z2 |--- V1 |
    __|Y2(7) (8)-Vs |--- -15V |
    | --------------------- |
    |_________________________________|

    The output that i got will be very inaccurate. For eg, when i pump in
    V1=0.406V and V2=0.008V, the answer =0.0433V. The answer should be
    0.019V. The measurements are done repeatedly and it shows consistent
    results.

    My circuit's connections are based on the data sheet of MPY634 at page
    7 (Basic Divider Connection). You can refer to the datasheet at the
    following webpage.

    www.dodeca.ru/files/bb/mpy634.pdf

    Does anybody know why this inaccuracy happen? Please advise.

    Thanks,
    Albert
     
  2. Your results are consistent with the input offset
    voltage specs for that part. The larger reason
    is that analog circuits are always inaccurate to
    some degree. It is up to the designer to make
    sure that the accuracy is appropriate for the
    application.

    You may want to add a trimmer or two, or
    determine what accuracy you really need
    and maybe find another technique. Be sure
    to consider temperature (in)stability before
    deciding that trimmers will do the job.
     
  3. Guest

    Thanks, Larry.

    What do you mean by "consistent with the input offset
    voltage specs" and "add a trimmer"? Can you explain by giving an
    example?

    So, my connections are correct right?

    sorry if my questions are stupid.
     
  4. You are welcome.
    Go to the spec sheet and find the guarantees placed
    upon input offset voltage by the part manufacturer.
    Measure some actual (Xin, Yin, Vout) triplets. You
    will find that there is a pair (Xio, Yio) such that for
    each triplet, Xin+Xio times Yin+Yio is very close
    to Vout, (much closer than the errors you posted),
    and both Xio and Yio are within the input offset
    specification. You can think of the input offset as
    a secret voltage that is added to each input with
    no extra parts or effort. Your trimmer would be an
    explicit circuit to add a seperate constant voltage
    to each inputs. That can be easily done (for your
    circuit) by putting a high value pot between the
    supply rails, dividing the wiper with a resistor
    divider that reduces voltage by a factor of 500
    relative to ground, and using the result in place
    of the grounded side of each differential input.
    (See figure 2 in the spec for an almost usable
    example. Just don't ground the divider output
    as appears in the diagram.)
    I did not check them, but your results were close
    enough to right that I had no reason to doubt the
    connection. The input offset issue is adequate
    to explain what you see and the probability of
    getting there with wrong connections is very low.
    And since there is a simple divider hookup in
    the datasheet, and your schematic resembles
    it closely, I figured you had not botched it.
    They are not stupid. They are evidence of
    some learning opportunities.
     
  5. Rich Grise

    Rich Grise Guest

    The classic answer to this is, "The only stupid question is the one
    you don't ask," but recently I saw a beer commercial that started out
    looking like a laxative or hardon pill ad, but about "Taste Loss".
    One of the characters asked something stupid, and the voice-over says,
    "There are no stupid questions when it comes to Taste Loss..." and the
    other character says, "Can you get Taste Loss from a Foosball table?"
    And the voice-over says, "Let me rephrase that..."

    Cheers!
    Rich
     
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