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Number of bits and resolution my answer VS my teacher's

  1. Mar 14, 2013 #1

    Femme_physics

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    Gold Member

    1. The problem statement, all variables and given/known data

    A load cell converts weight to voltage. It's known that the transmission of a certain measurement system is 5 mV/kgf


    In a computerized control system, it is necessary to translate the analog signal, which the transducer sends off, to a digital signal through an A/D converter.

    In a certain weight system, it is necessarily to measure weight in the range of 0-200 kgf, whereas the needed accuracy of the measurement is 0.5 kgf

    The A/D converter which is in the conversion system, is supplied by a 5V DC (Direct Current) voltage.

    1) Pick the appropriate A/D converter for this measurement system (what's the required "data bits"?)



    3. The attempt at a solution

    What bothers me is that I got n = 10 and he got n = 11. What gives?

    My solution:

    http://img827.imageshack.us/img827/7513/myann.jpg [Broken]

    My teacher solution: (Don't mind the Hebrew words)

    http://img703.imageshack.us/img703/5646/teacherans.jpg [Broken]
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Mar 14, 2013 #2

    NascentOxygen

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    Staff: Mentor

    Hi FP. It is just an arithmetic error (on your part) I think. :smile:

    0.0025 ≤ 5/(2n - 1)

    2n - 1 ≥ 2 x 103

    if n=10, LHS = 1023
    if n=11, LHS = ...
     
  4. Mar 14, 2013 #3

    I like Serena

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    Homework Helper

    How did you find your n=9.64?
     
  5. Mar 14, 2013 #4

    rude man

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    Gold Member

    Has nothing to do with the 5V a/d converter reference. We are interested in resolving 0.5kg/200 kg. If the LSB is 0.25kg and the actual weight is say 10.25 kg then we could get a reading of either 10.00 or 10.50 which is at the limit of requirement, so you should resolve to 0.125 kg which implies 200/.125 = 1600 LSB's or n = 11.
     
  6. Mar 18, 2013 #5

    Femme_physics

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    Nascent is right, I just reset my calculator and recalculated. It is indeed n =11 :-) Thank you for the replies.
     
  7. Mar 18, 2013 #6

    jedishrfu

    Staff: Mentor

    Bad calculator! Bad! Switch to a slide-rule. :-)
     
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