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Determining engineering.design tolerances/margins

Discussion in 'Electronic Design' started by kean, Jun 11, 2004.

  1. kean

    kean Guest

    Hi,
    can anyone point out what are the usual methods in calculating the
    design tolerances of a critical section of a circuit. For example, i
    have a maximum limit for a voltage for a particular signal, so how do
    i determine what are the margins i need to design in order to be
    reasonably sure that the signal will always be below this level?

    What is the usual industry practice? Blind rule of thumb or is there
    standard estimation procedures?

    Thanks.
     
  2. Joerg

    Joerg Guest

    Hi Kean,
    You'd have to be more specific. There usually is not only a maximum but
    a minimum as well. Then you have to calculate all the tolerances in your
    circuits, 1% resistors, capacitive loads and their tolerances and so on.

    The way most engineers tackle this is to design a circuit with common
    parts. With some experience their tolerance will be almost where it
    needs to be, or better. Then they go back and calculate worst cases, all
    parts in the direction of lowest gain and then all parts in the other
    direction.

    Then there is the Monte Carlo method which is often used in chip design
    (yield calcs etc.). But in discrete designs that can be, well, what the
    name says. There are large casinos in the city state of Monte Carlo :).

    Regards, Joerg
     
  3. John Larkin

    John Larkin Guest

    Two methods:

    1. Add up all component tolerances worst-case, and ensure that they
    can't hit your overall target.

    or

    2. Assume all components are worst-case, but that their RMS sum, or
    some scaled version of same, doesn't hit the limit. Monte Carlo is
    essentially a numerical simulation of this situation. *Some*
    production units will miss the spec, but not many; fix them, throw
    them away, or just sell them anyhow.



    2) is more realistic for comercial products where large numbers of
    parts are involved. 1) is hyper-conservative, for critical systems or
    pickey customers.

    "Blind rule of thumb" is nobody's engineering practice.

    John
     
  4. Ted Wilson

    Ted Wilson Guest

    It's not just a case of the component tolerances - you need to work
    out the coefficients for the contribution of each of the tolerances if
    you want a rigourous analysis. A 1% resistor does not necessarily
    generate a 1% error in the signal of interest.

    I've always used partial differentiation down the years and found it
    to be an extremely usefull tool

    Regards



    There is a tide in the affairs of men, which, taken at the flood, . . . suggests we should not inhabit certain low-lying coastal regions.
     
  5. I think this is what is called a sensitivity analysis. It tells you how
    the output of a system varies for changes in one or a group of
    parameters.
     
  6. Terry Given

    Terry Given Guest

    Bingo. and bloody useful, too. it allows you to concentrate effort where
    best results will be achieved.....

    Cheers
    Terry
     
  7. Bill Sloman

    Bill Sloman Guest

    Me too. I was using it for a couple of years before I realised that I
    had become an electronic engineer ...
     
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