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High impedance pins are susceptible to leakage?

Discussion in 'Electronic Design' started by panfilero, Mar 5, 2013.

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  1. panfilero

    panfilero Guest

    Hello,

    I was about to begin laying out a part and I read the layout considerationsand I don't know what they are talking about, can anyone tell me what thismeans

    "Due to the high impedances on the SD, VDD, and GATE pins, these pins are susceptible to leakages to ground. For example, a leakage to ground on SD will activate the shutdown state if greater than 1.6µA. Providing adequate spacing away from grounded traces and adding conformal coating on exposed pins lowers the risk that leakage current will interrupt system operation."

    What does it mean that a high impedance pin is susceptible to leakage? A high impedance pin... like the input to an op-amp, I would think would have very little leakage... and what do they mean, "adequate spacing from grounded traces?" I shouldn't route these pins close to ground?

    this came from this datasheet, pg 16

    http://cds.linear.com/docs/en/datasheet/436612fc.pdf


    much thanks!
     
  2. The pin is at some voltage but with a high source impedance. (So a
    voltage source with a soucre impedance of 100Meg ohm (or something))
    If you have a ground trace near it, then you may get leakage currents
    between the pin and ground... becasue of the high source imepadance
    the pin voltage drops... and unwanted things happen.

    (You should think not only about ground traces but any trace at a
    different voltage level... I guess ground is the most common.) (NPI)

    George H.
     
  3. The crud seems to make dirty boards hydrophilic or something.. breath
    on them and you get will get a big decrease in resistance.

    --sp
     
  4. If you talk with ultra high vacuum people, they will tell you there is
    a mono-layer of water coating everything. (Hence the bake-out to
    150C) I'm not sure if it's true or not... but it gives you a
    different perspective on the world.

    George H.
     
  5. Cool! Is it only metals or do dielectrics get a layer too?
    (I guess I was wondering what is the bonding mechanism?)

    Molecular sieve certainly gobbles up water. (but that may be a
    different mechanism.)

    George H.
     
  6. Water chemically bonded with clay doesn't scoot off until well over
    400°C. I guess the clay dessicant packets can't be recycled like the
    silica gel ones can, unless you open them and repackage the material.
     
  7. Hmm, As a grad student we used Linde molecular sieve (I can't recall
    the size maybe 13X?) for cryo-pumping.
    The prescription was to heat it to 250C to drive off the water. I made
    this little furnace from fire brick and an old toaster.

    George H.
     
  8. Bill Sloman

    Bill Sloman Guest

    http://en.wikipedia.org/wiki/BET_theory

    I did an experiment based on their numbers as a final year chemistry
    undergraduate, and it stuck in my mind. BET stands for Stephen
    Brunauer, Paul Hugh Emmett, and Edward Teller, with the Edward Teller
    being the one who went on to work on the H-bomb.

    The mono-layer of water atoms is stuck to surface by rather stronger
    bonds than those that bond subsequent layers of water atoms to
    underlying layers of water.

    Bill Sloman, Sydney (but in New Zealand at the moment).
     
  9. Neat, that's pretty cool Bill, I went on to this,
    http://en.wikipedia.org/wiki/Langmuir_equation
    (You can get lost for hours in wikipedia... )

    As an aside, let me say I welcome your 'science' presence here on SED.

    George H.
     
  10. Thanks Phil, No problem, I gather it's related to hydrogen bonding...
    dipole dipole interactions... all very confusing to me, since I've
    never studied it.

    (In a metal you can imagine a dipole /induced dipole interaction.)

    George H.
     
  11. Uwe Hercksen

    Uwe Hercksen Guest

    Hello,

    somebody said: God made solids, but surfaces are a work of the devil!

    Bye
     
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