[sdiy] Static Damage to ICs

[patchell]

    I am not sure how the inputs on a JFET opamp get damaged, but I was using
some JFET opamps that had bias currents down in the fempto-amp range that
were being used in a sample hold circuit.  When the boards got zapped, the
droop on the sample hold would increase dramatically.  This was how I could
tell when they were damaged.  Replacing the amplifier would always fix the
problem.

René Schmitz wrote:

> At 07:24 13.03.02 -0800, you wrote:
> >    However, with opamps and the like, the failure is generally fairly
> >subtle.  For Fet opamps, a lot of time it increases the leakage current
> >in the inputs.  A TL072, while it does have fairly low bias currents, it
> >is really not all that low.  So, you may or may not notice the damage
> >right away.
>
> A TL072 has JFET inputs, how do you blow away its gate "insulation",
> shouldn't
> it zehner long before?! I was always under the impression that bipolar
> transistors and JFETs were pretty immune to static discharge.

    This is a very common misconception.  3M came to our plant to demo their
anti static equipment.  For their demo, they would put a JFET into an
instrument to measure the gate leakage current.  I can't remember the FET,
but the bias current was something like 1 pico amp.  They would take the part
and put it into a "pink" anti static bag (this was the product they were
competing against).  The would then take a roll of scotch tape, and pull off
a strip of it about 1 foot away from the pink bag.  They would then take the
FET back out of the bag and put it into the test instrument, and the Gate
leakage would be about 1 nano amp.  The same experiment using the 3M metal
coated bag would result in no increase in the gate leakage.  Mind you, nobody
ever touched the fets physically to ZAP them.

    Even things like good old LS TTL, which is pretty darn rugged, was rated
at only 100V (by NASA, for whatever that is worth).

>
>
> Cheers,
>  René
>
> --
> uzs159 at uni-bonn.de
> http://www.uni-bonn.de/~uzs159
>
>

--
 -Jim
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