Re: gEDA-user: Opamps et al...

[Stuart Brorson <sdb@xxxxxxxxxx>]

> If I have an op-amp left over unused in a package, what is the
> recommended way of tying its inputs to avoid noise / power consumption?
> I figured connecting the inverting input to the O/P, then the
> non-inverting input to GND. Perhaps I should just use the second op-amp
> to drive a non 50Ohm output?

I generally tie unused inputs & outputs to GND through 1K resistors.
Whether this is the most optimal practice for noise considerations is
not known to me, although I suspect that it is good enough for most
purposes.  There *are* good reasons to do this (or something similar):

*  During board DVT, it's nice to have an op-amp ready for blue wire
rework if you find you've made a mistake and need an additional amp
for some reason.  In that case, you can just depop the 1K GND
resistors and use the spare op-amp for whatever use you have in mind.

*  Many in-circuit testers want to have a resistance between an unused
pin and GND so they can bias up the pins and verify that they are
connected & functional.

> Are there any gotchas to watch out for when using opamps to buffer at
> low gain?.. I can vaguely recall there being an issue with low gain and
> some amps (depending on their internal compensation).

Look for the phrase "unity gain stable" in your op amp's datasheet if
you plan on using it at low gain and large bandwidth.

Cheers,

Stuart


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