[sdiy] unused comparators

Magnus Danielson magnus at rubidium.dyndns.org
Sun Nov 3 04:49:14 CET 2013

On 11/02/2013 10:03 PM, David G Dixon wrote:
> Hey Team,
> I gotta simple question: What's the best thing to do with an unused
> comparator on, say, an LM339?  I'm thinking ground the input pins and pull
> up the output.  Can it just be left unconnected?
In general, unconnected is a big no-no. Some may be designed for it.
Both to ground is another particularly bad idea.

What you don't want is your unused comparator to operate as a
free-wheeling amplifier stage biased just right for linear operation.
I've seen such a thing in one of our products, when a hardware designer
had set on of the inputs to the comparator right at one of the levels,
when we had signal in no problem, when we had no signal... it was
picking up noise from the board and amplifying it... it was a beast to
find, but when I realized it, it made sense and explained all the issues.

So, unless the datasheet makes specific recommendations, make sure that
the inputs is well away from each other such that the output gets safely
stuck in either high or low state.


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