[sdiy] unused comparators

[David G Dixon]

Yes (based on what Magnus says below), the more I think about it the more it
seems dumb to ground the inputs of an unused comparator, since the output
state will be indeterminate.  Better to connect the rails to the respective
inputs, or one rail and ground at least, and to arrange it so that the input
doesn't consume current.

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of 
> Magnus Danielson
> Sent: Saturday, November 02, 2013 8:49 PM
> To: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] unused comparators
> 
> 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.
> 
> Cheers,
> Magnus
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