[sdiy] unused comparators

Magnus Danielson magnus at rubidium.dyndns.org
Sun Nov 3 12:50:58 CET 2013


On 11/03/2013 05:39 AM, David G Dixon wrote:
>> On 11/03/2013 04:55 AM, David G Dixon wrote:
>>> 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.
>> Beware, railing them may not be a guarantee for stability. 
>> That is a feature of a particular design. Another feature may 
>> be that the valid input range may include the rail... such 
>> that railing both inputs is just as dumb as grounding them. 
>> When you don't know better, park them at distinct levels from 
>> each other!
>>
>> Just did a quick search for "comparator rail to rail input" 
>> and found a bunch of them.
>>
>> Cheers,
>> Magnus
> So, what you're saying is: comparators suck!  Yeah, I concur.  In any case,
> if I did have to leave one unused (which I don't), I would hook the inputs
> up to +5V and GND.  Since, I'm running some TTL off them anyway, I have a 5V
> source in the circuit, so yeah.  I wouldn't hook it up to the rails.
>
No, I'm not saying that they suck. Actually, when they do not have an
input range of rail-to-rail they suck, but these days they do handle that.
What I am saying is that you need to understand the tool, and handle it
with care. I give advice which may apply generally, but is also saying
that you might use it differently, if you read the fine-print in the
manual (datasheet). My general recommendation will however be wise to
follow, as it allows you to drop in a different comparator and it will
still be stable in this case.

Learn to use the tool wisely, and here are my thoughts about what that
means for comparators.

Cheers,
Magnus



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