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'[EE] Comparator inputs are equal voltages'
2010\12\16@131552 by V G

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Hey all,

I tried googling this, but got no answers.

What happens when both inputs of an analog voltage comparator are equal in
voltage? What will the output be? Wikipedia says:

- > + : output is negative
+ > - : output is floating

But it doesn't say what happens when the inputs are equal

2010\12\16@132944 by Bob Ammerman

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Any voltage difference within the "input offset" specification may result in High/Low or something in between or even oscillations. That is why you generally use some positive feedback around a comparator.

-- Bob Ammerman
RAm Systems

{Original Message removed}

2010\12\16@133734 by Olin Lathrop

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V G wrote:
> What happens when both inputs of an analog voltage comparator are
> equal in voltage? What will the output be? Wikipedia says:
>
> - > + : output is negative
> + > - : output is floating

This is misleading since it assumes a open collector output.  Many
comparators do have open collector outputs, but that is a implementation
detail not inherent to being a comparator.

> But it doesn't say what happens when the inputs are equal.

There is no such thing as two equal voltages.  The comparator output will be
high when + > -, and low when - > +.  For a ideal comparator, there is no
equals case to consider.  Every voltage has some inherent noise, even if
it's just the noise inside the chip that is measuring it.  A perfect
comparator with the inputs tied together will therefore produce a random
sequence of high and low outputs as the two signals after the noise is added
flip between the +>- and ->+ cases.

However, real comparators have a characteristic known as the input offset
voltage.  That is the error band within which the output value is not
guaranteed.  For example, if a comparator has a 2mV input offset voltage,
then there is no guarantee what the output will do when one input is at
1.000V and the other at 1.001V since they are within the 2mV limit of each
other.  The comparator still has a threshold somewhere, but you don't know
where it is this minute at this power voltage at this common mode voltage
and this temperature within the input offset voltage band specified in the
datasheet.

Real comparators also have finite gain, unlike a ideal comparator, which has
infinite gain.  If you managed to hold the two inputs of a real comparator
just at the threshold, then you will see the inherent noise amplified on the
output.  It could also oscillate, since some portion of the noise comes from
coupling from the output.

In short, you don't know what will happen within the input offset voltage
band, and need to design your circuit accordingly.


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2010\12\16@133908 by Bob Blick

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On Thu, 16 Dec 2010 13:15:36 -0500, "V G" said:
> Hey all,
>
> I tried googling this, but got no answers.
>
> What happens when both inputs of an analog voltage comparator are equal
> in
> voltage? What will the output be? Wikipedia says:
>
> - > + : output is negative
> + > - : output is floating
>
> But it doesn't say what happens when the inputs are equal.

Typically there would be no change, but different architectures will
have different responses. Many will have a tiny bit of hysteresis, some
will have a linear range. Others will oscillate. It's up to the designer
to insure the comparator works predictably, and that usually means you
will add positive feedback to get a calculated amount of hysteresis.

Bob

-- http://www.fastmail.fm - The way an email service should be

2010\12\16@134159 by Isaac Marino Bavaresco

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Em 16/12/2010 16:15, V G escreveu:
> Hey all,
>
> I tried googling this, but got no answers.
>
> What happens when both inputs of an analog voltage comparator are equal in
> voltage? What will the output be? Wikipedia says:
>
> - > + : output is negative
> + > - : output is floating
>
> But it doesn't say what happens when the inputs are equal.


It is very unlikely that both inputs will be at *exactly* the same
voltage. Any micro-volt may be enough to offset the result to one or the
other value.

More, there is unbalance in the comparator inputs (sometimes several
tens o millivolt) and sometimes one input is above the other and the
comparator sees the opposite.

If your inputs are really very, very close (as seen by the comparator),
any noise may make the comparator change state and you may get oscillations..


Best regards,

Isaac

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2010\12\16@135346 by Michael Watterson

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On 16/12/2010 18:15, V G wrote:
> Hey all,
>
> I tried googling this, but got no answers.
>
> What happens when both inputs of an analog voltage comparator are equal in
> voltage? What will the output be? Wikipedia says:
>
> ->  + : output is negative
> +>  - : output is floating
>
> But it doesn't say what happens when the inputs are equal.
with hysteresis it depends on last state. i.e. you have to have a certain  offset to change state,

Without hysteresis, in the "real world" you get noise or oscillation. Depending on layout, power decoupling etc

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