A comparator works by sending its output positive if the noninverting
input is more positive than the inverting input.
A comparator with its inverting input tied to ground, and
noninverting input tied to a signal, will have a positive output when
the signal is above ground.
A comparator with its noninverting input tied to ground, and
inverting input tied to a signal, will have a positive output when
the signal is below ground.
This assumes you have a comparator capable of working with negative
voltage inputs. Some are spec'ed for only a 0-5v input range.
There are a couple of practical refinements: instead of tying the
inputs in the above examples directly to ground, it's better to tie
them slightly above and slightly below ground respectively, so that
you have a little noise immunity. You can also add hysteresis (beyond
the scope of my reply here) to accomplish the same thing.
Moe
--- In motm@y..., mark@i... wrote:
> Do they make
> comparators that can be configured to detect negative or positive
> voltages?? That way I can get two (or more) of the same kind one
chip. I
> also wonder how I can connect output of the same differentiator to
two
> different comparators without having them interfere with each other.
>