MOTM

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.
>