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From: firstname.lastname@example.org (Tim Shoppa)
Subject: Re: Comparator snap on-off ??
Date: 15 Oct 2002 12:18:01 -0700
NNTP-Posting-Date: 15 Oct 2002 19:18:01 GMT
email@example.com (frank) wrote in message news:...
> I am working on a comparator circuit that is used as a window
> comparator to determine if a temperature is within spec. The reference
> voltages to the comparators are only 300 mV apart. Comparators are
> AD790 types. Example:
> Nominal temp input voltage: 1.5 V
> Reference High level : 1.65 V
> Reference Low level : 1.35 V
> The comparator signal is monitored with an ADC and also drives a pair
> of status LED's for the operator to monitor. It would be desirable for
> the LED's to "snap" on or off as the levels are reached. As it is now,
> the LED's slowly turn on and off as the level is being detected. The
> output of the comparators drive an Allegro 2981 relay driver to turn
> the LED's on. Any ideas/suggestions? Let me know if more details are
What you probably want is a "Schmitt Trigger". Feed a little bit of
the output back to one of the comparator inputs and you get hysterisis.
But also: the fuzz may not be because of lack of "snap-on-off", but because
either the signal or the reference has a high-frequency AC noise component. A
Schmitt Trigger does help here too, but if the noise is bigger than the size
of the hysteresis the Schmitt Trigger won't help an awful lot. Maybe some
simple low pass filtering (to the appropriate ground!) will help. Maybe
going to differential signal inputs will help.
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