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From: firstname.lastname@example.org (frank)
Subject: Re: Comparator snap on-off ??
Date: 15 Oct 2002 12:20:09 -0700
NNTP-Posting-Date: 15 Oct 2002 19:20:09 GMT
Fred Bloggs wrote in message news:<3DAC2B57.email@example.com>...
> frank wrote:
> > 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
> > needed.
> > Thanks,
> > Frank
> You have two reference sources for upper and lower boundaries of the
> window. In order to stop the oscillation at threshold crossing you need
> to feedback voltage that lowers the upper threshold reference and
> increases the lower threshold reference, or, alternatively, adds to the
> signal upon upper threshold crossing and subtracts from the signal on
> lower threshold crossing. These feedback voltages need only be a few
> millivolts in most cases.
I understand that the circuit requires hysteresis to function better
and I have tried to apply it to the circuit. Applying it to the
inverting input section works fine but when I try to apply it to the
non-inverting input also it messes the inverting section up. The
reference levels are provided by a resistor divider network and trim
pots for fine adjustment. The input signal is provided by the ouptput
stage of an op-amp that amplifies the thermistor being monitored. The
level changes very slowly and as such causes oscillation at the
transition points. What I am ultimately trying to find out is: How can
I apply hysteresis to both sections at the smae time without
interaction between the two?? Everything I have tried so far makes one
section work as desired but not the other. Any ideas ??
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