Brian Kraut wrote...

>I am working on an alarm that trigers off of an analog voltage.  I need
>a very high input impedance on the analog voltage so I won't affect the
>circuitry down the line from where I am picking the voltage off.  My
>thought was to use an instrumentation amplifier in the standard
>configuration with three op amps.

>I originally tried the circuit in Electronics Workbench and breadboarded
>with an LM324 op amp.  I just wound up with 1.8V on the final outpur no
>matter what the input was.  When I use some different amplifiers the
>circuit works.

>I realize that the LM324 is not a true instrumentation amplifier, but I
>would think that it would still work.  Any suggestions on why it
>doesn't. =20

There are several things that can go wrong with this.

=46irst, I don't know what source impedance the voltage you're monitoring
has, but the LM324 has a maximum input bias current of 100 nanoamps.
Thus, for every megohm of source impedance the Ib of the opamp can cause
as much as a hundred millivolts of input error.  If your source
impedance is many megohms, you can be wiped out completely.

Second, the input common-mode range of the LM324 only goes up to within
1.5 volts of Vcc; if either of your inputs goes higher than that, the
circuit will no longer function.

I don't know what your circuit is, but those are two possibilities.  The
fact that you get a constant 1.8 volts out of this thing, however, makes
me suspicious: are you sure it's wired right, and that your LM324 isn't
dead?  It's unusual for it to do nothing.

>Also can someone suggest a low cost single supply amplifier
>that whould work good.  I would like to use a 14 pin amp in the same
>configuration as a standard op amp.  Accuracy isn't too important.  I
>will feed the output to a comparator so I can alarm at a voltage that
>will be set with a trim pot.

=46or nearly all of my work--at least, anything that doesn't require
extreme precision or speed--I've settled on National's LMC6484 (quad)
and LMC6482 (dual) CMOS opamps.  They have rail-to-rail inputs and
outputs, which makes them good for working off a single +5V supply, they
have very low input bias currents (just a few dozen femtoamps) and most
other specs are as good as or better than the LM324.  They're not cheap,
but I find the savings in design time due to these parts' flexibility
more than offsets their higher cost.  Pinout is standard, same as the
LM324 and LM358.

Hope this helps...

Dave

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