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.  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.

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