The 1.8V was in an Electronics Workbench simulation.  The real thing is
about .8V.  I am using an adjustable power supply as the input now so
input impedance is not a problem.  The real circuit will have a 24V
supply and an input that will vary between about three volts and 15 volts.

Dave Dilatush wrote:

>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.
>>
>
>There are several things that can go wrong with this.
>
>First, 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.
>>
>
>For 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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