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'[EE]: Instrumentation amplifier'
2002\02\09@135045 by Brian Kraut

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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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2002\02\09@144301 by Cosmin Buhu

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       Analog Devices has a lot of IAs products.
AD620 for 1-10k gain range, AD622, AD623 for 1-1k. Prices around 7
USD a piece. Newark usually has them in stock.

Cosmin



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.
..................

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2002\02\09@144440 by Dave Dilatush

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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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2002\02\09@152546 by Brian Kraut

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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:

{Quote hidden}

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2002\02\09@161230 by Dwayne Reid

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At 03:29 PM 2/8/02 -0800, Brian Kraut wrote:
>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.

Something else to try with the LM324 is to add pull-down resistors to the
outputs of the op-amps - something around 4k7 should be fine.  There can be
a problem in the output stage near the class A - class B crossover (very
well documented in NS data sheets).  Its worth a try.

dwayne


Dwayne Reid   <dwaynerspamspam_OUTplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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2002\02\11@141226 by Peter L. Peres

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The LM324 requires a coulpe of tricks to make it work well. One of them
requires that you ensure that none of the inputs ever go over 3.5V or so
because the output will become +Voutmax immediately. If it is used with a
circuit that allows any of the inputs to become more positive than this
Voutmax then this can cause it to latch up.

There are a number of fixes that remove this problem but you need to work
them out yourself for your circuit.

hope this helps,

Peter

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2002\02\12@023246 by Vasile Surducan

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Peter, LM324 is the last operational amplifier I'll use for this purpose,
because the operationals are catching one each other and the input
performances are poor. There are some beautifull and exorbitant expensive
BURR-BROWN instrumentation amplifiers.
I like ( and use intensively till now ) LM308/LM301 or TL081/TL084
best regards, Vasile

On Sun, 10 Feb 2002, Peter L. Peres wrote:

{Quote hidden}

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