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Instrumentation amplifier - simulation vs practical results

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nothing9099

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I'm testing a basic 3 op amp instrumentation amplifier.

I simulated the circuit firstly and then tested it using practical equipment.

If expected the results would differ slightly, but I want to know why. What factors are causing the results to differ slightly between a computer simulation and practical test?
 

KlausST

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Hi,

If expected the results would differ slightly, but I want to know why.
This statement is extremely vague ..... you should know that the answer can't be more exact than your given informations.

What answer do you expect?
I assume there will be a difference. Maybe wrong values, or bad wiring...how can we know?

Klaus
 

nothing9099

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I don't understand how that statement is vague. every circuit that is simulated and then tested practically will have slightly different values because of different factors, no?

when i simulated the instrumentation amplifier and then tested it practically, the values i got are correct, but they're obviously not exactly the simulated values. my question is why? what factors are causing the results to differ slightly?
 

dick_freebird

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This goes to the qualities of the simulation. Which may
or may not be obvious.

One critical thing is, some 3-op-amp instrumentation
amplifier schemes have an issue with the A=2 amp
clipping / limiting and the A=-1 amp then is added to
a gimp signal and you get a wrong result, from signal
that is within the common mode input range of the
amps (but not common mode output range when
A>1).

A simulation using controlled sources and no limiting
function, will give you clean operation while a real
(or really well modeled at the individual amplifier level)
will show you the bind.

Now slight differences, like a 2*VIO2X-VIOn1X+VIOSUM
offset, may just be about real statistical scatter vs
models with no variability. Gain@VCM vs ideal simulation
model gain (towards the rails, block gain drops a lot).

The less your model looks like transistors with foundry
fully parameterized .model definitions, and more like
B sources / vcvs and passives, the more you are
going to be living in a fantasy land. How to tell, is on
you.
 

KlausST

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Hi,

I don't understand how that statement is vague.
It is vague in reagard of what "parameter" (time, voltage, CMRR, noise, offset, overshot, supply current, distortion...)
And it is vague in ragard of the deviation.( 100%, 0.1%, ppm...)

If supply current deviates by 10% .... I'd say: don't care
If gain deviates by 10% .....I'd say there is something wrong
If gain deviates by 10ppm ... I'd say don't care

Klaus
 

barry

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I'm testing a basic 3 op amp instrumentation amplifier.

I simulated the circuit firstly and then tested it using practical equipment.

If expected the results would differ slightly, but I want to know why. What factors are causing the results to differ slightly between a computer simulation and practical test?

They differ because you can't buy ideal components like your simulator uses. Unless your simulator accounts for component tolerances, parasitics, board layout, bad soldering, etc., etc.,
 

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