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gain of instrumentation amplifier falls

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Junior Member level 1
Jun 16, 2008
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instrumentation amplifier

I'm trying to connect opamp and inamp like on schematic attached, but the gain of the instrum. amplifier falls down. When a source of stabilized voltage is connected with instr. amp. instead of LEM transducer, everything works fine. What's the problem?

lem cas-6np

First of all, PNG is a much better format for uploading puctures/schematics. It takes less space and you can wiew it directly in the browser.



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gain of instrumentation operational amplifier

thanks, i didn't know how to attach the picture so it could be visible - i'm new in this forum :D

Do you have any suggestion, how to fix the problem? Whatever i did gain is fixed around 10... :(

derivation of gain of instrumentation amplifier

Get the datasheet for LM324N. Look for a testing circuit in the datasheet and mount it on a breadboard. Or simply just replace it the ic.

You sure it is the amplifier that doesn't work properly? maybe the Rm is all wrong. Have you measured signal on Out?

Have you any connections that isn't shown in the schematic?


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instrumentation amplifier oscillation

No, except 2.5V, which i didn't draw... but it's all drawn as it is. Simply the gain extremly falls down. I was reading about source impedance vs. load impedance, but even after that still doesn't work as it should. Whatever i tried, gain can't be more than 10.
Amplifier works fine. Whole mistery is about source of noninverted voltage, at input of inamp...

gain of instrumentation amplifier

Do you have opportunity to send in a sine wave with fixed amplitude and frequenzy? As mentioned before, can you use a scope and read the signal several places? Espacial where you have pin 11.

Also I see you don't have any negative voltage supply. Maybe that's the problem. When you don't have, it means that neither of the opamps is capable of handling negative voltages.


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instrumentation amplifier schematics

When you asked if there is any connection that is not drawn, yes there is, 2.5V is supplied from voltage devider, 10V devided by 4700Ohm and resistance from potentiometer (about 17KOhm)... if that means something... I cant send any information about signals, due the lack of equipment :(

instrumentation amplifier derivation

wizardgrt said:
I cant send any information about signals, due the lack of equipment :(
Need that in order to locate the failure.

And once again: Have you the ability to use negative voltage?


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every thing about instrumentation amplifier

The circuit can work without negative supply. As a main restriction, it has only unipolar output (e.g. positive sensor currents)
and it can't measure values near zero. If these restriction are acceptable, the circuit is my opinion.

I understand, that the output is intended to drive a PIC ADC (0..5V range), so negative outputs won't be meaningful anyway.
You may want to adjust the reference voltage to include the sensor zero in the observable measurement range.

The reason for the said "gain fall" can't be seen from the circuit, I think. You should mention the connected LEM sensor type,
but common types should work with the circuit. Most likely there's a problem that's hidden in your circuit (e.g. missing ground
connection, or similar).


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for the negative voltage - no, i'm using battery voltage of 12V stabilized to 10V, to eliminate the possibility of oscillation... Now i have a new problem which is same type - when trying to gain up the signal from voltage devider 21V/0.1V and then 0.1V to 5V with ordinary opamp gain is ok and is 10. In second case, when the voltage from shunt resistor (0.05 Ohm * 3 A = 0.15 V) is being gained to 5V, cant get more than 3V??????
Does this have something to do with matching source (shunt res as voltage source) and load (opamp) impedance? If does, how can it be achived? I've also attached specification of transducer LEM CAS-6NP

Problem solved :), thank's guys for all suggestion, and comments. Mistake that i made was that inputs weren't connected to supply voltage, so measured voltage was floating, thus nothing, (or something :D) gained up. When I did a simulation, both ends of shunt resistor, and transducer were connected, (across some resistance) with voltage supply and ground, and it worked fine. Helped button is pressed for everyone! cheers!

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