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Problem with opamp and instrumentation amplifier

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wizardgrt

New Member
:confused:Hi,

I'm trying to power up the signal with instrumentation amplifier, that comes from operation amplifier. Problem - gain loss! When a source of stabilized voltage is connected with instr. amp. instead of LEM transducer, everything works fine. What's the problem? :confused:
 

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crutschow

Well-Known Member
Most Helpful Member
What is the signal level out of the op amp (AC and DC offset) and what gain levels are you using in the instrumentation amp?

Why are you using an instrumentation amp configuration? Since you are using it as a single-ended input, not differential you could just use a single op amp.
 

wizardgrt

New Member
Here is complete specification of transducer, with all parameters. Point is that the transducer gives 2.524V at 0A. I have to do some limitations, and it would be much easier when it would be 0V at 0A, and of course 5V at maximum current (3A), which is not satisfied, because transducer gives 4,627V at I=20A, so i can only operate with about 0.212V... If using simple voltage subtractor (diferential amplifier) the CMRR gets high, so at 0A have 0,6V. I tried R2 adjustments, but it affects only about 0.1V...
Of course, as i wrote before, when i connect in amp to indipendent regulated voltage source - no problem...
2.5V is supplied from voltage devider, 10V devided by 4700Ohm and resistance from potentiometer (about 17KOhm)...

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, can't 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?
 

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wizardgrt

New Member
Problem solved :), thank's guys for all suggestion, and comments. Mistake i made was, that inputs weren't connected to supply voltage, so measured voltage was floating, thus nothing, (or something ) gained up. When I did a simulation, both ends of shunt resistor, and transducer were connected, (across resistance of 10K) with voltage supply and ground and it worked fine. Cheers! :)
 
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