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Variable gain for a Instrumentation amplifier


ROOSIE

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I'm trying to build a Instrumentation amplifier that would do a 10uV -> 10mVamplification. The motivation is to measure uA currents on a small enough shunt resistor (1-10Ohm).

For a proof of concept, I've built a circuit like the one below(png1.)with a distinction that I'm powering it with two 9V batteries and am using LM2902N as the op amps and Rg is somewhat different. It has much less that the required 1000x gain.

However, when measuring the results for different input voltages, the gain seems to vary: png2.

I was expecting it to be constant.

· What is a reasonable place to look for the causes of the variations?

· Cheap resistors?

· Inadequate op amp?

· Breadboard prototyping?

· 2x9V batteries as power supply?

· other?

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your mid range input voltage is 25mV.  I am presuming this is RMS. . if you gain was designed to be 1000, then you are asking it to output 25 volts...RMS this is like +/-17V peak.
Are you sure you are not hitting the DC rails?   Unless your X axis is marked incorrectly... are  you inputting mV or uV?  

 

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