Different input voltages and different the gain, why is that?

Question

Hello, I'm new here. 

I'm trying to build a Instrumentation amplifier that would do a 10uV -> 10mV amplification. 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 with a distinction that I'm powering it with two 9V batteries and am using LM2902N (datasheet here: http://www.kynix.com/uploadfiles/pdf9675/LM2902N.pdf)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: 

I was expecting it to be constant. 
And here are my questions: 
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?

Answer 1

The PLECS forum may not be the best place to discuss real hardware.

However, now that you're here: You're probably having an issue with the input offset voltage, which may be as high as 7 mV and therefore has the same order of magnitude as your input voltage range. Or, in other words, your OpAmp is not suitable for this task.

Comments

Hi Oliver, thanks for your reply and tip, that makes sense, so you mean i should replace the opamp with another one? Any suggestions?

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Yes, you should definitely replace it, but I don't have any suggestions for OpAmps suitable for Microvolt amplifications. Try the search engines from Analog Devices or TI, for example.