How can I find out the causes of the variations of input voltages in my amplifier?

F

fannling

New Member
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 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:


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?
 
heckler73

heckler73

Well-Known Member
The batteries are actually a better source of power than a regular power supply (if the concern is noise).
What are the tolerances on the resistors? That would the most likely cause of imbalance in the circuit.
The breadboard will also contribute to the noise (especially if you're dealing with microamps).
Noise is tricky, and it could be from a myriad of external sources, too. Do you have dangling wires everywhere?
 
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