I'm observing nonlinearities in microvolt range inputs on an instrumentation amplifier. Here is the amplififer datasheet and its gain is supposed to be:
G = 1 + (49.4 kΩ/Rg)
And the inputs to this amplifier(from a strain-gage) will be in -/+2mV range in my case.
So I wanted to see the gain at different differential inputs in simulation.
Below the inputs to the amplifier are +/-1mV, so the differential input signal Vsig = (+Sig) -(-Sig) = 2mV and Vout = 1.6V. Here the gain is then around 780. Below is the plot for these:
And this time below the inputs to the amplifier are +/-1μV, so the Vsig = 0.2μV and Vout = -38mV. But this time the gain is around 190k(LTspice shows 19k). Below is the plot for this case:
To see this non-linearity I made a DC sweep for 0.1mV...2mV range and here is what I get:
Am I doing something wrong here? If not would this nonlinearity have bad results for my -/+2mV range strain-gage inputs to the amplifier.
Common mode voltage added to the inputs as will exist in real scenario: