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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:

enter image description here

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:

enter image description here

To see this non-linearity I made a DC sweep for 0.1mV...2mV range and here is what I get:

enter image description here

Am I doing something wrong here? If not would this nonlinearity have bad results for my -/+2mV range strain-gage inputs to the amplifier.

Edit:

Common mode voltage added to the inputs as will exist in real scenario:

enter image description here

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  • \$\begingroup\$ AD8221 has up to 25 uV offset voltage. That alone will produce a nonlinear response, probably noticeable for input amplitudes below 100 uV. \$\endgroup\$ – The Photon Apr 3 '17 at 17:48
  • \$\begingroup\$ @ThePhoton Does that mean for small strain changes there will be great error in measurements? What could be the workaround for these in electrical side besides using more sensitive gages? \$\endgroup\$ – floppy380 Apr 3 '17 at 17:53
  • \$\begingroup\$ What are your power supplies, and are you good with the allowed common mode input ranges? \$\endgroup\$ – Scott Seidman Apr 3 '17 at 17:53
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    \$\begingroup\$ Yes, that's close enough to what I meant. If you have solved your problem, be sure to post an answer explaining the solution. \$\endgroup\$ – The Photon Apr 3 '17 at 20:39
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    \$\begingroup\$ @Enric Blanco This is my first answer. I also added some extra I would be glad if you agree my answer. Its not practical to tie the inputs all the time. \$\endgroup\$ – floppy380 Apr 3 '17 at 21:12
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I followed ThePhoton's suggestion in the comments.

Here is how to solve this:

First take offset by tying inputs together and to the GND and measure the output voltage Vout. In this case -39.3478mV as shown below:

enter image description here

So this should be subtracted from any output voltage after any measurement. And when I do that gain looks 800 constant:

enter image description here

*I suppose if a strain_gage is mounted and fixed, an offset measurement can be taken before any moving measurement(without the procedure above). Voffset is the output voltage when the inputs are tied to GND.

Imagine when mounted the offset stress of the gage is X amount(Newton) and the voltage is Vxo = V1+Voffset . If the stress is varied ΔX amount there will be increase in output voltage as ΔV and the output will be V2=(V1+Voffset)+ΔV. Now V2-V1 = ΔV. So the amplifier's Voffset is included in the first offset already.

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