Help with INA122

I am building an interface for a PT100 temperature sensor and I need an instrumentation amplifier.

I came across the INA122 which looked ideal, only one component to set the gain, single supply and rail-to-rail output swing.

For the inputs I have a reference voltage of 0.1V and a signal voltage that swings between 0.1V and 0.1384V. The output needs to be in the 0-1V range so the gain of the amplifier needs to be around 26.

Now I simulated this all on Proteus and it works great when the input is at 0.1384V giving a good 0.99V output. However down at an input of 0.1V I get an output of 0.33V, not the 0V I was expecting.

What gets me is that the data sheet says rail-to-rail output but it isn't. Is this an error with the Proteus simulation or just a limitation of the device?

I guess if I can't solve it with one this device I will have to level shift, amplify and then level shift back down, yuck.

Thanks for you help.

You have to compensate for input offset of 0.1V applying exactly that voltage to the second non-inverting input (the first one receives your signal)

Here is an example for input offset correction of a pressure sensor which supplies 40mV at zero pressure.

Ignore the output offset correction if not required.

Boncuk