AD620 spice model input impedance


I'm trying to check the differential and common-mode impedance modelled on the spice model of AD620.I'm using Pspice.

By using a DC sweep, looking at the input current and take the slope, I found about 5 MOhms which is very smaller than the datasheet (10 GOhms).

Could somebody help me to find if the model is wrong or if it's my method of measuring input impedance.

Below the circuit for differential impedance measurement