We built this circuit. When at the "Vin1" pint I apply 100mVpp, 100Hz sinus (output impedance HighZ on signal generator), on pin7 OUT I have the same sinus wave with approx 100mVpp amplitude, which seems to be OK.
If I apply to point "Vin2" (200Mohm in series with voltage generator) the same 100mVpp sinus, I have no signal on the output, just noise.
If I measure with a multimeter (measure current ~0.5uA) between pin 4 (IN+) and pin6 (REF - connected to GND) I measure 1,3 MegaOhm (my expectation was 500Mohm). At this measurement the input signal generator was removed, Vin1 and Vin2 float. IN- was connected to GND. If IN- was connected to Vout, than on VOUT was a +/- 2.5V, approx 1 MHz square wave (I guess self oscillating).
From this measurement it seems for me, that IN+ to Vref input impedance is in range of 1,3Mohm, but from datasheet this values shall be in range of 100 GOhm.