AD620 reference terminal problem
The AD620 negative supply (-Vs) is at 0 volts but if you do this, you need to respect the input common mode range and that is specified as: -

−Vs + 1.9 to +Vs − 1.2 




In other words, tying pin 2 (one of your inputs) to ground is an illegal condition and all bets are off. The lowest voltage allowed on your inputs is 1.9 volts.

Just in case it's important to your design, you should also consider that the output signal is limited to this following range: -

−Vs + 1.1 to +Vs − 1.2


So, with a single 5 volt supply, the output can typically be between 1.1 volts and 3.8 volts but, over the full temperature range this might reduce to 2.1 volts to 3.6 volts.