I assume the instrumentation amp runs on a dual rail supply, e.g. +12volt and -12volt.
In that case, you can't connect the output of the instrumentation amp directly to an Arduino input.
The Arduino pin could get damaged by higher voltages than it's supply, and lower voltages than Arduino ground.
You must use protection resistors (voltage dividers) and maybe schottky clamping diodes.
And ofcourse, connect grounds.
Leo..