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Hello afnan, There are a couple things. First, the REF pin needs to be attached to a low impedance voltage such as ground. If you do not connect it, the subtractor see AD datasheet for circuit details becomes a gain of 2 for the positive path and a gain of -1 for the negative path, since these no longer match, you do not get the common-mode rejection from the subtractor.
Second, in addition to standard input range and output range limits, three-op-amp type instrumentation amplifiers such as the AD have internal op-amp nodes that can saturate.
Because of this, the output swing depends on the common-mode voltage as well as the differential voltage, gain, and reference pin voltage. We have this design tool to help with this. If you want to learn more about instrumentation amplifiers, I highly recommend reading this book , which is free on the web. The different models of the AD that we offer are listed on the product page. Also AD is an instrumentation amplifier, not just an op amp.
With an op-amp, you might want to match the resistance at the input terminals to reduce bias current errors. With an in-amp, it might be for protection from voltages beyond the supplies or for a filter to reduce RF interference. All of this is discussed in the book I linked above. I hope this helps.
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