How do I find op amp specifications regarding voltage limits?
for sensible results the signals on the input pins of the op-amp must be between Vee -0.3 and and about Vcc -2 (this from memory it will be in the data-sheet sowewhere)

Assuming you're using the normal unity gain circuit with four equal resistors 
you'll hit that limit when the high input is about twice the supply voltage.







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