thermocouple interfacing to instrumenation amplifer

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mess123

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I constructed an instrumentation amplifier using three op-amps and connected the thermocouple o/p to the +ve terminals of the first two op-amp of the input stage. I didn't obtain the gain of the instrumentation amplifier as according to the formula (1+2R/Rg). What could be the reason? Is this gain practically not achievable? Please reply me.
 

You must guarantee a common mode input voltage within allowed limits. Usually this doesn't work with a floating source. You need at least high-ohmic bias resistors at the input. Consult a datasheet of a typical monolithic instrumentation amplifier for reference.
 

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