Instrumentation Amplifier Reference Node - Integrator vs Buffer
This INA has the unity gain Diff Amp Output stage with laser trimmed R's for high CMRR*.

Thus for single supply REF input = +Vs/2*

Since REF needs to go to a Zout=0  the AC coupled Op Amp in Figure 64 meets that criteria,  thus +Vs/2 goes to the external OA +in  = Vref.*

AC coupling for INA is mandatory for ECG , EEG applications due to galvanic skin response  which create a pressure sensitive DC voltage. * 



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