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[Consultation] How are the input impedances of OpAmp meatured?

Other Parts Discussed in Thread: OPA2356

Hi guys,

OpAmp has differential input impedance(Rd) and common(Rc) input impedance. Well, I think Rd=4*Rc. However, specifications in datasheet of opa2356 indicates that Rd=Rc. 

So, how are the input impedances of OpAmp meatured?