OpAmp has differential input impedance(Rd) and common input impedance(Rc). In my opinion, Rd=4*Rc. However, datasheet of OpAmp indicates that Rd is not simply related to Rc this way. Well, how are input impedances of OpAmp meatured or tested?
Can anyone help me? Thanks in advance!
How the impedances are related is presumably dependent upon the architecture of the amplifier. You really should read the forum guidelines and post properly framed questions, otherwise you really won't get anyone being able to help you.