Do you understand this? Several basic op amp questions that netizens are "struck" by in interviews

I am usually interested in op amps and I study them after work. Today, I was frustrated when I went to the interview. I came to the forum to ask two simple questions. Please give me some advice! As shown in the picture

1. The impedance of the inverting input terminal is Ri;
2. The second question is that this resistor is to eliminate the influence of the bias current.
When Rs is added, the offset voltage generated by the input bias current Ib is 0.
Vo=(1+Rf/Ri)*Vi + [Rf -Rs(1+Rf/Ri)]*Ib - [Rf+Rs(1+Rf/Ri)]*Iio/2

Thank you for your reply. I remember very clearly that during the interview, I said that the reverse input impedance was Ri, but was looked down upon. He said it was Ri+(Rcm//Rf). One path is from the op amp input terminal Rcm to ground, and the other path is through Rf to the input terminal to ground. The output impedance is very small. Since the common-mode input impedance of Rcm is very large, it can be considered as Ri+Rf. I thought it was right at the time, but if I use the method of calculating the input resistance, adding a voltage source combined with a virtual short, the reverse input impedance is Ri. In addition, I have used pspice to simulate, and in the low frequency band, the input impedance is indeed Ri. I would like to ask the experts to explain which one is correct?

Enthusiastic netizen: The input resistance should be Ri.

Consider the circuit as a two-terminal input and a two-terminal output, so the following conditions apply: Input voltage Ui
Output voltage Uo = (-Rf/Ri)*Ui
Input current Ii = Ui/Ri
Output current Io = Ui/Ri
Therefore,
the input resistance is Ui/Ii, that is, Ri. If Rcm must be considered, then the input resistance should be Ri//Rcm. When Rcm tends to infinity, the input resistance is still Ri.
In summary, the input resistance must be Ri.

Further reading

If you still have questions about the above issues, you can read this article-------"TI Signal Chain Blog Collection - See how an old TI engineer tamed the precision amplifier" (please click "Read the original text").