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OPA627: question on equivalent input current noise

Part Number: OPA627

Hi there,  I am watching TIPL video section 8.3. on page6 of the slide(as below), en_i = in*Req. This seems correct, but I'm not very sure how this is strictly derived. Could someone explain?


by the way, I also googled some similar info from this extended link (by Art Kay), page99(as below), it shows Voltage across R1 is 0V (red circle ) and Vo=in*Rf,why? I thought the current In should both flow into R1 and Rf, as I marked In = I1+If.  

Then, based on this page, the next slide (page100) indeed derived the equation en_i = In*Req, which is straightforward.

Thanks a lot !