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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 !  

  • These are drawn poorly, there is a current noise separately on each input pin, here is a more thorough treatment, For the inverting current noise with the V+ node grounded, the V- node is at 0V due to loop gain, then, nothing can go towards a grounded Rg element and it simply flows through the Feedback Rf to the output voltage. 

    https://www.ti.com/lit/an/sboa066a/sboa066a.pdf

  • Hi Rust,

    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.  

    The ideal OPAmp keeps the -input on virtual ground Relaxed

    Kai

  • Hi,

    Michael's Application Report that he provided is as in-depth and concise treatment about op amp noise analysis that you will find. The TIPL sessions are not to the same technical depth and serve as as an introduction and quick study. They provide sufficient information to allow one to become familiar and more comfortable with the particular subject matter being presented.

    Do you now have the information you need?

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Thanks Tom, when I wrote that one for BurrBrown in 1996, I was actually reprising an earlier one I had written for Comlinear Corp - that was focused on the current feedback op amp noise analysis, which was relatively new at the time, 

    Kumen redid it after I left for BurrBrown in 1995, but the original OA-12 was something I had done as apps manager at Comlinear,

    https://ieee.li/pdf/essay/oa-12.pdf

  • Hi Thomas, yes, and thanks to all of you. 

    Do you now have the information you need?