Op amp summing non inverting

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activee

Joined Jan 16, 2014
39
Hey I don't understand this summing non inverting OP amp. (I understand the inverting one).
The final equation is Ve = V1 + v2 + v3 . I don't know how to get that answer.

Thanks in advance.
 

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crutschow

Joined Mar 14, 2008
34,201
...............
V+ = V1 +V2 +V3 is that correct ?
No. The plus input is not a summing junction so it has a very high impedance, thus the input voltage is the parallel value of the three voltages through the three resistors. You need to solve that with some type of nodal equation such as Kirchoff's.
 

atferrari

Joined Jan 6, 2004
4,763
When I had to find the voltage value at the non inverting input I did it as explained in the attached document.

Please do not complain about the quality of the images. (I am sure that somebody will, anyway).

I do not have time to redraw them properly. The .pdf was assembled directly from a former .htm file.
 

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Fibonacci

Joined May 23, 2014
25
thank you but I don't think I understand still!

V+ = V1 +V2 +V3 is that correct ?
You must follow the reference that makes shteii01; in this case you have three input resistors. Using the superposition theorem, a voltage divider and paralell resistors we find three contributions to v+ equal to Vi/3, i=1,2,3. Therefore, the voltage at noinverting terminal v+ is (V1+V2+V3)/3, and the output voltage is equal to (1+R/R1)(V1+V2+V3)/3.
 
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