# quick op-amp question

Discussion in 'General Electronics Chat' started by veritas, Dec 10, 2008.

1. ### veritas Thread Starter Active Member

Feb 7, 2008
167
0
Is it possible to set up an op-amp circuit as a non-inverting summer?

I know the inverting summing circuit as well as the non-inverting amp....

*edit* after 10 more seconds of searching, I see the schematic is in the AAC book, so nevermind.

May 16, 2005
5,072
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3. ### veritas Thread Starter Active Member

Feb 7, 2008
167
0
I notice that in that section, the description of the equations for an inverting summing amplifier are not very robust. V1, V2, and V3 can each be scaled independently of one another, given by the equation:

Vout = (-Rf/R1)V1 + (-Rf/R2)V2 + (-Rf/R3)V3, where Rf is the feedback resistor.

That handy information is not apparent in the article.

4. ### hgmjr Moderator

Jan 28, 2005
9,030
214
The scaling feature of the inverting opamp summer you have mentioned is a handy thing to recognize.

As shown in the AAC ebook non-inverting summer example, one can keep the math simple if the resistors in series with V1, V2, and V3 are chosen to be equal in value and the gain set equal to the number of input signals.

While it is feasible to implement a non-inverting summer with scaling, the math can become somewhat complex since the three voltages interact with each other through the three resistors that make up the input summing network. The gain is set to 3 in this case to make up for the attenuation that occurs in the input summing network composed of equal R values.

hgmjr