# Summing amplifier

Discussion in 'General Electronics Chat' started by Dritech, Mar 29, 2014.

1. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
Hi all,

I want to sum three different analog voltages using a summing amplifier. When I designed the summing amplifier on a simulator and set two from the three inputs as 0V, I was getting a wrong output voltage. When I increased to values of R5, R2 and R1 (please see link below) to 200kΩ the problem was solved.
Can someone please explain to me how to determine the right values for these resistors ? and what exactly are they used for if no ratio is required?

2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,581
389
hi,
What were the resistor values in the circuit that gave an error?
E

3. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
Hi ericgibbs. The resistors were 1kΩ before I replaced them with 200kΩ resistors.

4. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,581
389

For the non inverting summer the source impedance's driving the R1,R2,R5 inputs should be identical or at least within 1%.

What are the input sources for R1,2,5.??

EDIT:
I would use 100K for R1,R2,R5 and ensure that the driving source impedance of each source is 1K or less, this will give the best accuracy.

Last edited: Mar 29, 2014
5. ### LvW Active Member

Jun 13, 2013
674
100
What means "wrong"? What did you expect?

Example:
For R1=R2=R5 and R4=2*R3 the output should simply be vout=v1+v2+v3.

6. ### atferrari AAC Fanatic!

Jan 6, 2004
2,663
783
Were the two inputs effectively to 0V?

Last edited: Mar 30, 2014