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?

Thanks in advance.

Link:
http://masteringelectronicsdesign.com/wp-content/uploads/2010/01/3_input-summing-amplifier.png

 

When I increased to values of R5, R2 and R1 (please see link below) to 200kΩ the problem was solved.

hi,
What were the resistor values in the circuit that gave an error?
E

 

hi,
What were the resistor values in the circuit that gave an error?
E

Hi ericgibbs. The resistors were 1kΩ before I replaced them with 200kΩ resistors.

 

Hi ericgibbs. The resistors were 1kΩ before I replaced them with 200kΩ resistors.

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.

 

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.
http://masteringelectronicsdesign.com/wp-content/uploads/2010/01/3_input-summing-amplifier.png

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.

 

Were the two inputs effectively to 0V?