Summing amplifier

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

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    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?

    Thanks in advance.

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

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    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
    2,503
    380

    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
    2,648
    764
    Were the two inputs effectively to 0V?
     
    Last edited: Mar 30, 2014
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