Noise in summing op amp circuit (741)

Discussion in 'General Electronics Chat' started by sotinam, Apr 20, 2016.

  1. sotinam

    Thread Starter New Member

    Apr 20, 2016
    10
    0
    Hi,

    Can anyone please help me to figure out why I have lots of noise in my summing voltage circuit?

    I am using summing amplifier to add two sine and square voltages. I am attaching two screenshots of the output signal as well as a schematic of the circuit.

    Any help and advice is highly appreciated.

    Thanks,
    S
     
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  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,801
    1,105
    Welcome to AAC!
    Building the circuit on a breadboard, using long connecting wires, using unscreened signal wires, operating the circuit in an electrically-noisy place, lack of or inadequate supply decoupling, use of a cheap switch-mode power supply can all contribute to noise pick-up.
     
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  3. #12

    Expert

    Nov 30, 2010
    16,316
    6,818
    Oh my, that is nasty!
    You have the signal floating on the power line frequency and a lot of hash and ringing.
    Of course, you gave us nothing about the resistance used, the power supply voltages, the biasing method.
    Start over and build this nice and tight with good shielding and capacitance right at the chip to help it deliver fast current when it needs to.
     
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  4. crutschow

    Expert

    Mar 14, 2008
    13,024
    3,236
    Post a picture of the actual circuit.
     
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  5. dannyf

    Well-Known Member

    Sep 13, 2015
    1,824
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    Too little information to help you more specifically but looks like the opamp is oscillating.
     
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  6. sotinam

    Thread Starter New Member

    Apr 20, 2016
    10
    0
    Thank you guys for your reply. I appreciate that.

    Actually I made it better by replacing the power supplies with higher quality ones. The output signal is much better.
    I am attaching a schematic of my circuit as well as screenshot of the oscilloscope, and a picture of the circuit.

    Would you please have a look at the attached? As you see the noise is much less than before. Any comments on how to make it better (if it is possible)? Maybe moving every thing on PCB and soldering wires will improve the circuit performance?

    Thanks again,
    S
     
  7. #12

    Expert

    Nov 30, 2010
    16,316
    6,818
    That looks like a faster square wave riding on a slower sine wave. That's what you wanted, isn't it?
     
  8. sotinam

    Thread Starter New Member

    Apr 20, 2016
    10
    0
    Yes
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,024
    3,236
    Add some 0.1μF ceramic caps directly from each op amp power pin to ground.
     
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,392
    1,606
    What does the output look like with both inputs shorted to ground?

    What does the output look like with just V1 shorted?

    What does the output look like with just V2 shorted?

    What does V1 and V2 look like?

    These are things you should be thinking of when sitting at your setup to try. Instead of asking us ask your circuit where comes this noise.

    You can ask us too but your circuit knows all.
     
  11. sotinam

    Thread Starter New Member

    Apr 20, 2016
    10
    0
    Just wanted to thank you guys again. Power supplies were the problem as I said.
    Sorry for silly question, its my first time working with op-amps.
     
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