Op-amp help (adding two waveforms)

Discussion in 'General Electronics Chat' started by Clive80, Feb 25, 2012.

  1. Clive80

    Thread Starter New Member

    Dec 23, 2011
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    Sorry for the mass posting tonight but I’m stuck on a problem and wondered if anyone on here could help.
    I’m trying to add a positive and negative offset to a triangle wave by adding it to a square wave through a 741 op-amp, the problem is the offset is not constant and the triangle waveform slowly drops. See attached jpeg.
    Am I doing something wrong of is there a better way of doing this rather than using a summing op-amp?
    Also please see my other post in the projects forum regarding control of a H-Bridge to achieve an AC PWM or modified sine wave output.
    Thanks
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    It's hard to answer without your circuit schematic.
     
  3. Clive80

    Thread Starter New Member

    Dec 23, 2011
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    Just a basic op-amp supplied by two signal generators via a couple of resistors.

    See attached
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    As you suggest its a pretty basic circuit. Must be something wrong with the simulation setup.
     
  5. Ron H

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    Apr 14, 2005
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    If you are probing with a 10X probe, it looks like the probe compensation is screwed up.

    EDIT: AC coupling could also do it, but I don't think you would see that much droop at 5ms/div.

    EDIT: I thought your scope picture was of an actual circuit. Apparently it is a simulation?

    EDIT: Your schematic shows that both inputs are coming from the triangle outputs of the generators.
     
    Last edited: Feb 25, 2012
  6. Clive80

    Thread Starter New Member

    Dec 23, 2011
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    Thanks for your replies.

    Ron H

    The signal generators are set up correctly, the leads are connected to the positive terminal.
    I can measure both waveforms independantly with the scope before the op-amp.

    The circuit is a simulation in multisim (pspice).

    Any further thoughts world be welcome.

    Thanks
     
  7. Ron H

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    Apr 14, 2005
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    OK, I was interpreting the function generator icon incorrectly.
    Are you overdriving the op amp? Its output is limited to less than ±20V by the power supply rails.
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    The frequency response of a 741 is worse than whale turds at low tide. It is pretty much done by the the time you get to 9 kHz. or so.
     
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