Op Amp Integrator Problems

Discussion in 'General Electronics Chat' started by Management, Sep 25, 2007.

  1. Management

    Thread Starter Active Member

    Sep 18, 2007
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    I am current studying the opertion of a particular circuit board that I have. What is driving me up the wall is the use of two integrators in to different locations on the board. I have tested and print out waveforums of the real circuit behavior so that I can have a reference for comparison in Spice. All waveforms are behaving the way they should except for the outputs of bought integrators.

    What is really troubling me is that one of the integrators does not have a resistor across the feedback capacitor. I though that was needed because it is the real world and you cannot expect to have an ideal integrator. But for some reason this board uses a standard ideal configuration op amp integrator.

    This integrator is suppose to provide a constant DC averaging of the input signal. I have searched the the internet but I am not sure how an op amp integrator is suppose to output the average DC value of the input. I understand the other operations of the integrator; such as waveform generation and AC signal integration (sin -> cos), but I do not understand this average voltage output. All I get is saturation.

    Can someone explain to me how I get this output? Thank you in advance.

    -Andrew
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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  3. Management

    Thread Starter Active Member

    Sep 18, 2007
    306
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    Thank you for the link but I have read that page a few times and about a few more scattered over the web. I think I understand out the integrator works. Plus I just don:t understand why I do not get in spice what I am getting when I use the osclliscope on the board. Everything else is ok but the integrators.

    Additionally I am not applying a constant DC voltage to the input of the integrator. I will post a circuit later when I get a picture of it and a picture of what the input and out waveforms of what I get with the oscillscope and what I get in spice.

    Thank you for replying.
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    An integrator will convert a sine wave input to a cosine wave output, but only if the RC time constant of the integrator is more than about thirty times the period of the input. The closer the RC time constant comes to the period of the input, the lower the phase shift.

    Which does not explain either output saturation nor the notion of integrating an average. This is an intriguing puzzle. I look forward to seeing the schematic.
     
  5. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    I have seen integrators used in SMPS for average current mode control.
     
  6. Analogarsonist

    New Member

    Jan 22, 2010
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    At DC you are operating in open loop (only have feedback with dv/dt) so if the inverting input is higher than the non-inverting input (reference) you saturate to VEE rail and vice versa if your input were below the reference pin. It's basically acting like a comparator.. all you get is high or low. Also, due to non-ideal characteristics such as input offset voltage, even if you think your input is zero, that error will integrate over time and saturate your opamp.
     
  7. Analogarsonist

    New Member

    Jan 22, 2010
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    Also, think about it, averaging a DC signal is area under the curve so multiply amplitude by time.. you quickly saturate! Without feedback it's useless.
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Good input, but a three year old thread.
     
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