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sine wave notches

Discussion in 'General Electronics Discussion' started by pccmd, Sep 14, 2009.

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  1. pccmd

    pccmd

    8
    0
    Jul 28, 2009
    I'm prototyping a sine wave generator using AD8620(dual, low noise, high bandwidth(+/-) ) op amp. The 'A' side is a relaxation oscillator and 'B' side is a Sallen-Keys LPF.

    Well, the first .jpg attachment herein is initial circuit(using 10uF caps. where the supply connects to breadboard(+ and - ), the top image is output of first stage and lower is the sine wave o/p. WHAT is producing the square wave's distortion AND the notches in the sine wave(did improve with the power supply filtering mentioned above) ?

    Now, I put a 0.1uF cap. between output of first stage and ground, and low and behold the notches improved as in the second attachment. I understand why the 'square wave' has become triangular, but HOW did the cap. get rid(nearly) of the notches, and WHY is the frequency now so much lower?

    Thank you.
     

    Attached Files:

    Last edited: Sep 14, 2009
  2. amdNRA

    amdNRA

    50
    0
    Sep 3, 2009
    See if you post the schematic...It really is easier to see what's going on. My $0.02: But it seems that what you were doing/have to do is to tame the opamp. See if adding a 75-100ohms resistor on the output of the opamp helps. The notches appear to be produced by the nonlinearity of the switching square/triangular wave. The cap loads down the output. No wonder they say "Art and science of analog circuit design"
    Analog= the wonderful world between 1 and 0. :)
    Measure the power supply ripple on an oscilloscope and also check for spurious oscillations.
    Later, Gil.
     
  3. pccmd

    pccmd

    8
    0
    Jul 28, 2009
    Gil,

    Thanks for response re: notched sine wave using AD8620.

    The schematic is full page along with notes I'd made, and it's too large for attaching herein.

    If you are comfortable with the idea, I'd give you my Email and you can respond....I'll answer back with the file attachment that way. HOWEVER: this forum wont' allow this because I've not yet posted enough.

    CC
     
  4. pccmd

    pccmd

    8
    0
    Jul 28, 2009
    taming op amp

    Gil,

    Oh...where to put that resistor.....at second output(sine wave) or between A output and B input, between the two halves of the 8620?

    Thanks,
    CC
     
  5. amdNRA

    amdNRA

    50
    0
    Sep 3, 2009
    Experiment...it usually goes on the output to deal with capacitive loads. Also,try putting a compensating capacitor accross the feedback resistor of around 50-100pf.
    The Problem
    The cause of the capacitive load stability problem in most amplifiers is the pole formed by the load capacitance and the open-loop output impedance of the amplifier. This output pole increases the phase lag around the loop which reduces the phase margin of the amplifier. If the phase lag is great enough the amplifier will oscillate.
    External networks can be used to improve the amplifier’s stability with a capacitive load.
    For instance, most designers are familiar with the use of a series resistor RS between the load and the
    amplifier output. The optimum value of RS depends on the load capacitance.
    See if this helps, Gil. :)
     
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