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Oscillisope AC/DC button question

  1. Jan 30, 2008 #1
    [SOLVED] oscillisope AC/DC button question


    with regards to the AC/DC button, what does it do... i mean i can google it and get stuff like:

    "a capacitor is placed in the signal path. the capacitor blocks DC signals but allows AC signals to pass."

    so ya, if i remember right, a capacitor is a open circuit to DC... but visually... how does this affect the display? Also why use this mode?

    in other sites i get this:
    "AC – DC switch
    The switch should normally be set to DC, even when the oscilloscope is used for AC. In the AC position there is a capacitor in series with the input and this will separate the AC component from a wave-form such as the one in the sketch. The AC position of the switch should be used only for this purpose. "

    ... once again i'm lost for imagining what the display looks like in AC mode.

    the sketch their referring to: http://www.practicalphysics.org/imageLibrary/jpeg200/2238.jpg

    that website: http://www.practicalphysics.org/go/Apparatus_188.html
    Last edited: Jan 30, 2008
  2. jcsd
  3. Jan 30, 2008 #2


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    Imagine feeding the oscilloscope a signal -- a sine wave that varies between 2V and 4V. This signal could be said to be composed to two parts summed together: a 1V-amplitude sine wave, centered around 0V, added to a 3V dc offset voltage.

    If you put the 'scope in dc mode, you'll actually see the signal varying between 2V and 4V on the screen, relative to ground.

    If you put the 'scope in ac mode, you'll see it varying from -1V to 1V instead. It'll look the same, but it'll appear at a different vertical position. The capacitor is blocking the dc component, so the signal appears to have zero offset.

    The choice between ac and dc modes is usually one of preference. If you have a bunch of signals with different offsets, it might be convenient to use ac mode -- you won't have to keep messing with the vertical shift knob, but you will not be able to visually see the offsets. If the offsets are important to you, then you should use dc mode exclusively.

    - Warren
  4. Jan 30, 2008 #3


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    Just one more point to add to chroot's explanation. On many oscilloscopes (especially the digital scopes we tend to use in lab work), you get better control of the trigger when in DC mode. For many signals, you want to adjust the trigger for some particular thing in the waveform that interests you, and in DC trigger mode, you can see the vertical trip point marker moving up and down on the waveform as you adjust the DC trigger level.
  5. Jan 30, 2008 #4
    Last edited: Jan 30, 2008
  6. Jan 30, 2008 #5


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    Yep, you got it.

    - Warren
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