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DC or AC in Oscilloscope.

  1. Nov 14, 2011 #1
    Hello experts!

    I am the student of electronics engineering.
    I have an question related to AC and DC button in Oscilloscope.
    When I am performing practical then I don't know whether I switch oscilloscope output to DC or AC mode. When I ask to teacher they don't give me any satisfaction for my question.

    Let say I am performing a practical of comparators. So should I switch oscilloscope output AC or DC? and why?

    Thanks in advance.
  2. jcsd
  3. Nov 14, 2011 #2
    That sets how the signal is coupled to the rest of the oscilloscope.

    DC coupling will let all of the signal through, and display the exact value of the voltage on the screen.

    AC coupling sends the signal through a high pas filter first to remove any DC component.

    One application for AC coupling is if you have a small AC signal riding on top of a large DC offset. On DC coupling, it won't be easy to display since turning the V/DIV knob will send the signal off the screen. AC coupling removed the DC offet and centers the AC waveform around 0 so it's easier to see.

    DC coupling would have to be used for slow changing signal.

    Does that help.
  4. Nov 14, 2011 #3
    Thanks for answer.

    But my question is still there. First I want to know in which mode we set oscilloscope during performing the practical of comparators. AC or DC. And why? How did you know that set an oscilloscope either in AC or DC for that particular practical? What is the basic key to this?
  5. Nov 14, 2011 #4
    What type of signals would you say a Comparator produces?
    What is it that you are expecting to see on the scope (AC or DC signals)?

    Based on your answers to those questions, and everything J-N said, which type of setting do you think is appropriate?
  6. Nov 14, 2011 #5

    We get an AC signal with high gain that is a saturated output on oscilloscope. And If it according to our setting then why my teacher said to switch it to DC rather than AC?
    While in AC mode I was receiving a shifted level. But I know not both of them are correct results but just one is correct AC or DC. Why it is DC so?
  7. Nov 14, 2011 #6


    User Avatar

    Staff: Mentor

    You will almost always use DC coupling for your oscilloscope measurements. The case that Jiggy-Ninja mentioned is one of the rare times you would switch to AC coupling.

    Just keep your oscilloscope on DC coupling for all of your lab work, and you should be fine.

    Here is a set of oscilloscope tutorials from Tektronix:


    You have to register to download them, but they are free, and very useful.
  8. Nov 14, 2011 #7
    As a specific example, I am using an FSK generator with a really small output; 100mV peak to peak. That's riding on top of a 4.5V DC offset from the chip itself. If I want to analyze that signal, DC coupling is impossible since the offset is 40 times greater than the signal. AC coupling is necessary.
  9. Nov 15, 2011 #8
    So as this is the case. If I use DC mode then what signal should I get? is this shifted? If so then how much?
  10. Nov 15, 2011 #9


    User Avatar

    Staff: Mentor

    If you use DC mode on Jiggy-Ninja's signal, you will see a flat line at 4.5V, with maybe a little bit of fuzz on it (the AC part of the signal). You will not be able to see the character of the AC signal unless you can zoom in and adjust your DC offset to keep the signal centered on the screen, or unless you switch to AC coupling and zoom in.
  11. Nov 15, 2011 #10
    OK. Now I have got my answer. Thanks for all of you friends. God bless you.
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