It’s not intended to remove non-zero, low frequency components. This could be something like a power rail or a signal with a very low frequency drift. Use AC coupling if you have a high frequency signal with a DC offset. Keeping this in mind, there are times when you should and shouldn’t use AC coupling. To learn more about the ins and outs of bandwidth and why this occurs, check out the article “ What is Bandwidth? How Much Do You Need?” A 100 Hz oscilloscope can still see a 150 Hz sine wave, it’ll just be attenuated according to the roll-off characteristic of the oscilloscope. Why is this happening? Because bandwidth isn’t as simple as we sometimes like to think. This filters out all the DC components.įigure 4: The same 1 V sine wave, but in DC coupling shows the proper 1 Vpp When you enable AC coupling on an oscilloscope channel, you’re switching in a high-pass filter on the channel’s input signal path. DC coupling allows you to see all signals from 0 Hz up to the max bandwidth of your scope. In the “coupling menu” you can switch between DC coupling and AC coupling. There’s often a 20 MHz bandwidth filter, probe settings, input impedance settings, and coupling settings. In the channel menu of your oscilloscope, you can turn on/off a couple of different settings. An oscilloscope’s offset range is dictated by the volt/div setting, so getting a usable view of the ripple is often impossible in DC coupling mode.

To see this ripple, you need a sensitive volt/div setting and a huge offset. keep learning, sign up for free oscilloscope courses! What you really care about is the AC component around 12V. When should you use AC coupling? The Reason for AC Coupling on an OscilloscopeĪC coupling is intended for AC signals with a large DC offset.įor example, what if you want to look at ripple on a 12V power rail? You don’t really care about seeing the 12V DC component. I’ve heard this comment before, and it’s dead wrong: “ You should use AC coupling when looking at an AC signal.” Following this false AC coupling adage is an easy way to make some very wrong measurements.

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