AC Coupling vs. DC Coupling on an Oscilloscope

An oscilloscope's AC and DC input coupling lets you work with different signal types.

If you're learning electronics, knowing how to use an oscilloscope will help you in building and repairing circuits. The oscilloscope's AC and DC coupling options let you examine different kinds of electrical signals conveniently.

Description

Every oscilloscope has at least one input channel. You can set it to read direct current (DC) or alternating current (AC) signals by pushing a switch marked "coupling."

DC Signals

With DC coupling, the oscilloscope acts as a voltmeter. The screen will show a flat trace moved up or down by the DC voltage you're measuring. By touching the scope probe to different circuit points, you can get rough voltage readings.

DC offset

A signal with AC and DC combined, where a waveform "floats" on top of a constant voltage, has what's called DC offset. With DC coupling, the offset will push the waveform offscreen. AC coupling removes this offset.

AC Signals

You can use either coupling type to see AC signals. By pressing the switch to go from AC to DC and back again, you can detect small offsets. A signal with offset will move vertically on the screen.

Low Frequencies

Because of the vague distinction between DC and low-frequency AC, AC coupling may not be useful for frequencies below a few cycles per second. Use DC coupling instead.

References

• National Instruments: AC and DC coupling
• Hobby Projects: Oscilloscope Vertical Controls