The fundamental difference between AC and DC coupling on an oscilloscope lies in how the instrument handles the DC component of the input signal. DC coupling shows the entire signal, including its DC offset, while AC coupling blocks the DC component, allowing you to focus on the time-varying part of the signal.
Understanding Oscilloscope Coupling
Oscilloscope coupling determines how the input signal is connected to the vertical amplifier of the instrument. The two primary types are DC (Direct Current) coupling and AC (Alternating Current) coupling.
DC Coupling
When an oscilloscope channel is set to DC coupling, the signal from the probe is passed directly to the vertical amplifier. This means that the oscilloscope displays the entire signal, including any DC offset present.
According to the reference, DC coupling allows you to see all signals from 0 Hz up to the max bandwidth of your scope. This makes it suitable for viewing signals where the DC level is important, such as power supply voltages, battery voltages, or signals that swing both positive and negative relative to ground where the exact voltage levels are critical.
AC Coupling
In contrast, when AC coupling is enabled, a capacitor is placed in series with the input signal path before it reaches the vertical amplifier. This capacitor acts as a filter.
As the reference states, AC coupling filters out DC components. When you enable AC coupling on an oscilloscope channel, you're switching in a high-pass filter on the channel's input signal path. This filters out all the DC components. This means the oscilloscope only displays the time-varying (AC) portion of the signal, centering it around zero volts on the screen, regardless of the original DC offset.
Key Differences Summarized
Here's a quick comparison of the two coupling types:
| Feature | DC Coupling | AC Coupling |
|---|---|---|
| DC Component | Passed through; displayed | Filtered out; not displayed |
| AC Component | Passed through; displayed | Passed through; displayed |
| Frequency | All frequencies from 0 Hz up to max bandwidth | Time-varying frequencies (AC) above the filter's cutoff frequency |
| Signal Display | Shows total voltage (AC + DC) | Shows only the varying part (AC), centered around zero |
| Internal Filter | None (direct connection) | High-pass filter (capacitor) |
When to Use Each Coupling Type
Choosing between AC and DC coupling depends on what aspect of the signal you need to analyze.
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Use DC Coupling When:
- You need to see the absolute voltage levels of the signal, including the DC offset.
- You are measuring DC voltages or slowly changing signals (near 0 Hz).
- You need to analyze power supply ripple riding on a DC voltage.
- You are looking at signals that are entirely DC or vary significantly around a non-zero average voltage where the average matters.
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Use AC Coupling When:
- You want to examine a small AC signal that is riding on a large DC offset, and the DC offset is making the AC variations too small to see clearly on the screen. AC coupling removes the large DC offset, allowing you to use a more sensitive vertical scale to magnify the AC component.
- You are measuring audio signals, communications signals, or other signals where only the alternating component is relevant.
- You need to determine the peak-to-peak voltage of the AC portion of a signal, independent of its DC level.
In essence, DC coupling gives you the full picture, while AC coupling helps you zoom in on the changes within the signal by ignoring the steady DC part.
