AC and DC Coupling

Overview

Each input can be selectively AC or DC coupled. DC coupling allows both DC and AC signals through, while AC coupling accepts only AC signals. Figure 4-15a shows a waveform containing both AC and DC. If the oscilloscope is DC coupled, then the waveform is displayed as drawn in Figure 4-15a. If the oscilloscope is AC coupled, then the DC portion of the waveform is blocked and only the AC portion is displayed as shown in Figure 4-15b.
The previous example seems very straightforward, but the issue of AC coupling may show up in other unexpected ways. Consider the pulse waveform in Figure 4-16a, shown as a DC coupled scope would display it. This waveform appears to be a typical AC waveform so one might think that it would be unaffected by coupling. However, when the scope is AC coupled, the display does change. The waveform shifts down by about one-third of its original zero-to-peak value (Figure 4-16b). The original waveform did have some DC present in it (remember, DC is just the average value of the waveform). The AC coupling removed the DC, leaving a waveform whose average value is zero. Notice that the waveform is not centered exactly on zero volts, since its duty cycle is 1/3. AC coupling may also cause voltage "droop" or "sag" in the waveform (Figure 4-16c), due to the loss of low frequencies.


Most oscilloscopes have a convenient means of grounding the input (usually a switch near the connector). This is symptomatic of one of the most confusing things in using an analog scope—where is zero volts on the display? The ground switch allows the user to quickly ground the input and observe the flat trace on the display which is now at zero volts. The line may then be set anywhere on the display that is convenient, using the scope's position controls. Knowing where zero is defined along with the volts per division selection determines the scale on the display.

Many scopes provide a BANDWIDTH LIMIT control which activates a fixed-frequency low-pass filter in the vertical amplifier. This has the effect of limiting the bandwidth of the scope (typically to about 20 MHz). Since bandwidth is such a desirable thing, it may seem odd to limit it intentionally. Figure 4-17a shows an oscilloscope display of a sine wave with a noticeable amount of high-frequency noise riding on it. When the bandwidth limit control is switched on (Figure 4-17b), the high-frequency noise is eliminated, but the original sine wave remains uncorrupted. Of course, this works only when the interfering noise is mostly outside the bandwidth of the filter and the desired signal is inside the filter bandwidth.

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