This is an important subject to understand when working in the signal processing field.
In terms of AC and DC Coupling, what is Coupling?
Quite simply, coupling is the transfer of energy from one medium to another, for example, from a metallic wire to an optical cable. This transfer of energy may be desirable or undesirable.
What is DC coupling?
DC coupling allows both AC (alternating current) and DC (direct current) signals to pass through a connection.
What is AC coupling?
AC coupling allows only AC signals to pass through a connection.
A DC source can be considered to be like a battery, in that there is a constant negative charge from the battery’s negative terminal to the positive terminal. An example of such a source is shown in Figure 1.
An AC source is one that is constantly changing from positive to negative. A sine wave is an example of an AC signal, such as the one shown in Figure 2.
DC coupling effectively does nothing. It does not affect the signal at all; everything that goes in comes out again.
AC coupling will remove the DC component from a signal.
For example, a sine wave with a mean of zero and an amplitude of +/-5 volts, as shown in Figure 2, would be unaffected by either a DC or an AC coupling. But a sine wave with a mean of 5 volts and an amplitude of +/-5 volts would be affected by an AC coupling but not a DC coupling. The example shown in Figure 3, when passed through a DC coupled connection would be unaffected. But if the signal shown in Figure 3 were to be passed through an AC coupled connection, the 5 volt mean content would be removed and the signal would match that shown in Figure 2.
So AC coupling rejects the DC component of the signal. Effectively, this normalizes the signal to a zero mean.
The DC component of a signal can be considered to be 0Hz and any AC component of a signal to be above 0Hz. For example, an AC coupling filter might have a cut off frequency of 1 Hz. Therefore, passing a signal through this AC coupling filter would result in the frequency content of the signal below 1Hz being removed.
James Wren was Sales & Marketing Manager for Prosig Ltd until 2019. James graduated from Portsmouth University in 2001, with a Masters degree in Electronic Engineering. He is a Chartered Engineer and a registered Eur Ing. He has been involved with motorsport from a very early age with a special interest in data acquisition. James is a founder member of the Dalmeny Racing team.