
4–82
Altera Corporation
Stratix GX Device Handbook, Volume 3
February 2005
Miscellaneous
Figure 4–80. Smith Chart
AC Versus DC Coupling
AC coupling refers to the use of a series capacitor on a signal to block the
DC signals from going through. DC coupling refers to the case where this
capacitor is not present and the signal passes through without any
interruption. In AC coupling, a DC restore circuit is generally required
after the capacitor to ensure that the common mode voltage requirements
of the receiver are met. Sometimes, as in the Stratix GX transceiver inputs,
the DC restore circuitry is built into the device. In that case, external DC
restore circuitry is not necessary. DC coupling works only in cases where
the output common mode voltage of the transmitter is in the required
range of the input common mode voltage of the receiver.