A particular circuit may have a mix of both DC and AC voltage (explained in AC
Circuits). When such a circuit is coupled to another circuit, the DC voltages
are often different and must be isolated while allowing the AC signal to pass.
If the DC voltage is not blocked, either circuit will cause the DC voltages in
the other circuit to change from their required
levels. When measuring AC voltages in such circuits, the DC voltage may
cause erroneous readings in the AC measuring equipment. The measuring equipment
may also take enough DC current to cause a drop in the DC voltage, altering the
circuit operation. This is called DC loading.
A capacitor is often used to block DC voltage from passing between circuits
or from reaching test equipment when measuring AC voltage in such a circuit.
Such capacitors are called coupling capacitors.