Coupling Capacitors in Op Amp Circuits
How does a designer isolate DC voltages in op amp circuits?
A coupling capacitor, Cin, is shown in many of the schematics in the Filter Wizard and Single Supply Filter
Expert. This coupling capacitor is used to isolate the Vcc/2 DC potential created by the Virtual Ground circuitry from the rest of
the system. This virtual ground potential would otherwise be shorted to ground through the circuit or device that is acting as a source.
Because the coupling device is a capacitor, it will act as a high pass filter with the input impedance of the op amp stage being designed. This cannot be avoided, but
the coupling capacitor can be made large enough that it will pass all frequencies required by the system with little noticable roll-off. An audio system, for example,
can be designed with a coupling capacitor that allow frequencies above 20 Hz to be passed. The designer is not concerned with lower frequencies that are below the range
of human hearing.
Some circuits, such as high pass filters, may already incorporate a capacitor on the input to the stage as part of the filter response characteristic. No coupling
capacitor is required, because the filter capacitor already provides DC blocking.
All of the single supply circuits shown in the Filter Wizard and Single Supply Expert have the Virtual Ground potential, Vcc/2, present on their output. This
because there is no way to know the requirements of the next stage. Many systems use more than one op amp stage. One op amp might be used for filtering, another used
for gain, and still another for DC offset. The next stage is commonly an A/D converter which requires the Vcc/2 potential anyway. There are many instances where the
next stage can utilize the Vcc/2 potential as its own virtual ground potential. If the next stage involves gain, however, the Vcc/2 potential would be amplified! It is
up to the designer to know that the Vcc/2 potential exists on the output, and supply the necessary capacitor on the output, if it is required.