Capacitive coupling
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In electronics, capacitive coupling is the transfer of energy from one circuit to another by means of the mutual capacitance between the circuits. This can be an intentional or accidental effect. It only works well enough to be of practical use over relatively short ranges, such as some RFID applications. Capacitive coupling is typically achieved by placing a capacitor in series with the signal to be coupled.
"Gimmick" is the name of a very simple kind of capacitive coupling. It is a piece of wire that is placed in close proximity to another one, providing a capacitive coupling between the two circuits of a few picofarads in value. Sometimes the wires are twisted together for physical stability.
Use in Digital Circuits
In digital design, capacitive coupling is typically referred to as AC-Coupling, which allows a signal to maintain different voltage levels at different ICs. For example, to route a signal from a +3.3v to a +5v device, one can place a capacitor inline with the signal and place a pull-up resistor on the +5v side of the capacitor. The inline capacitor acts as a high-pass filter and blocks the DC offset. Placing weak pull-down resistors on each side can also help increase signal fall-time, although this increases signal distortion..