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 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 analogue circuits
In analogue circuits a coupling capacitor is used to connect two circuits such that only the AC signal from the first circuit can pass through to the next while DC is blocked. This technique helps to avoid altering the DC bias settings of each unit when they are interconnected. Thus capacitive coupling is also known as AC coupling and the capacitor used for the purpose is known as a coupling or DC blocking capacitor. This method of coupling has the disadvantage of degrading the low frequency performance of a system containing capacitively coupled units, since each coupling capacitor along with the input impedance of the next stage forms a high-pass filter and each successive filter results in a cumulative filter with a -3dB frequency that may be higher than each individual filter. So for adequate low frequency response the capacitors used must be high enough, such that the reactance of each is at least ten times the input impedance of each stage, at the lowest frequency of interest. This disadvantage of capacitively coupling is largely minimised in directly coupled designs.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 integrated circuits. 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..References
See also
Direct couplingExternal links
- [Protecting Signals from EMI: Introduction and Capacitive Coupling]
- [Capacitance and Uses of Capacitors]
- [When to use AC coupling]
- [DC Blocking Capacitor Value]
- [AC-Coupling Between Differential LVPECL, LVDS, HSTL, and CML]
- [DC-Coupling Between Differential LVPECL, LVDS, HSTL, and CML]
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