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Capacitive coupling
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In electronics, capacitive coupling is the transfer of energy within an electrical network by means of the capacitance between circuit nodes. This coupling can be an intentional or accidental effect. Capacitive coupling is typically achieved by placing a capacitor in series with the signal to be coupled.
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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 isolate the DC bias settings of of the two coupled circuits. Capacitive coupling is also known as AC coupling and the capacitor used for the purpose is known as a coupling or DC blocking capacitor. Capacitative coupling has the disadvantage of degrading the low frequency performance of a system containing capacitively coupled units. 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 have high capacitance ratings. They should be high enough 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 minimized in directly coupled designs.
A "gimmick" is a very simple kind of capacitive coupling: a piece of wire that is placed in proximity to another one, providing a capacitive coupling between two nodes of a few picofarads in value. Sometimes the wires are twisted together for physical stability.[1][2]
Capacitive coupling is often unintended, such as the capacitance between two wires or PCB traces that are next to each other. Often one signal can capacitively couple with another and cause what appears to be noise. To reduce coupling, wires or traces are often separated as much as possible, or ground lines or ground planes are run in between signals that might affect each other. Breadboards are particularly prone to these issues due to the long pieces of metal that line every row creating a several-picofarad capacitor between lines. To prototype high-frequency (10s of MHz) or high-gain analog circuits, often the circuits are built over a ground plane so that the signals couple to ground more than to each other. If a high-gain amplifier's output capacitively couples to its input it often becomes an oscillator.
- ^ Bernard Grob and Milton Sol Kiver (1960). Applications of Electronics. McGraw–Hill, 300–301.
- ^ Forrest M. Mims (2000). The Forrest Mims Circuit Scrapbook. Newnes, 95–96. ISBN 1878707485.
- 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
