Decoupling capacitor

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A Decoupling capacitor is a capacitor used to decouple one part of a electrical network (circuit) from another. That means it makes one part of a circuit unaffected by things going on in another part of the circuit.

One common kind of decoupling is of a powered circuit from signals in the power supply. Sometimes for various reasons a power supply supplies an AC signal superimposed on the DC power line. Such a signal is often undesirable in the powered circuit. A decoupling capacitor can prevent the powered circuit from seeing that signal, thus decoupling it from that aspect of the power supply circuit.

Another kind of decoupling is stopping a piece of a circuit from being affected by switching that happens in another part. Switching in subcircuit A may cause fluctuations in power supply or other electrical lines, but you don't want subcircuit B, which has nothing to do with that switching, to be affected. A decoupling capacitor can decouple subcircuits A and B so that B doesn't see any effects of the switching.

To decouple a subcircuit from AC signals or voltage spikes on a power supply or other line, a bypass capacitor is often used. A bypass capacitor is one whose job is to shunt energy from those signals or transients past the subcircuit to be decoupled, right to the return path. For a power supply line, you would connect a bypass capacitor from the supply voltage line to the power supply return (ground). You connect it between the power supply and the circuit to be decoupled. High frequencies and transients flow through a capacitor, in this case in preference to the harder path through the decoupled circuit, but DC power cannot go through the capacitor, so continues on to the decoupled circuit. The bypass capacitor is, therefore, a low-pass filter.

A similar form of decoupling is sort of the reverse. You have a switching subcircuit and you don't want its switching to mess up the power supply upon which other subcircuits depend. (The same applies to other electrical lines shared by other subcircuits, but we'll limit the explanation to a classic power supply). When you suddenly switch a load into a circuit, the circuit tries to suddenly increase its current draw, but the inductance in the power supply line acts to oppose that increase. It opposes it by lowering the voltage the power line supplies. This is not just the voltage that the load in question sees, but the voltage that every other subcircuit that shares that power supply line sees. This is only temporary -- the inductance ultimately loses the battle and the voltage comes back to normal. But even a temporary reduction in voltage can mess up other subcircuits.

To decouple other subcircuits from the effect of the sudden current demand, you place a decoupling capacitor between the supply voltage line and its reference (ground). You connect it right next to the switched load. While the load is switched out, the capacitor charges up to full power supply voltage and otherwise does nothing. When you switch the load in, and the power supply baulks at supplying the demanded current, the capacitor supplies it. By the time the capacitor runs out of charge, the power supply line inductance has lost its battle to maintain the previous current, so the load can draw full current at normal voltage from the power supply (and the capacitor can recharge too). The actual behavior is of course continuous, such that the voltage dip is reduced but not eliminated; i.e. the decoupling is not perfect.

You need transient load decoupling as described above when you have a large load that gets switched quickly (the larger and faster, the larger decoupling capacitor you need; if it's small and slow enough, parasitic capacitance in the circuit may be plenty).

Logic circuits tend to do sudden switching (an ideal logic circuit would switch from low voltage to high voltage instantaneously, with no middle voltage ever observable). So logic circuit boards often have a decoupling capacitor between power supply and ground right next to each logic IC. These capacitors decouple every IC from every other IC in terms of supply voltage dips.

[edit] Placement

A transient load decoupling capacitor should be placed as close as possible to the device requiring the decoupled signal. The goal is to minimize the amount of line inductance and series resistance between the decoupling capacitor and that device, and the longer the conductor between the capacitor and the device, the more inductance there is. ^[1].

A power supply decoupling bypass capacitor should be placed as close to the voltage/current source as possible. The idea is to maximize the line inductance and series resistance between the capacitor and the supplied devices.

[edit] References

1. ^ Capacitor Design Data, and Decoupling Placement, How-to on Leroy's Engineering Web Site

A transient load decoupling capacitor should be placed as close as possible to the device requiring the decoupled signal. The goal is to minimize the amount of line inductance and series resistance between the decoupling capacitor and that device, and the longer the conductor between the capacitor and the device, the more inductance there is. [1].

A power supply decoupling bypass capacitor should be placed as close to the voltage/current source as possible. The idea is to (maximize) the line inductance and series resistance between the capacitor and the supplied devices.

[edit] External links

• Xilinx application note – an article from Xilinx describing how to use decoupling capacitors in 3.3-V systems.
• Power Supply Noise Reduction – How to design effective supply bypassing and decoupling networks

• Cypress application note – an article from Cypress describing how to use decoupling capacitors.