Differences Between Bypass And Decoupling Capacitors

The terms “bypass capacitor” and “decoupling capacitor” are used interchangeably, though there are definite differences between them.

Let’s first understand the context in which the need for bypassing arises. When powering any active device the prime requirement is that the point of entry of the power supply (“power rail”) be as low an impedance (relative to ground) as possible (preferably zero ohms though this can never be achieved in practice). This requirement ensures the stability of the circuit.

The bypass capacitor (“bypass”) helps us meet this requirement by constraining the unwanted communications a.k.a. the “noise” emanating from the power line to the electronic circuit in question. Any glitch or noise appearing on the power line is immediately bypassed into the chassis ground (“GND”) and thus prevented from entering into the system, hence the name bypass capacitor.

For different devices within an electronic system or for different components within the same integrated circuit (“IC”), the bypass capacitor suppresses inter-system or intra-system noise. This situation arises because of the commonality in the form of a shared power mail.  Needless to say, at all operating frequencies, the impact of noise should be contained.

As far as their physical location in the design is concerned, bypass capacitors are placed close to the power supplies and the power supply pins of the connectors. These caps allow alternating current (“AC”) to pass through and maintain direct current (“DC”) within the active block.

Fig. 1: Basic Implementation of a Bypass Capacitor

As shown in Fig. 1, the simplest form of the bypass capacitor is a cap connected directly to the power source (“VCC”) and to GND. The nature of the connection will allow the AC component of VCC to pass through to GND. The cap acts like a reserve of current. The charged capacitor helps to fill in any ‘dips’ in the voltage VCC by releasing its charge when the voltage drops. The size of the capacitor determines how big of a ‘dip’ it can fill. The larger the capacitor, the larger the sudden drop in voltage that the capacitor can handle. Typical values of capacitor are .1uF capacitor and .01uF.

As to the question of how many bypass capacitors need to be used in a design, the thumb rule is as many as the number of ICs in the design. As mentioned earlier, the bypass cap so it is directly connected to the VCC and GND pins. While using that many bypass capacitors might sound like overkill, in essence, this helps us guarantee design reliability. It has become commonplace for designs to use DIP sockets that have the bypass caps built in when the number of capacitors per square inch reaches a certain threshold.

Decoupling capacitors (“decap”), on the other hand, are used to isolate two stages of a circuit so that these two stages don’t have any DC effect on each other.

In reality, decoupling is a refined version of bypassing. Because of bypassing’s finite limitations in creating the ideal voltage source, the “decoupling”, or isolation of adjacent noise sources is often required. A decoupling capacitor is used to separate the DC voltage and AC voltage and as such is located between the output of one stage and input of the next stage.

Decoupling capacitors tend to be polarized and act mainly act as charge buckets. This helps to maintain the potential near the respective power pins of the components. This, in turn, prevents the potential from dropping down below the supply threshold whenever the component(s) switches at considerable speeds or whenever there is simultaneous switching happening on the board. Ultimately, this brings down the demand for extra power from the power supplies.

A bypass capacitor usually takes the form of a shunt capacitor was placed across the power rail as shown in Fig. 2. The decoupling completes the implied “RC” (LC) part of the network: the series element–as in a low-pass filter.

Fig. 2: Basic Implementation of a Decoupling Capacitor

Decoupling may also be accomplished by using a Voltage Regulator in place of the LC network as shown in Fig. 3.

Fig. 3: Use of Voltage Regulator as a substitute for a Decoupling Capacitor

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