A Low-Impedance Path to Silence

Have you ever turned on the TV and seen the static flashes on the screen? The static noise is like heavy rain on the tin sheet. THAT is what we call noise. Noise refers to unwanted signals and disruptions in actual signals that deviate from the actual output of the signal. This deviation can cause many errors, such as changing the bit from “0“ to “1” in a binary system inside an IC(integrated circuit), or inducing a high-pitched, shrill noise in an audio signal. The origin/causes of these noises can vary from radio signals around us to high current running in electric lines or from the fast switching in the microprocessors. Broadly, these noises are categorized into two types: 1) Internal noise, 2) External Noise (Both of which are self-explanatory).

So we conclude these noises are bad for a circuit. But how do we prevent them from disrupting our signals and messing with our output? That’s where our bypass capacitor comes into the picture. The Bypass capacitor has the ability to filter out all the high-frequency noise that is superimposed on the signal and is disrupting it. It blocks the high-frequency signal and lets our low-frequency signals pass through. But how does a capacitor achieve this function?

This is achieved by what is called “IMPEDENCE“. Now the impedance of a capacitor is its capacitive resistance (The ability of the capacitor to resist the flow of current). Capacitive Resistance is calculated by the formula.

$$Xc = 1 / (2πfC)$$

where, Xc → Capacitive Resistance
f → Frequency of the wave
C → Capacitance

From the formula, we can conclude that as the frequency increases, capacitance decreases. This means for the higher frequency noise, the capacitor provides a low impedance path, or in other words, a short circuit-like path, which is much easier for the wave to take to GND(0 V). But what happens when the frequency becomes 0? Xc becomes infinite, the capacitor works as a short circuit, and does not allow any current to pass through it. This is the exact case with DC current. As DC is a constant current, the frequency remains zero, and thus a bypass capacitor is also used to separate the AC noise from the DC input.

One very interesting thing about bypass capacitors is that they are often confused with decoupling capacitors. Bypass capacitors help reduce the high-frequency noise, whereas decoupling capacitors are used as an energy reservoir, which helps in smoothing the voltage fluctuations. Often, one capacitor is only used, which fulfills both requirements.

Thus, in a world of noise and fluctuations, a bypass capacitor helps us in getting what we really want, what we really seek. Thus, it is important to add a bypass capacitor between power and GND in parallel to ICs and other electronic components.