Type of capacitors is used for decoupling

 In this article we are going to discuss about what type of capacitors is used for decoupling and how do you place a decoupling capacitor?

Types of capacitors is used for decoupling: -

Electrolytic capacitors

Larger electrolytic capacitors (1 to 100 μF) are used to decouple low-frequency noise. These capacitors act as charge reservoirs to fulfill the instantaneous charge requirements of the circuit. Such capacitors should not be placed more than 2 inches away from the IC. Since all electrolytic capacitors are polarized,  they cannot withstand more than 1 volt of reverse bias without damage. They have relatively high leakage currents, which depend upon the design, electrical size, and voltage rating vs. applied voltage. Nonetheless, leakage current does not significantly affect decoupling.

 

Ceramic capacitors 

Low inductance surface-mount ceramic capacitors (0.01 μF – 0.1 μF) are used to decouple high-frequency power supply noise. These capacitors are connected directly to the power supply pins of the IC.

Ceramic capacitors are compact and have a low loss. They have wide temperature tolerance, low ESR/ESL, stability, reliability, and can withstand wide voltage ranges. X7R, Z5U, and Y5V capacitor types are available in values up to several μF with high dielectric constant and voltage ratings up to 200 V. The X7R-type ceramic capacitor is preferred because it shows less change in capacitance as a function of DC bias voltage compared to Z5U and Y5U.

Additionally, NP0 (COG) ceramic capacitors (0.1 μF or less) are also used because of their lower dielectric constant formulation and low voltage coefficient.

 

Multilayer ceramic (MLCC) surface-mount capacitors

MLCCs are used for bypassing and filtering at 10MHz or more because of their low inductance design.


How do you place a decoupling capacitor?

The placement of the decoupling capacitor is crucial because it reduces the impedance of power supply rails. Ideally, it should maximize the capacitance and minimize the resistance and inductance. Components like ICs depend on their input voltage for being as steady as possible while operating. 

A decoupling capacitor should be placed as close as possible to an IC because it protects these sensitive chips by filtering out any excessive noise. The farther away they are, the less effective they will be.


 

How do you place a decoupling capacitor?

In the figure on the left (as shown above), the connection to both the power pin and the ground is made as short as possible. It is the most effective arrangement. In the figure on the right (as shown above), the PCB trace may cause interference issues by forming a loop. This arrangement is less effective because of the excess inductance and resistance of the PCB trace.

Always connect decoupling capacitors between the power source and load/IC in parallel to one another.

Placing a capacitor in series with input and output signal traces removes low-frequency transients from the input and output signals.

Placing a capacitor in parallel with a resistor reduces high-frequency EMI.

When using vias to reach the power plane, connect the capacitor to the component pin, then to the via to ensure current flows through the plane.

 

decoupling capacitor layout

·s  Decoupling capacitors are also effective for isolating analog and digital signals. It is achieved by connecting a capacitor between AC and digital PCB ground pours.

Make sure that power and ground planes are continuous and adjacent: Placing capacitors close to the power and ground pins of ICs is crucial. It makes circuit paths to ground and power planes as short as possible.

The symmetrical placement of adjacent power and ground planes: Adjacent power and ground planes should be placed symmetrically. It is also advised to minimize the number of layers between the planes and the decoupling capacitors.

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