Capacitor
Max Value Table

Footprint Package Impedance Table

Capacitor
Types & Attributes
Table

General
Capacitor
Notes

References

Bypass (Decoupling) Capacitor Design and Selection

Model for a Non-Ideal Capacitor

An Ideal capacitor behaves only as a capicitor at all frequencies. A real capacitor has the following attributes that tend to limit effectiveness, especially at higher frequencies:
  • Equivalent Series Inductance (ESL)
  • Equivalent Series Resistance (ESR)
These can be represented as shown:

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Frequency Vs Impedance for Bypass (Decoupling) Capacitors

In an Ideal Capacitor, the impedance would continuously drop as the frequency goes up. As a result of ESL and ESR however, the impedance of a real capacitor rises with frequency after reaching a minumum:


The image below shows the Frequency characteristics of the Murata GRM15 Series of capacitor: (1)(2)



Based on the previous data, here is a table showing the approximate maximum frequency for bypass (decoupling) capacitor values:

Capacitor Value Frequency Max Table

Capacitor

0.1 uF

0.01 uF

0.001 uF

~Max Freq.

25 MHz

75 MHz

115 MHz

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ESL & ESR For Different Package Footprint Sizes


Other factors being equal, ESL and ESR tend to be equivalent for different capacitor values given the same footprint package size:


The ESL and ESR is reduced in smaller footprint package sizes given the same capacitance value:


Capacitor Footprint Size Impedance Table (3)

Package

ESL (pH)

0201

400

0402

550

0603

700

0805

800

1206

1250

0612

63

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Effective decoupling coverage can obtained by using smaller footprint packages for the smaller capacitor values to filter out the higher frequencies. The red line represents the approximate parallel capacitance impedance for the 1.0uF, 0.1uF, and 0.01uF capacitors in parallel:


Capacitor Types & General Properties

Capacitor Types & Attributes Table (3)

Type

Cap Range

ESR

Voltage Rating

Notes

Ceramic

uF to pF

Low

High

Multipurpose,
Inexpensive

Mica

pF to nF

Low
0.01 to 0.1
Ohms

High

Good for RF Filters, Expensive, Stable

Plastic Film
(polyethylene
polystyrene)

uF's

Medium

High

Inexpensive, Low Freq.

Tantalum

uF's

High
0.5 to 5.0
Ohms

Lowest

Expensive, Non-Linear, Not good for Audio

Aluminum
Electrolytic

High uF's

High
0.05 to 2.0
Ohms

Low

Inexpensive, Low Freq.

OSCON

uF's

Low
0.01 to 0.5
Ohms

Low

Expensive, Very High Quality

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General Capacitor Notes

  • Long & thin PCB traces increase ESL & ESR. Bypass Caps should be placed as close as possible to the VDD pins of componants as possible, with as thick as possible traces.
  • Smaller footprint package sizes result in lower ESL. The higher frequency bypass caps are best selected in smaller packages.
  • Generally speaking, putting capacitors of the same value in parallel reduces the ESL and ESR of the combined set. The overall capacitance value will be increased, i.e. capacitors in parallel combine in the same way that resistors combine in series.
  • Tantalum and Aluminum Electrolytic capacitors should have a voltage rating at least twice the voltage used in the circuit application. They are also polorized--they must be installed in the proper orientation during assembly. Tantalums are especially sensitive to this.



References

(1) Murata GRM Series Data Sheet

(2) Murata Application Manual C39E

(3) Intersil Application Note 1325

(4) Xilinx Application Note Xapp623

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