Capacitor Data

Compound Capacitor By-Passing

Capacitor Bypassing using multiple capacitors

An Integrated Circuit [IC] is normally decoupled using one bypass capacitor from 0.01uf to 0.1uf.
Using the graph below, the 0.1uf response would result in the first dip at f1 followed by the dotted single line.
Placing a 0.01uf capacitor in parallel with the first cap also provides the second dip at f2 increasing the frequency response of the pair of caps.

Frequency Response for two combined capacitors

The graphic above shows the benefit of placing two different value capacitors in parallel. The impedance is reduced across a band of frequencies, adding the two nulls.
The first null is developed from the larger value capacitor, and the second null from the smaller value capacitor.
Placing a large value Tantalum capacitor next to a smaller value ceramic will cover a wide range of frequencies.
Placing two ceramic capacitors, a decade apart in value, in parallel will have the same effect but over a different frequency range.

Many Application notes require two or more capacitors in parallel each a decade apart in value to produce the desired effect. App notes for larger chips, FPGA's for example, require compound by-passing.
Back to the main Capacitor Design page, or the Capacitor manufacturer page, or Glossary of Capacitor Terms.

Engineer Design Key words: Capacitor, Military, MIL, Parallel Capacitor Filter, Spec, Specification, Military, Frequency Response, Engineering Design, Decade, Dielectric Types, Dielectric Material, Preferred


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