Parallel Capacitor Frequency Response
Capacitor Data
Paralleling 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.
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.
IC's benefit from using parallel capacitors [compound by-passing] because the the smaller capacitor is able to react very quickly to variations in voltage because of its low ESR and lead inductance. While the larger capacitor is able to hold a larger charge to supply to the IC when required. Of course the two capacitors are also able to operate over a wider frequency range as shown in the graphic above.
Normal interface ICs that drive long traces, heavy loads, or backplanes may also require compound or parallel capacitors.
A bus driver IC would require compound by-passing to handle the two main functions of a by-pass capacitor.
First the smaller ceramic capacitor would suppress fast transients on the Vcc line [Voltage variations].
The ceramic capacitor would also be used to prevent Voltage Droop between nearby IC signaling [ICs on the same board].
While the larger tantalum capacitor would be used to supply larger amounts of current as the IC switched signals over the backplane [or cable].
Because the backplane represents a much heaver load the IC would tend to switch slower, which would tend to be a benefit because the larger tantalum capacitor would also be reacting slower.
Board-to-Board communications always require a larger capacitor.
Design Hint; Remember to add in the trace and connector capacitance when calculating By-pass capacitors when driving off-board.
So, in this example the Parallel Capacitors solve two design issues.
The capacitor pair both reacts quickly to small transient voltage variations on the voltage plane [by-pass capacitor] and accommodates large current requirements from the Integrated Circuit [IC], [Decoupling Capacitor].
Design Hint; note the placement of the two capacitors to IC 'B' and the location of the via.
Never place a trace via between an IC and its By-pass capacitor.
A via represents a low-pass filter and would inhibit the reaction time of the ceramic capacitor [defeating its purpose].
Back to the main Capacitor Design page, or the Capacitor manufacturer page, or Glossary of Capacitor Terms.
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Design Note; the terms By-pass capacitor and Decoupling capacitor are used interchangeably and refer to the same action. To decouple the IC from the power supply, having the decoupling capacitor providing any required short term current demands, which would otherwise cause a voltage reduction on the Vcc line. Or to by-pass any voltage variations on the Vcc line away from the IC, so that the line remains at its steady state condition
Design Recommendation; When the design will only result in a deliverable quantity of a few dozen boards, add the larger parallel capacitor. Adding the tantalum only results in a few extra cents to the design cost. Better yet, if the tantalum is not required it does not have to be installed and the ceramic capacitor still ends up next to the IC. At the same time the pads are still on the board to accept the tantalum in the event it is needed.
However if the deliverable quantity is in the thousands than more analysis should be done to determine if the additional capacitor is required. A proto-type board would answer any questions regarding the IC's needs for bypassing.
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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