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AR# 43283

Spartan-6 UG393 - Decoupling Capacitor Frequency Range

Description

In the Spartan-6 PCB Design and Pin Planning Guide, there is a section on PCB decoupling capacitors as they relate to the Power Distribution System (PDS).

Table 2-1 shows the required number of decoupling capacitors for each supply per device.

Table 2-2 gives additional specifications such as ESL (Equivalent Series Inductance) and ESR (Equivalent Series Resistance).

The table does not explicitly tell you the frequency range that these specifications apply to.

Solution

The larger 100uF is the bulk capacitor. Its purpose is to decouple noise with a frequency above that at which the output capacitor of the voltage regulator stops working, and to decouple noise below where the high frequency capacitors begin to work.

In the document, the frequency range for the 100uF capacitor is given as approximately 100kHz up to around 2MHz.

The document also says that the impedance network should work up to 500MHz.

This gives you the operating range for the high frequency capacitors.

The 4.7uF and 0.47uF capacitor will need to operate in the range 2MHz up to 500MHz.

To summarize:

  • 100uF Bulk Decoupling Cap: 100kHz <= Fbulk <= 2MHz
  • 4.7uF High Freq Decoupling Cap: 2MHz <= Fhigh <= 500MHz
  • 0.47uF High Freq Decoupling Cap: 2MHz <= Fhigh <= 500MHz
AR# 43283
Date Created 12/12/2012
Last Updated 12/12/2012
Status Active
Type Documentation Changes
Devices
  • Spartan-6