What factors should be considered when choosing decoupling capacitors?

[d.arbitman]

I have seen power rails bypassed with parallel capacitors which are at least an order of magnitude apart in capacitance value. From what I understand, one capacitor filters high frequency noise and the other filters low frequency noise.
1. Which capacitor does what? The low value capacitor filters low frequency noise?
2. What physical property of a capacitor is responsible for the difference in frequency response?

 

[Averagesupernova]

Do the math. Xc = 1/(2 * pi * f * c)
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So, a lower capacitance offers a higher impedance compared to a higher capacitance.

 

[d.arbitman]

Do the math. Xc = 1/(2 * pi * f * c)
-
So, a lower capacitance offers a higher impedance compared to a higher capacitance.

Yes, so why not just put a huge capacitance to create a "perfect short" to ground for noise?

 

[The Electrician]

Have a look at:

http://www.interfacebus.com/Compound_Capacitor_By-Passing.html

 

[Averagesupernova]

Large electrolytic capacitors may hold a very large charge but that is not to say that have a small enough ESL ( equivalent series inductance) or ESR (equivalent series resistance) to provide a low AC impedance that is suitable for bypass.

 

[d.arbitman]

Have a look at:

http://www.interfacebus.com/Compound_Capacitor_By-Passing.html

Thank you. That explains it well.

 

[the_emi_guy]

The interfacebus.com link is inaccurate, the situation is actually way more complicated.

Using different value capacitors does create series resonances at staggered frequencies (as shown in figure at interfacebus.com), but they also introduce nasty parallel resonances (see figure 5 in attached link).

This is a topic that created a great deal of discussion in our industry about a decade ago. If you want to see the real impact of your capacitor choices you will need to use a power integrity tool. These tools include important factors such as plane to plane capacitance, and inductance of traces that feed the capacitors.

I have seen boards that used half dozen different value decoupling capacitors, then hooked them up with 100mil long 10mil traces.