On 07:14 PM 4/06/2003, John Sheahan said:
On Wed, Jun 04, 2003 at 09:34:18AM +0200, Norbert Hoppe wrote:
> When selecting parallel caps, it is important to remember that as the larger
> value capacitor goes inductive, the smaller value cap is still capacitive.
> At a particular frequency, a LC circuit is developed between the 2 caps.
> An infinite impedance could be generated with no decoupling benefit provided.
> When this occurs, single-capacitor decoupling is all that one can use
> for this application.

actually - if you look at ESL graphs for multilayer SMD caps - you will
see it depends much more on case size than on capacitance.
So the 100n tends to win.
This quote I think may be  older wisdom for thruhole components.

The first resonance (at least) of a cap is series, so looks like a short circuit. By adding a number of different valued caps you can scatter a number of these nice AC shorts around your board and around your frequencies of interest.

Above resonance the reactive impedance starts to rise as the impedance characteristic is now inductive. In many cases this is not an issue, as the effective reactance is still low in the frequencies of interest. In other situations, though, it is a critical issue and hence designers have used, and will continue to use, a variety of values in parallel - very common in RF environments.

However, big small caps, or is that small big caps, you know ... large capacitance in small volume, have pretty cruddy material, X7R if you are lucky or Z5U if capacitance is big. These materials have pretty poor, and frequency dependent, ESR which decrease their value as decouplers. Due largely to the effects of the lossy material, a Kemet, for example, X7R shows a sloppy self resonance and the following series impedance at 100 MHz:

Value   Size  Impedance
103      0603    ~1 ohm
103     0805    ~0.5 ohm
103     1206    ~0.3 ohm
104     0805    ~1 ohm
104     1206    ~1 ohm

So if you spec a 10nF 0603 you have a resistor, not a decoupler, at 100 MHz. According to Kemet, the 0603 only performs better than the other sizes at over the narrow freq range of about 10 to 30 MHz. The lossy dielectric, and the need to use thinner metal in the large capacitances (to keep the pkg the same), is killing the performance. Maybe that is an overstatement - but compare the self resonance curves of a COG/NPO material to that of a X7R, or worse Z5U, you can see the dramatic effect the losses have on the resonance shape. COG/NPO has dissipation factors in the order or 0.1% while the other materials are between about 2.5 to 5% or more. (In fact, the lower Q of the high capaciatnce devices is partially a good thing. Having high Q resonances around a board is a shocker when you find you fail EMC.)

Note also that the 100n 0805 has roughly *twice* the impedance @ 100 MHz than the cap *one tenth* the value in the same pkg! In this case, if you are operating above 30 MHz the 1206 10n wins, followed closely by the 0805 10n. 100n in any pkg and 10n in 0603 have about twice the impedance.

See figures 4, 5 & 6 of:

There is always progress in material science so the small-packaged, larger capacitance devices get better over time.

For modern high speed decoupling - I use lots of 10n devices, a few bulk devices and good (hopefully) layer stackup and split plane arrangement. Specific devices operating at speed will have special treatment. Currently, I side with the get to the plane fast crowd and have my supply via close to the power pads and then decoupling caps strung to these same vias with nice fat tracks. I don't, usually, have a via, track, cap, then component pad arrangement - though my guess is, with the right sort of component selection either arrangement can be done well.

Ian Wilson

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