placing addiional caps across existing caps to reduce noise



To the subject at hand: placing additional caps across existing caps does not reduce the noise (unless the dominant cause is lack of adequate capacitance).

The reason why the noise is bad is that the L (as in Ldi/dt) is most likely the largest, and most dominant factor, in the form of the via and traces to the bypass capacitor.

Many times people have placed additional caps on top of the the existing caps and wondered why the noise is not reduced: well, you did not change the L in the equation, did you. So why did you expect V to change?

You may have moved the resonant frequency (more often not), but often people make the mistake of assuming that a 0.1uF requires a 0.01uF and a 0.001uF in parallel. You can see that if the series L is dominant, you haven't even moved the frequency by more than a few percent by the small amount of additional capacitance.

Unfortunately, once the via and trace L is large, there is no way to make the noise less, withpout making a whole new pcb (re-layout).

More that once we have had to inform a customer that there is "no hope" for their pcb because the series L in their layout is dominant, and there is no way to reduce it.

And, we have then helped them re-layout their pcb and making their system work just fine (as, if you know what you are doing, this is not a hard problem to solve).

Mark Alexander's power distribution application note represents the latest state of our power distribution sysem "knowledge."

As we learn more, we will certainly update the applications note.

Again, my apologies to the group for a new thread, on an old subject.

Austin
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