Raveninghorde said:
I read conflicting advice on decoupling.
frequently I see suggestions that ICs etc should be decoupled by, for
example, 1nF, 10nF and 100nF in parallel. Is this just a hang over
from PTH days or is there still some advantage in doing this with
surface mount X7Rs?
Hmm, possible. Back then, a 1nF was physically smaller than 100nF, so the
ESL was lower.
Today, they all fit into the same 0603, even 0402 at low voltages, so as
soon as Xc --> 0, impedance is dominated by ESR and ESL, which are
geometry defined. ESL is by length / width, while ESR is by electrode
resistance (and some from dielectric losses).
Simulating a crude backplane is illustrative. More caps can actually make
things worse, with staggered values or otherwise.
Aluminum polymer caps are generally a bad thing, because their super low
ESR enhances the resonances due to ESL. Standard tantalums are perfect
because they tend to damp things, as Bill said. Special low-ESL tantalums
fall somewhere between alpoly and regular, probably on the low side for
most purposes. (Save the low ESR caps for where it matters: power
supplies with big peaky currents!)
To give you some understanding of the system, the ground plane generally
looks like a short circuit across its width. It has very low inductance
between points, and a small capacitance to ground, which dominates at high
frequencies (over 50MHz or so). The sheer existence of the ground plane
dominates HF performance, and in many cases you might easily survive
without any tiny caps at all!
When you add a bypass cap, it pokes down to the planes with vias. The
via-cap-via loop is on the order of 2-3nH. This is worsened with
connecting traces. Via pairs, placed directly aside the pads, with
minimal trace length, are best. Since Xc --> 0 at high frequencies, any
cap looks like an inductance, which makes the ground plane's capacitance
resonate -- *worsening* things!
When you add a cap with ESR, it acts in parallel with the ground plane C.
This resistance has to be connected with a sufficiently small ESL, so ESL
must be very small on the damping components. Thus, you want to use a
few, scattered about (where doesn't really matter!), and perhaps use extra
vias with them.
You could easily distribute 100nF's with 1 ohm chip R's to accomplish the
same thing, but you might as well take the additional bulk capacitance in
stride.
When low noise is desirable (or required!), of course, you'll want a good
bit of ESL between the noisy PSU and the ground plane. Ferrite beads are
handy little buggers.
Tim