RE: [SI-LIST] : High Permittivity Board Level Decoupling and rela

Andrew Ingraham (Andrew.Ingraham@digital.com)
Sat, 20 Jun 1998 16:41:07 -0400

Re: high permittivity dielectrics:

I have heard about very high dielectrics for making "capacitor layers",
but have no experience with them.

In addition to the very good points already made, I have a
question/comment. Is the material used over the whole area of the
board, even around signal vias? If so, what effect does it have on the
via? Might it turn a via from 0.5pF to 20pF or so? Or does the layer
have cut-outs around each via?

Re: analog decoupling:

The principles are somewhat the same, rise time and switching currents,
but your circuit's susceptibility to noise is different and quite
variable. In digital logic, noise is considered too much when it flips
a bit. In analog, it depends what your signals are, how much noise you
can tolerate, how much is in your desired signal bandwidth, etc. "Rules
of thumb" tend to be things such as, use X decoupling caps, of value Y
uF, per Z op-amps; because someone found that it seems to work well
(probably at avoiding oscillations). An exact analysis is more
difficult.

For low frequency (op-amp) work, an old starting point is an application
note by Paul Brokaw at Analog Devices Inc. titled something like "User's
guide to grounding, decoupling, and making things go right for a
change."

Regards,
Andy Ingraham

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