[SI-LIST] Power Supply Impedance and Decoupling

  • From: "Landrum, Chris" <chris_landrum@xxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Fri, 12 Mar 2004 08:11:50 -0800

I have been tasked to cost reduce a PCB with special focus in the DC-DC
power supply decoupling/bypass circuits.  In the past the "standard" rules
(which we know by now are all bad) were used to sprinkle decoupling caps
throughout the power planes of this particular PCB.  One cap per power pin,
at least one decade apart, blah, blah, blah.  
I was wondering if someone might be able to suggest an efficient way to
evalutae this.  The problem is that the IC's being used are proprietary, and
the specs have very little information describing power consumption, etc.
This makes working the problem from a purely theoretical viewpoint
impossible.  I do have the benefit of have a board or two to play with.

While, I could go in there and simply remove caps until it breaks...I would
rather take a more reliable and scientific approach.

Here's what I am thinking.  I can measure the impedance of the plane
structures on a PCB without any components, and then on one with only caps
using a Spectrum Analyzer.  This can give me an idea for the Impedance of
the PCB over Freq.  I then need to come up with a strategy for determining
what freq's I am most interested in.  Any suggestions here?

Thanks. 



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