[SI-LIST] Re: Why is capacitor with high ESR

  • From: steve weir <weirsp@xxxxxxxxxx>
  • To: zhang_kun@xxxxxxxxxx, si-list@xxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Tue, 07 Dec 2004 16:48:51 -0800

Zhangkun, some comments-
At 07:46 AM 12/8/2004 +0800, zhangkun 29902 wrote:
>Dear all
>
>This morning, I spend half one hour to review all the mail about this 
>topic. According to my experience, there is the following views:
>    1.Dency via will not absorb or scatter the EM energy. We have made 
> testing PCB to study this problem. When there is no decoupling caps, the 
> dency via will affect the resonance. While there is decoupling caps on 
> the PCB, the effect of dency via could be ignored. In DesignCon 2004, we 
> have proposed one paper.

Please forgive me for the language issue, do you mean densely populated 
vias when you say "dency via?"  If so, this would again depend on the 
stack-up.  If your test case had Vcc on an outside layer, then ground 
stitch vias without decoupling capacitors would leave an open cavity.

>    2.Via along the edge of PCB will not help to eliminate the far field 
> EMI. The via has no effect on reflecting EM energy unless the power and 
> ground plane is connected by via. The voltage source will be shorted. We 
> have made testing PCB to study this problem.

For any structure that the via leaves open, agreed.  However one could 
construct as Chris suggested with ground on the outside plane layers, in 
which case the via fence does create a screen.  For Vcc on the outside it 
is only as good as the inductance loop through the local decoupling capacitors.

>    3.Steve and Chris have proposed seven cases about the PCB decoupling. 
> We have measured almost the seven cases. The result is that the caps with 
> high ESR help on eliminate power ground noise.

Well, I am not surprised, but am most interested in your test case set-ups 
and data for any  comparative  4/5/6/7, especially 5 and 6.

>    4.I have done some simulation about SI+PI. I do not see any great 
> difference when the signal referenced to power or ground. There is a 
> little difference because of the displacement current. I think power and 
> ground are same but the voltage potential.

The issue is that when you reference to power, you can excite the resonant 
cavity formed by the power and ground layers.


>Best Regards
>
>Zhangkun
>2004.12.8


Steve

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