[SI-LIST] Re: Power Integrity Question
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Avtaar Singh <avtaarenator@xxxxxxxxx>
- Date: Thu, 06 Nov 2008 05:07:25 -0800
The simulators are reporting correctly. Your understanding of physics
theory ( actually Calculus ) is incorrect. A convenient way to think
about the impedance anywhere is as a result of the characteristic
impedance and the Q. In the original case, the impedance significantly
away from the original pole approximates the characteristic impedance.
At each of the two peaks, when the impedance is reactive, Q > 1, and the
peak impedance is substantially greater than the characteristic
impedance. Therefore if you've got peaks at all the impedance will
always be above the original curve surrounding each one.
Steve.
Avtaar Singh wrote:
> Dear All:
> I have a very basic Power Integrity question.
>
> Lets say a Power Delivery Network (PDN) has an impedance peak at 100MHz. I
> add a capacitor that dips the impedance exactly at 100 MHz, and creates 2
> smaller peaks at, lets say, 20 MHz and 250 MHz. According to the theories of
> Physics, these peaks should have their impedance value little bit lesser
> than the impedance value of the PDN, at those 2 frequency points, before
> this capacitor was added. But we do not always see that in simulation tools.
> Sometimes we see that the 2 peaks that are formed have higher impedance than
> the impedance that existed at those frequency points before this last
> capacitor was added. This seems weird to me, because even though the ESR of
> the caps should decide how sharp or blunt the Z peaks are, but an added Z in
> parallel should always mean lesser impedance rather than more impedance. Can
> someone please explain this paradox?
>
> Thanks for the kind guidance.
>
> Regards,
> Avtaar Singh
>
>
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