[SI-LIST] Re: Power Integrity Question
- From: istvan Novak <Istvan.Novak@xxxxxxx>
- To: Avtaar Singh <avtaarenator@xxxxxxxxx>
- Date: Thu, 06 Nov 2008 08:06:06 -0500
Avtaar,
Parallel resonance between the different capacitors inductance and
capacitance is what
raises the impedance above the floor you had previously.
I suggest a very simple exercise: you can do it either analytically, or
by using simulators
or simply by measuring it. You can start out with an arbitrary flat
impedance: it is a
resistance R. Add the first capacitor with ESR R1<R, this will drive a
notch in the
impedance profile, but will not add a peak above R. Now add a second
capacitor
with a different series resonance frequency and ESR R2<R. Dependent on the
separation of the two resonance frequencies, you will get a peak with
value >R.
Regards,
Istvan
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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