# Re: [SI-LIST] : A Question About Power Noise.

Larry Smith ([email protected])
Wed, 17 Mar 1999 16:47:06 -0800 (PST)

Yes, it is true that the impedance of the lower valued capacitor is
underneath that of the higher valued capacitor for just a brief span of
frequency (resonance). But it is more than 10x lower in this
particular place. That means that you would have to put 10x the
quantity of higher valued capacitors in parallel to achieve the same
impedance at 1 GHz as you get from one resonating capacitor. This is
certainly useful when trying to sit on an EMI problem frequency. It
can also be used to our advantage when trying to achieve a low impedance
across a broad frequency range with a minimum number of capacitors.

regards,
Larry Smith
Sun Microsystems

> Date: Wed, 17 Mar 1999 16:18:25 -0800
> From: Vinu Arumugham <[email protected]>
> MIME-Version: 1.0
> To: Larry Smith <[email protected]>
> CC: [email protected], [email protected]
> Subject: Re: [SI-LIST] : A Question About Power Noise.
> Content-Transfer-Encoding: 7bit
>
> Larry,
>
> Looking at the impedance/frequency plot of say a 0.01uF and a 0.1 uF capacitor in
> the same package/layout, one finds that the impedance of the 0.01 uF capacitor only
> briefly dips below the inductive region of the 0.1uF line. If the goal is broadband
> low impedance for the power distribution system, then the larger value is a better
> choice. On the other hand, if you are trying to cover a narrow band of frequencies,
> then a carefully chosen smaller value may be better.
>
> Thanks,
> Vinu
>
> Larry Smith wrote:
>
> > Yes, I believe a low value of capacitance is important. A good
> > model for a decoupling capacitor is a series RLC circuit. The
> > circuit resonates (becomes a low impedance) at the frequency
> >
> > f0 = 1/(2pi*sqrt(LC))
> >
> > At that frequency, the capacitive reactance and the inductive reactance
> > cancel each other and you are left with the ESR of the capacitor as the
> > impedance.
> >
> > The total inductance (reference Ray Anderson's note on this thread) of
> > an 805 or 603 capacitor mounted on carefully designed pads can be 1 nH
> > or less. Working backwards in the above formula, you would want a 25
> > pF capacitor to decouple a 1 GHz frequency. Our studies show that an
> > NPO 27 pF capacitor has an ESR of about 0.46 Ohms
> >
> > The inductive reactance jwL = j6.28 Ohms and the capacitive reactance
> > 1/jwC = -j6.28 Ohms at 1 GHz. In the series RLC, those reactances
> > cancel at resonance and we are left with the 0.46 Ohm ESR, far less
> > than the reactances.
> >
> > This resonance has a pretty sharp Q and you have to dial in the decap
> > value pretty carefully if you want to hit an EMI problem frequency. A
> > 25 pF capcitor mounted on a padset that gives a total of 1 nH
> > inductance provides a far smaller decoupling impdeance than a larger
> > valued capacitor.
> >
> > regards,
> > Larry Smith
> > Sun Microsystems
> >
> > > Date: Wed, 17 Mar 1999 09:39:23 -0800
> > > From: Vinu Arumugham <[email protected]>
> > > MIME-Version: 1.0
> > > To: Mark Randol <[email protected]>
> > > CC: "[email protected]" <[email protected]>
> > > Subject: Re: [SI-LIST] : A Question About Power Noise.
> > > Content-Transfer-Encoding: 7bit
> > >
> > > Mark,
> > >
> > > Is a low value of capacitance really important? At 1GHz+, the package
> > inductance dominates the characteristics and so the impedance of a 0.1uF and a
> > 100pF capacitor in an 0603 package will not be very different. As you point out,
> > > for better performance at higher frequencies one would need a lower inductance
> > 0402.
> > >
> > > Thanks,
> > > Vinu
> > >
> > > Mark Randol wrote:
> > >
> > > > SMT 0603 ceramic caps are still pretty good at 1GHz+ as long as you keep
> > > > the value under a couple 100pF. 0402's go up through a couple GHz.
> > > >
> > > > --
> > > > ---------------------------------------------------------------
> > > > Mark Randol, RF Systems Engineer | Motorola SPS, Inc.
> > > > (602)413-8052 Voice | M/S EL379
> > > > (602)413-4150 FAX | 2100 E. Elliot Road
> > > > [email protected] | Tempe, AZ 85284
> > > > ---------------------------------------------------------------
> > > >
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>
>
>

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