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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