Re: [SI-LIST] : Capacitor dielectric type

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From: Larry Smith (ldsmith@lisboa.eng.sun.com)
Date: Fri Nov 17 2000 - 07:15:45 PST


TK - For some values, such as your 470pF, you have a choice to use X7R
or NPO. I always use NPO which has a much lower ESR. You can get a
much lower impedance vs frequency by using lower ESR parts.

But, before you can use the low ESR caps, you must have a low "mounted
inductance". That has a lot to do with your pad layout, via design and
PCB stackup. The total mounted inductance should be under 1 nH. The Q
of the mounted capacitor is omega*L/R. Omega is the resonant frequency
of the capacitor, 1/sqrt(LC). If you are going to use a low ESR part,
you have to have a low inductance mount to keep the Q down. I find that
Q's less than 5 can be managed well. Another definition of Q is the
reactive impedance divided by the resistive impedance = X/R =
sqrt(L/C)/R at resonance. This essentially establishes the "best" ESR
for each capacitance value. It depends greatly on how aggressive you
are in lowering the inductance of your mount.

You must also use many values of C in parallel to achieve a flat
impedance over a broad frequency range. Each capacitor value holds the
impedance down in a certain frequency range. If you don't use enough
values, you will get high impedance "anti-resonances" at some
frequency, which can cause problems for your product.

This is the most efficient way to use capacitors to decouple low
impedance power systems. But high Q circuits can be dangerous. Don't
use low ESR parts unless you convince yourself that you haven't left
any high impedance resonances laying around.

regards,
Larry Smith
Sun Microsystems

> Date: Thu, 16 Nov 2000 15:47:28 -0800 (PST)
> From: Tae-Kwang Jeon <tkjeon@vps6.lsil.com>
> Subject: [SI-LIST] : Capacitor dielectric type
> To: si-list@silab.eng.sun.com
> MIME-Version: 1.0
> Content-MD5: aTdedr8fnd0PynvUNsn4Ng==
>
> Hello,
>
> I'm trying to put multiple decoupling capacitos on my board mixing small and
> large values in order to minimize the ESL and ESR.
> What I'm wondering is when I use different types of dielectic for the
> capacitors. For instance, I'm going to use 0.022uF X7R type and 470pF of
> NPO type because NPO has the lowest ESR.
>
> Does anybody know whether or not it's not a big deal in terms of mixed
> dielectric material for high frequency decoupling?
>
> Thanks,
> TK
>
>
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