RE: [SI-LIST] : Capacitor dielectric type

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From: Bob Weber (rweber@txc.com)
Date: Tue Nov 21 2000 - 06:50:09 PST


RE: [SI-LIST] : Capacitor dielectric typeThe bypass cap ESR improves damping
(smoothing the impedance peaks & valleys) in the 50 to 250MHz range. The
degree of improvement varies depending on capacitance values (bypass and
plane capacitance).

I select the caps (voltage, dielectric, and package size) to make sure I get
enough ESR to be effective in damping. Obviously, small packages are good
(small ESL). I avoid capacitors that result in a "thick stack" (i.e. a many
layered MLC cap). The extra layers give more capacitance, reduce the L (a
small effect), and drive down the ESR. Cap sizes that need fewer layers will
in general have the most ESR.

Using the AVX SpiCap software, http://www.avxcorp.com/SpiApps/spicap/ ,
compare the ESL and ESR of a X7R cap at 1, 10, 47, and 100nF (100nF will
have to be a 16V cap). As the capacitance goes down, the ESR goes up. This
makes sense when you view the cap as a stack of smaller value layers. The
total capacitance is increased, but the ESR is reduced. A 100nF X7R cap is a
block, while the 1nF is a sliver. Z5U can be selected to give more ESR in a
100nF cap (fewer layers and the dielectric is more lossy).

Bob W.
  -----Original Message-----
  From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Larry Miller
  Sent: Monday, November 20, 2000 11:57 AM
  To: Bob Weber; Tae-Kwang Jeon; si-list
  Subject: RE: [SI-LIST] : Capacitor dielectric type

  Interesting point. R-C "snubber" networks have long been used to each
transient ringups in power transistor and SCR circuits at lower frequencies.

  What frequency range would you be considering this for?

  Larry Miller

    -----Original Message-----
    From: Bob Weber [SMTP:rweber@txc.com]
    Sent: Monday, November 20, 2000 6:11 AM
    To: Tae-Kwang Jeon; si-list
    Subject: RE: [SI-LIST] : Capacitor dielectric type

    TK,

    This will likely cause some discussion, but I avoid using NP0 or C0G
    capacitors for bypassing because of their low ESR. Small value (1nF) X7R
    capacitors have noticeable ESR (0.5ohm), but this is a good thing. It
    provides a damping element to absorb the ringing that develops on the
    power-ground planes due to parasitic resonances between the bypass
    capacitors and the unavoidable inductance of the connections.

    Bob W.

    -----Original Message-----
    From: owner-si-list@silab.eng.sun.com
    [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Tae-Kwang Jeon
    Sent: Thursday, November 16, 2000 6:47 PM
    To: si-list@silab.eng.sun.com
    Subject: [SI-LIST] : Capacitor dielectric type

    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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