[SI-LIST] Re: Decoupling capacitors
- From: Aleksandr Oysgelt <aleks@xxxxxxxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 16 May 2005 09:05:02 -0700
I'd also like to comment on this...
I've seen people use caps of smaller values because they have "higher
self-resonant frequency." While this statement is correct - what you are
really trying to achieve with decoupling capacitors is to lower the
impedance of your power system at frequency of interest. In the same
package (say 0603) at frequencies we are dealing with (say 10MHz+) a cap
with larger value will have lower impedance than a cap with smaller value.
Draw impedance graphs for two caps and you'll see...
Bottom line - pick largest cost-effective value in a package that you use.
Aleks
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
Sent: Monday, May 16, 2005 8:53 AM
To: Joe Paul M; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Decoupling capacitors
Joe "capacitors by the decade" can be used properly to theoretically
slightly reduce the number of capacitors or increase the ESR of the
network. In many common cases, it is just a rote practice that provides no
actual value. Life with SMT devices is much different today than when we
had leaded devices and those practices were first adopted.
Steve.
At 08:42 PM 5/16/2005 +0530, Joe Paul M wrote:
>Thanks Steve,
>
>Your explanations were really good and I did understand your
>explanation.
>
>But I have seen people using, 0.01 uF and 0.001 uF, even when 0.1 uF
>caps for the required voltage rating are available in same package.
>
>Is there reasoning behind the above approach?
>(I have not yet checked the ESL and ESR of these in their datasheets)
>But I understand that typically ESL remains same for same package &
>Dielectric, and ESR reduces with increase in Cap values( as per SICap3
>tool of AVX)
>
>Thanks
>Joe Paul
>
>
>-----Original Message-----
>From: steve weir [mailto:weirsi@xxxxxxxxxx]
>Sent: Monday, May 16, 2005 7:10 PM
>To: Joe Paul M; si-list@xxxxxxxxxxxxx
>Subject: Re: [SI-LIST] Decoupling capacitors
>
>Joe Paul, the ESL may be the same but the ESR of the 1uF in the same
>chemistry and voltage will definitely be about 35-40% that of the
>0.1uF. It is very unlikely that 0.1uF will provide any cost or
>performance
>advantage over 1uF in the same 0603 case from the same mfg at a low
>voltage
>rating. Due to cover layer considerations, depending on the voltage and
>
>chemistry, the 1uF may actually exhibit lower mounted inductance than
>the 0.1uF
>
>At 2ns/ 160MHz, both capacitors are fully inductive and cover layer
>issues
>aside will have very similar performance. The 1.0uF capacitor has the
>benefit of more capacitance which generally makes it easier to stabilize
>
>the transition from the bulk capacitors / VRM. Fans of the big "V" like
>
>Dr. Johnson, Istvan Novak, and myself will usually advise that at the
>same:
>cost, package size and chemistry, take the bigger capacitor. If you
>want
>to find out why some people do things differently, take a look at Larry
>Smith and company's papers on multipole capacitor networks. If nothing
>else, those papers should help you better understand what you are doing
>whether or not you elect to follow the methods they describe.
>
>If you really want to see how your capacitors perform and have access to
>a
>VNA, I suggest building a test board. You can get details on such a
>board
>from Istvan's papers on his web site, or from mine on the X2Y web
>site. You can put together a decent set of CPW test fixtures for under
>$200. cash and some time. Your biggest expense will be a pair of SMA
>connectors per fixture.
>
>If you want more information on bypass network design, Istvan's web
>site,
>the Teraspeed web site, and the X2Y web site all have papers on the
>subject.
>
>
>Steve.
>At 04:58 PM 5/16/2005 +0530, Joe Paul M wrote:
>
> >I have a doubt regarding decoupling capacitors.
> >
> >I have the option for using 1 uf9AVX 06036D105KAT2A) or 0.1uF (AVX
> >0603ZC104KAT2A) at same cost.
> >
> >Concerned rise time is about 2nS.
> >
> >Is there any issue in using 1uF caps, if it has same ESR and ESL and
> >package (0603) as 0.1uF.
> >
> >Thanks all
> >Joe Paul
> >
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