[SI-LIST] Re: Decoupling capacitors
- From: "Joe Paul M" <joepaul@xxxxxxxxxxx>
- To: "steve weir" <weirsi@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 16 May 2005 20:42:56 +0530
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]=20
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=20
chemistry and voltage will definitely be about 35-40% that of the=20
0.1uF. It is very unlikely that 0.1uF will provide any cost or
performance=20
advantage over 1uF in the same 0603 case from the same mfg at a low
voltage=20
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=20
aside will have very similar performance. The 1.0uF capacitor has the=20
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:=20
cost, package size and chemistry, take the bigger capacitor. If you
want=20
to find out why some people do things differently, take a look at Larry=20
Smith and company's papers on multipole capacitor networks. If nothing=20
else, those papers should help you better understand what you are doing=20
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=20
VNA, I suggest building a test board. You can get details on such a
board=20
from Istvan's papers on his web site, or from mine on the X2Y web=20
site. You can put together a decent set of CPW test fixtures for under=20
$200. cash and some time. Your biggest expense will be a pair of SMA=20
connectors per fixture.
If you want more information on bypass network design, Istvan's web
site,=20
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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