[SI-LIST] Re: Capacitors at high frequency
- From: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
- To: "'Alvaro alkschbirs'" <alvaro.alkschbirs@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Sat, 23 Oct 2010 12:29:47 -0700
Alvaro
Though the same capacitor may used in both locations the important
characteristics for each application is different. In a power supply
application the end result is a very low impedance power distribution
system. Low impedance could be 1 mOhm. To get the impedance that low
requires the impedance of the capacitors used in the PDS to be very low so
any amount of inductance quickly raises the PDS impedance. For theca
coupling application the impedance of the capacitor at a given frequency can
be much higher. These capacitors work in a 50 Ohm environment so Ohms of
impedance for the cap is not so significant. Thus more inductive reactance
can be tolerated.
Tom Dagostino
Teraspeed Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@xxxxxxxxxxxxx
www.teraspeed.com
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827
www.teraspeed.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Alvaro alkschbirs
Sent: Saturday, October 23, 2010 7:33 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Capacitors at high frequency
Hi all,
I have a question about power (de)coupling and ac coupling capacitors;
Situation 1:
Thinking about capacitors at high frequency, According to what I've read,
the power supply (de)coupling discrete capacitors works until some hundred
of MHz. Above this, the inductance goes dominant and the power charge
capacity goes down.
Situation 2:
Now, thinking about this same discrete capacitor working as AC coupling on
1Gb/s or 3,125Gbp/s signals. I have done this in my projects (using 0402
capacitors) and no error have been reported at this signals.
Q1. What do you think about it ?
Q2. Why the dominant inductance is a problem in situation 1, and not for 2 ?
Thank you very much,
Álvaro Alkschbirs
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