Re: [SI-LIST] : Power/Gnd Pin Input Impedance

Larry Smith (ldsmith@lisboa.eng.sun.com)
Tue, 12 Jan 1999 13:03:59 -0800

One way to get at the impedance looking into a chip's power supply is
go back to the basic definition of impedance, Z = V/I. If we know the
power supply voltage and the amount of current that a chip draws, it
is an easy calculation. This will be the impedance presented to the
power supply over a broad range of frequencies. At high frequencies
(probably over 10 MHz) the package inductance begins to be important
and there is a series inductance between the PCB and the chip. The
power supply impedance of a chip will change as the chip draws more
or less current as the activity level changes.

BTW, the impedance of the power distribution system should be less
than 5% of the impedance of the chip in order to have less than 5%
regulation.

regards,
Larry Smith
Sun Microsystems

It may change depending on the
function that the chip is performing at the moment.
> Date: Tue, 12 Jan 1999 09:21:48 +0800
> From: ycchien <ycchien@alpha1.srrc.gov.tw>
> X-Accept-Language: zh-TW,en
> Mime-Version: 1.0
> To: SI-LIST <si-list@silab.eng.sun.com>
> Subject: [SI-LIST] : Power/Gnd Pin Input Impedance
> Content-Transfer-Encoding: 7bit
>
> Dear SI-List members,
> What is the input impedance looking into IC's Power/Gnd pin? How to
> measure it?
> Thanks.
>
> Eric Chien
> Assistant Researcher
> Synchrotron Radiation Research Center
> Taiwan, R.O.C.
> ycchien@alpha1.srrc.gov.tw
>

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