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

Christian Schuster (schuster@iis.ee.ethz.ch)
Tue, 12 Jan 1999 10:04:44 +0100

Hi Eric and Dima,

may I add a little comment to the input impedance question?

> 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

> I think it should look like a small (I mean: very small)
> inductive value. If the pin is long enough, you should be
> able to observe it with a TDR.
>
> Thanks,
>
> -Dima
> ============================
> Dima Smolyansky
> TDA Systems, Inc.
> 7465 SW Elmwood St.
> Portland, OR 97223
> (503) 977-3629
> (503) 245-5684 (fax)
> dima@tdasystems.com
> http://www.tdasystems.com

There is a publication in 1994 which exactly addresses this
problem:

Chi-Taou Tsai, Package Inductance Characterization at High
Frequencies, IEEE Transactions on Components, Packaging and
Manufacturing Technology - Part B, Vol. 17, No. 2, PP 175-181,
May 1994.

The paper describes a measurement technique based on a vector
network analyser and gives examples as well (120 QFP, 296TAB tape,
224PGA). It turns out that the impedances of the packages
are described by inductive effects only for low frequencies
(about several nH). At higher frequencies capacitive effects
have to be taken into account as well. In the end even
resonances will show up (of course, these should be well beyond the
working range of frequencies).