In addition to what Mike has described, we also use a TDR approach to
measuring C_comp(IBIS for I/O pad capacitance).We use the Tektronix
IPA510(Interconnect Parametric Analyzer) instead of any LCR meter.
You launch your TDR pulse into the Device pin under test thru a
'very clean' test fixture. Your SMA connectors from the TDR head
(SD-24 TDR heads/11801B Tek scope)and into the jig needs to
be tourqued correctly so no loss occurs at the launch point and
again at the jig.
You use the IPA510(software on a PC)to capture the reflection profile
and do a 'Z-profile' analysis on this. THis will get rid of mulitple
or ghost reflections. Then place cursors and markers on
the capacitive dip part of the waveform to read off the Z, L and C
numbers. Since the measurements are done on a packaged unit, you
need to substract off the package capacitance to get the true
For more information visit, www.tektronix.com and enter IPA510 in
the search window.
National Semiconductor Corp.
> From owner-si-list@silab.Eng.Sun.COM Mon Apr 13 13:36:08 1998
> Date: Mon, 13 Apr 1998 12:43:51 -0700
> From: Mike Jenkins <email@example.com>
> Organization: LSI Logic
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> To: si-list@silab.Eng.Sun.COM
> Subject: [SI-LIST] : Re: measuring CMOS Cin with HP4275A
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> If you don't mind, I'd like to recommend an alternative method for
> measuring pin capacitance. L-C meters may be fine, (potentially)
> accurate instruments, but, as you've seen, they're not much good
> for telling you what's going on.
> My preferred method uses a TDR with a scope capable of waveform
> math (subtraction and integration).
> ____________________ ___
> | * | ___________
> without | * | ___________()--- +
> cap -->| * <-- with cap unit Zo |
> | * step C ===
> _________|* _|_ _|_
> Without the cap, the TDR shows the classical open-circuit doubling
> reflection. With the cap added, the reflection adopts an RC time
> constant. The time constant is Zo * C.
> Of course, real waveforms don't look like this -- ringing and other
> ugly stuff. However, the Zo*C time constant is still the integral
> of the difference between the "without" and "with" waveforms. This
> is exact, even with arbitrary incident step waveforms and even with
> inductance in series with the cap. And the TDR trace lets you see
> the effect of such inductance. The accuracy is as good as your
> equipment, probably limited by fixturing (including knowledge of Zo).
> I'll get off the soapbox, now.
> ps: If you're interested, the proof involves the LaPlace initial and
> final value theorems.
> Mike Jenkins Phone: 408.433.7901 _____
> LSI Logic Corp, ms/G750 Fax: 408.433.2840 LSI|LOGIC| (R)
> 1525 McCarthy Blvd. mailto:firstname.lastname@example.org | |
> Milpitas, CA 95035 http://www.lsilogic.com |_____|