FreeLists / si-list / [SI-LIST] low inductance resistor

["Eric Bogatin" <eric@xxxxxxxxxxxx>]

Richard-

GigaTest has done a number of measurements on the parasitics
associated with SMT terminating resistors. We have some app notes that
have the details of the measurements and the extracted models, posted
on our web site, www.gigatest.com.

The bottom line is that a simple RLC model fits a real resistor up to
about 5 GHz. The L and C depend a lot on the position of the return
path. To minimize L, be sure to route the return plane as close to the
surface as possible. When the return path plane is 32 mils away, the
series loop inductance is 1.8 nH and the capacitance for an 0603 body
is about  0.16 pF. By comparison, an axial lead resistor has a series
loop inductance of about 8 nH and capacitance of 0.4 pF.

If any of you are doing IBIS based or SPICE simulations, you might
check the parasitic values that are assigned to smt resistors. I was
pleased to see that Hyperlynx uses parasitic values very close to the
ones we measured.

If you would like more info on this and similar high bandwidth topics,
here are two immediate opportunities:

I am giving a talk at the next Ansoft user symposium in San Jose on
Oct 10 that includes some of the results from this work. The slides
from this talk are posted on our web site, www.gigatest.com as
application note #81. If you are interested in attending the
symposium, check the Ansoft web page for more info:
http://www.ansoft.com/thewave/locationDetail.cfm?eventID=12

The next public short course I'm doing is GTL250 High Speed Board
Design. It's an intermediate level rather than a basic class, in that
we go over a lot of practical details related to signal quality, lossy
lines, cross talk, switching noise, rail collapse and EMI. The details
are on the web site, www.gigatest.com

--eric

**************************************
Eric Bogatin
CTO, Giga Test Labs
v: 913-393-1305
f: 913-393-1306
e: eric@xxxxxxxxxxxx
26235 W. 110th Terr.  Olathe, KS  66061
corporate office:
408-524-2700
134 S. Wolfe Rd Sunnyvale, CA 94086
web: www.gigatest.com
**************************************


From: RMELLISON@xxxxxxx
Date: Mon, 1 Oct 2001 12:54:36 EDT
Subject: [SI-LIST] low inductance resistor

Hello SI Gurus
I'm working on a design for operation at 2.5GBaud.   In modeling the
path
from a Stratos Lightwave PECL driver to the TLK2501 receiver, I
noticed that
it is not as clean as in the other direction.  After much modelling,
I'm
convinced that the problem is two-fold.  First, the MLC driver is low
impedance and must be backmatched--a CML part with internal 50 ohms to
Vcc
would have been a better choice.  The backmatching resistor must be
placed as
close as possible to the source, but via inductance and driver pin
inductance
limits how effective this resistor is.  Second, and definitely the
worst
offender, is the fact that the TLK device requires external 50 ohm
resistors
for the end terminations--this leaves stubs that ring, caused by the
inductance and capacitance of the pins, bond wires and input
transistors.
Three questions:
       1)  Does anyone have any modelling information (internal pin
models,
driver characteristics, risetimes, etc.) on the Stratos MLC-25-7-2-TL
Optical
ATM OC-48 part?  I'm using Pspice, but IBIS would be a help.
       2)  Does anyone know if the CML version offered by Stratos has
internal 50 ohms to Vcc?  (I haven't had any success getting
information from
the apps engineer at Stratos).
       3)  I've been able to control the ringing caused by the pin
inductance
and input C of the TLK2501 by inserting a perfect 27 ohm resistor in
series
with the input pin (the 50 ohm terminations are placed on the bottom
of the
card, with the resistor being placed between the termination via and
the
input pad on top of the card).  The question is, what is the smallest
lowest-inductance part that I can use for this purpose?
Any help would be greatly appreciated.
Richard Ellison
Ellison Consulting
214-544-1920 bus
214-544-1924 fax


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