Re: [SI-LIST] : crosstalk through resistor networks?

Dennis Tomlinson (det@tellabs.com)
Fri, 14 Aug 1998 11:46:53 -0500

Andrew Phillips wrote:
>
> Hello,
>
> I am interested in using some small-package (QSOP) resistor networks for
> terminating a large number of bus-signals in a design. Signal rise-times
> range from ~200ps to 1ns or so.
>
> I have found some devices that look suitable here:
> http://www.bourns.com/cat/MNETXX1.htm
>
> I am wondering whether such an approach will have crosstalk problems.
> Has anyone analysed this and can give me some advice?
>
> I presume that if traces are suitably separated before and after the
> resistor package then the crosstalk induced between adjacent signals
> will be reduced. These signals are very close together for the 3mm or so
> across the resistor package - is this likely to cause problems?
>
> Thanks for any help,
>
> Andrew Phillips
> Supercomputing Systems AG
> Zurich, Switzerland
>

Hi Andrew,

I can't give you rigorous results from an exhaustive analysis, but perhaps
a rule of thumb will suffice.
Assuming backwards crosstalk dominates, the coupled voltage between two nets
is given by:
Vb = Kb*(2tp/tr)*dVs
Where:
Vb is the coupled voltage in the backwards direction,
Kb is the backwards coupling coefficient,
tp is the propagation time across the coupled length,
tr is the signal rise time, and
dVs is the source voltage swing of the aggressor.

The ratio 2tp/tr can be viewed as the percent of the source voltage
swing available to be coupled from aggressor to victim across the
coupled length. Assuming about 6 ps/mm propagation constant across
the 3 mm body width, this ratio takes on values from 18% for 200 ps to
3.6% for 1 ns.
Values of Kb, I can only guess at, but 0.25 is a rather high coupling
coefficient. Using this value gives Vb in the range of 0.045*dVs for
200 ps risetime, down to 0.009*dVs for 1 ns.

Also, for the QSOP package, you have an additional 1.5 mm pin length
from the seating plane to the body on each side of the package. The
above does not take pin-to-pin coupling into account.

BTW, is this R-pack used as a series terminator? Do all bus signals
switch at the same time - with some time lag before stability is
required? Do the signals all propagate the same direction down
the bus? If any/all the answers to the above are yes, you could have
errorless circuit operation with some rather high levels of crosstalk.

Cheers,

Dennis

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