[SI-LIST] Re: Substrate board impedance added to driver impedance for total output impedance of driver

Larry,
        Thanks for your response.  I wasn't clear in my last email, when
I said my frequency of interest was 350Mhz I meant that the knee
frequency as calculated from Johnson's book using the equation Fknee =3D
.5 * (1/(Trise)) came out to 350Mhz.  This signal of interest is
actually a 200Mhz digital clock and I assumed Fknee was the correct
frequency to use in my equations (I think I'm correct in assuming that
but maybe not).

I agree that on a half inch trace I wouldn't have to terminate, but I'd
really like to accurately provide the total impedance of our chips
output driver all the way out to the ball on the substrate board where
it will have to be terminated since it will be going out to the PCB
board on much longer traces.  I'm not saying that it will have a huge
impact, but I am interested in finding its contribution.

Thanks,
Eric


-----Original Message-----
From: Larry Miller [mailto:ldmiller@xxxxxxx]=20
Sent: Thursday, June 30, 2005 10:58 AM
To: steimle@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Substrate board impedance added to driver
impedance for total output impedance of driver

Eric,

The critical thing is not the 350 MHz, but the rise and fall times of =20
this signal.

If they are (let's say) 10% of the signal, then they are

1/350E6 * 1/10 =3D  2.8571428571e-10 sec (286 ps).

This is near the propagation velocity of a pcb trace (usually about =20
200 ps/in) so you are into  the "transmission line" mode as opposed =20
to lumped line mode.

If the wave is 350 MHz sinusoidal you would not have to worry about =20
it at all-- you are below the "wavelength/6" zone where you have to =20
start using transmission lines.

Get Howard Johnson's book "High Speed Digital Design". He explains =20
the why's and how's of this very well.

In any case a 1/2" trace will not need terminators or anything like =20
that. I think you are making more of a problem of it than you have.

Larry Miller

On Jun 30, 2005, at 7:58 AM, Eric Steimle wrote:

> I=92m have a chip that I=92ve done some analysis on to get its driver
> impedance.  However this chip is on a substrate board, and it=92s =20
> drivers
> run through traces to balls and to the board.  I have R, L, and C
> numbers for the trace/bondwire combination as part of a package model
> that was made by someone else.  The trace in question is =BD inch =20
> long and
> the frequency of interest is 350Mhz.  I=92ve made the assumption that =
I
> can use vector addition to add the inductive and resistive =20
> contributions
> together, but I don=92t know what to do with the capacitive part.  =
I=92m
> also assuming that once I=92ve vector added the inductive, resistive,  =

> and
> capacitive parts together for a total Zpackage that I can then =20
> linearly
> add Zpackage and the Zdriver from previous experiments.  Does that =20
> seem
> right?  I=92m just a little confused on the appropriate steps to =20
> take.  As
> always I appreciate the time you all take to read and respond to my
> questions.
>
>
> Thanks,
>
>
>
> Eric Steimle
>
>
>
>
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