[SI-LIST] Re: Driver Output Impedance am I doing this correctly?

The best method is to give the customer a SPICE model and let him figure out 
the output impedance based on his frequency of operation and application. 
The method I prefer ties an ideal open transmission line to the driver.  I 
like to make the length a few inches, but long enough so it is easy to see 
the voltage transitions at the end of the line.  Then solve for Zout:

Vout = 2 x VDD x Z0 / (Z0 + Zout)

where Vout is the pedestal at the end of the line and assuming VDD for a 
CMOS driver (for other driver types, use the voltage that would be used in a 
Thevenin equivalent of the driver)

Using a resistor to ground rather than an open transmission line keeps the 
DC currents high, which can change the output impedance.  In most 
applications, the DC currents are low, but use a topology that makes sense 
for the application.  Hence, I prefer getting the SPICE model.

Dennis



----- Original Message ----- 
From: "eric steimle" <ericsilist@xxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Wednesday, February 23, 2005 6:44 PM
Subject: [SI-LIST] Driver Output Impedance am I doing this correctly?


> Hi,
> Thanks in advance for anyone taking the time to read
> this, I know how valuable everyone's time is.  I'm
> trying to calculate a number for our driver output
> impedance that I can give to someone working with our
> chip.  I'm relatively new at this so I was wondering
> if my approach was flawed.
>
> My configuration is:
>
> Driver --> BondWire --> Substrate Board Trace --> Ball
>
> My driver's rise time is 500ps and my goal is to
> calculate our total impedance value in order to allow
> others to choose the appropriate series terminating
> resistor.
>
> Driver
> 1.) I used spice with a 50 Ohm load on the output and
> Ohms law to get the real impedance of the driver
> Zdriver (14.5Ohms).
>
> BondWire
> 2.) I read through the SI archives and other docs and
> then I assumed the inductance of the bondwire was
> 1nH/mm on a 2mm bondwire.  That means:
>
> BondWire = 2nH
>
> Then I took the Frequency of the output to be:
> F = .35/Tr
> F =  .35/500ps
> F = 700Mhz
>
> Then I used the impedance formula for an inductor (can
> I do this with the F I calculated or does that not
> make sense?).
>
> Z = 2pi * f * L
> Z = 2pi * 700Mhz * 2nH
> Z = 8.8Ohms
>
> Finally
> So I figured that the Zdriver + Zbondwire +Zimag(I'm
> missing that) would equal the output impedance of the
> driver itself, and I know the impedance of the
> substrate trace.
>
> Does that make sense, or am I going about this all
> wrong?  I still don't know how to calculate the
> frequency dependant part of the driver output
> impedance but I'm researching like crazy.
>
> Any feedback or new reference material would be
> greatly appreciated.
>
> Thanks,
> Eric
>
>
>
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