Mike,
When you are talking about output driver impedance and load impedance,
do you mean the differential component only of both, differential and
common?
I can imagine the driver in which the output impedance measured
differentially does not depend on the differential load (or dependence
is small). The example you provided is perfect. However, what if the
common mode load impedance can be anything between 25 Ohm and infinity?
As you mentioned:
> The common mode output voltage does need to be defined, as it does
affect output swing by perhaps as much as 20%
Just trying to understand. Doesn't the common mode impedance of the
load affect the common mode output voltage? Then, the common voltage
affects the swing, the swing affects the output differential impedance?
Vladimir
-----Original Message-----
From: ibis-macro-bounce@xxxxxxxxxxxxx on behalf of Mike Steinberger
Sent: Thu 1/13/2011 2:34 PM
To: ibis-macro@xxxxxxxxxxxxx
Subject: [ibis-macro] Re: BIRD-124: Dependency tables - question?
Arpad-
You're going off the rails pretty early in your discussion. So the rest
of your conclusions are not valid.
The bias point of the output transistors in a driver is affected only
minimally by the load impedance. That effect can be safely ignored in
most designs, and is clearly not a factor in AC coupled links.
We've run the SPICE decks for a lot of different drivers for a lot of
different swing settings and several different load impedances, and this
is what we see: The differential output impedance typically is about 100
ohms for low swing settings, going down to around 70 ohms (or lower) at
high swing settings. The common mode output voltage does need to be
defined, as it does affect output swing by perhaps as much as 20%;
however the load impedance has only a negligible effect on the output
impedance of the driver.
When the swing of a driver is set, it's set internal to the driver. In
many cases, the swing is set by a tail current source supplying a
differential output pair. The current supplied by the tail current
source sets the bias current for the output transistors. That bias
current pretty much directly determines the output impedance of the
transistor. The output impedance of the driver transistors usually ends
up in parallel with other, fixed circuit elements such as pull-up
resistors, and all of these elements in parallel determine the output
impedance.
What typically happens is that to produce higher output swings, the bias
current is increased, with the result that when the driver transistor
pulls down to a lower voltage, making it behave more like a triode and
less like a current source. That's why the output impedance goes down.
If you think something else is happening, then please show me the data.
Mike S.
On 01/13/2011 04:03 PM, Muranyi, Arpad wrote:
>
> Mike,
>
> Aside from the usefulness of the Dependency Table, there is
>
> a pretty fundamental technical problem here.
>
> You say that the most critical application of the Dependency
>
> Table is the driver impedance selection because the impedance
>
> of the driver changes with the bias point. (This basically
>
> means that the driver model is not linear, which I will come
>
> back to later). However, the bias point of a driver depends
>
> on the load impedance connected to the driver. The problem
>
> with selecting different impedance models using a model selector
>
> mechanism is that the controlling input to this model selector
>
> doesn't know what the load is on the driver. It is just a
>
> blind selection without actually solving the circuit operating
>
> point. The circuit's operating point can change drastically
>
> depending on what kind of termination is used (parallel, or series)
>
> and what the termination voltage and resistance values are. It
>
> also depends on whether the channel has series AC decoupling
>
> capacitors. None of this information is considered when the
>
> selection is made with the model selector (with or without the
>
> Dependency Table), so the results of the selection may end up
>
> being very inaccurate.
>
> An analogy is a simple voltage divider. Consider a Thevenin
>
> voltage source and resistor "driving" a load resistor. How
>
> can you solve for the correct voltage between the two
>
> resistors without knowing the load resistance?
>
> In the non-linear case this is even complicated further because
>
> the solution of the voltage divider is not just an algebraic
>
> equation.
>
> If you say that the driver impedance changes due to the bias
>
> voltage, you are talking about an I-V curve which is not a
>
> straight line. When you make a different linear model at
>
> different bias points, you are basically making a linearized
>
> model at the various points of the I-V curve. This is what
>
> a small signal AC analysis does in SPICE without the user
>
> having to go through a model selector mechanism. But this is
>
> also what can be done with our good old non-linear I-V curve
>
> based IBIS [Model]s.
>
> I wonder why are we then proposing linear S-parameter based
>
> buffer models with a bunch of highly inaccurate model selectors,
>
> when the I-V curve based model would do the same much more
>
> accurately automatically, without manual selections?
>
> The partial answer to this question is that an I-V curve describes
>
> the impedance as a function of voltage, but S-parameters describe
>
> the impedance as a function of frequency. It seems that we would
>
> like to have both. I wonder if there is a way to combine these
>
> two into a single representation...
>
> Arpad
>
> ===================================================================
>
> *From:*ibis-macro-bounce@xxxxxxxxxxxxx
> [mailto:ibis-macro-bounce@xxxxxxxxxxxxx] *On Behalf Of *Mike Steinberger
> *Sent:* Thursday, January 13, 2011 9:00 AM
> *To:* ibis-macro@xxxxxxxxxxxxx
> *Subject:* [ibis-macro] Re: BIRD-124: Dependency tables - question?
>
> Kukal-
>
> There are several useful applications for dependency tables; we do
> quite often use the application you mention It's more useful than you
> imagiine. The most critical application, however, is setting driver
> impedances.
>
> As you may know, the output impedance of a driver typically changes
> with the swing setting. That is, changing the swing setting changes
> the bias point, which changes the output impedance of the transistors.
> Therefore, as we do state in Opal(TM), both the algorithmic and the
> analog models need to know what the swing setting is. As a matter of
> good software engineering, it would be best if both models got their
> information from a single user input.
>
> One could imagine a number of ways to solve this problem, but
> dependency tables are a particularly simple and flexible solution.
>
> Mike S.
>
> On 1/13/2011 2:11 AM, Taranjit Kukal wrote:
>
> Hi,
>
> The dependency tables would be useful only if we want to keep same
> AMI-code (dll) and just change the dependency outside of the code
> using .ami file.
>
> However, I do not see this as common use-model - I would assume that
> all such dependency should be handled inside of c-code
> (Algorithmic...) and that we should avoid overloading of .ami file
> such tables. AMI-code should be able to handle such dependency by
> coding the right values for dependent parameters based on Key
> parameters in ami file.
>
> Please let me know if I am missing a use-case where this cannot be
> handled inside the dll.
>
> rgds
>
> ..kukal
>
> Taranjit Kukal | Product Engineering Architect
>
> P: 91 120 3984000 www.cadence.com <http://www.cadence.com/>
>
