I design very high speed digital interconnects. All the chips
that talk through the interconnects are custom designed & so
I have freedom to define how the driver needs to be characterized.
To facilitate design I rely on output impedence rather than
currents & also to avoid confusion I specify exactly what
should be the simulation environment & characterization
methodology for finding the output impedence of the drivers.
So it's no problem comparing the strengths of two different
drivers & also , for example, it's clear that a 75 ohm
driver will not be able to drive a 50 ohm xmission line
when incident wave signaling is in mind.
> From Greg.Edlund@digital.com Tue Mar 3 05:38:13 1998
> From: Greg Edlund <Greg.Edlund@digital.com>
> To: "'Mark Nass'" <firstname.lastname@example.org>
> Cc: "'email@example.com'" <si-list@silab.Eng.Sun.COM>
> Subject: RE: [SI-LIST] : Driver Strength
> Date: Tue, 3 Mar 1998 08:05:33 -0500
> X-Priority: 3
> MIME-Version: 1.0
> X-Mailer: Internet Mail Service (5.0.1458.49)
> Sender: owner-si-list@silab.Eng.Sun.COM
> I have found that driver current ratings don't mean a whole lot today.
> They are a hold-over from TTL days, when each receiver drew a certain dc
> current and a driver had to be able to source the dc current of all its
> loads. In a CMOS environment, they are useful in a relative sense, i.e.
> you can guess that a 4 mA driver will probably have a lower output
> impedance than a 2 mA driver - within the same part family and the same
> vendor. However, don't expect vendor A's 4 mA driver to have the same
> output impedance as vendor B's 4 mA driver.
> Output impedance tends to be a much more useful parameter in a
> transmission line environment. Unfortunately, most vendors don't spec
> output impedance. If you have a non-linear output IV curve, it does
> vary over loading, making it a little difficult to spec. The vendor
> would have to guess what kind of environment driver will see and spec,
> say, a 60 Ohm resistive load to ground and Vdd. Even this information
> is more useful than nothing.
> The best thing you can do is to ask for an IBIS model of your driver
> that is verified against lab data.
> How many loads you can expect to drive varies widely with load
> capacitance, net topology, and timing. A behavioral simulator is an
> excellent tool for prototyping nets and answering these kinds of
> questions. It's important not to rely on the simulator to do all your
> work for you, though. I find it necessary to go back to transmission
> line theory (reflection coefficients, etc.) and make sure what I'm
> simulating makes sense!
> Greg Edlund, Principal Engineer
> Server Product Development
> Digital Equipment Corp.
> 129 Parker St. PKO3-1/20C
> Maynard, MA 01754
> (978) 493-4157 voice
> (978) 493-0941 FAX
> From: Mark Nass[SMTP:firstname.lastname@example.org]
> Sent: Monday, March 02, 1998 10:35 PM
> To: si-list@silab.Eng.Sun.COM
> Subject: [SI-LIST] : Driver Strength
> Can someone explain to me what driver strength means? When
> a driver is spec'd by an ASIC vendor as a 12ma, 6ma, etc what
> that mean as far as its expected VI curve and how many loads
> I can expect it to drive?
> Mark Nass