[SI-LIST] FW: Re: Series Termination Question
- From: Mike Cantwell <Mike.Cantwell@xxxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 18 Apr 2002 09:06:05 -0500
Having spent many of my past years on the EMC side of the world, I can say
that sometimes your line of thinking bcomes more of how to troubleshoot
emissions problems rather than true design. The only reasoning I can see for
R3 is to isolate that piece of etch to determine if it radiates. I agree
that it won't necessarily help signal integrity or performance.
The other thing that happens very frequently on the EMC side is that
designers don't take the time to explain why they've designed it that way.
As such, it's rather difficult for EMC engineers to keep up with
technologies and design techniques. The things one needs to know as a
designer are different than those for an EMC engineer. They are two
professions in their own rights.
What I would recommend is to share your simulation results or to forward one
of the good SI articles in your stash with this regulatory person and take
the time to discuss it with them. It will inevitably be a learning
experience and will provide all with a better working relationship.
All this said from someone who has been on both sides of the fence with
respect for both sides.
Mike
-----Original Message-----
From: Zabinski, Patrick J. [mailto:zabinski.patrick@xxxxxxxx]
Sent: Thursday, April 18, 2002 8:44 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Series Termination Question
Bill,
Assuming the receiver has a high input impedance, I can
see no benefit (other than political) in R3. With
a high input impedance, there should be no current flowing
in R3, so it's useless.
If your customer insists on R3, then make it 0.1 ohms. ;)
Pat
> I'm presently designing a board that will be used by company "X". =
> Basically, this is to be one of our storage routers in a form factor =
> that will fit within their product. In going though the
> schematics to =
> ensure that we're going to be giving them what they want, their =
> regulatory engineer made a request that I can't quite get my mind =
> around. I was hoping that someone could give me some insight
> into this =
> request so I can understand why he is asking for this (i.e.,
> what good =
> does it actually do).
>
> Normally, we use series terminators in this fashion with R
> chosen based =
> on driver and trace impedance (as well as what seems to work in the =
> lab):
> R1 __________
> |\__/\/\/\__| Trace |__|\
> Driver |/ |__________| |/ Receiver
>
>
> But the company "X" regulatory guy is asking for this on
> clock signals:
>
> R2 __________ R3
> |\__/\/\/\__| Trace |__/\/\/\__|\
> Driver |/ |__________| |/ Receiver
>
>
> So, what would be the advantage of using two resistors where
> one seems =
> to do the job? And it seems that, if we went with the second
> circuit, R2 =
> + R3 =3D R1 but what other constraints would make sense? For clock =
> signals, we have 40 MHz, 66 MHz, 85 MHz, and 100 MHz.
>
>
> I'm rather caught between not understanding the utility of
> his request =
> and two basic business axioms:
> * The customer is always right.
> * Give the customer what she needs (not necessarily what she wants).
>
> Thanks in advance for your advice.
>
> _______________________________________
> | |
> | Bill Reams - Sr. Hardware Engineer |
> | 512-928-7201 (direct) |
> | 512-349-0300 (Main) |
> | breams@xxxxxxxxxxxxxx (e-mail) |
> |______________________________________|
>
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