[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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