[SI-LIST] Re: Why 50 ohms
- From: Patrick_Carrier@xxxxxxxx
- To: Alfredo.Moncayo@xxxxxxxxxxxx, Patrick_Carrier@xxxxxxxx,siman_74@xxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Mon, 21 Jan 2002 14:07:57 -0600
Sorry, didn't mean to imply there was no good reason for the 27-ohm
solution. I'm sure a lot of design consideration went into it. It's just
that RAMBUS is much different in nature than most other drivers, and in that
respect is quite unique, as is its 27-ohm impedance.
--Pat
-----Original Message-----
From: Alfredo.Moncayo@xxxxxxxxxxxx [mailto:Alfredo.Moncayo@xxxxxxxxxxxx]
Sent: Monday, January 21, 2002 11:59 AM
To: Patrick_Carrier@xxxxxxxxxxxxxxxxx; siman_74@xxxxxxxxxxxxxx;
si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Why 50 ohms
Pat,
The Rambus example you cite is not at all a random case. You'd be rather
surprised to know exactly how that came about. In summary, it was the
essence of engineering - optimization of many variables to achieve the best
possible balance of electrical behavior, ease of implementation, and cost.
Oh, and yes, a significant part of this optimization effort had to do with
the driver.
Alfredo
-----Original Message-----
From: Patrick_Carrier@xxxxxxxx [mailto:Patrick_Carrier@xxxxxxxx]
Sent: Saturday, January 19, 2002 7:25 AM
To: siman_74@xxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Why 50 ohms
Mani--
PCB impedances are usually determined to closely match the output impedance
of a signal driver. This eliminates reflections from occurring and
distorting the transmitted signal.
Also, typical PCB trace widths and dielectric heights will give you a 50-ohm
line fairly easily, since impedance is a function of trace geometry.
The 27 ohms for RAMBUS is kind of a random one, and mostly had to do with
the fact the driver was much different than normal signal drivers.
Good question!
--Pat
-----Original Message-----
From: mani kantan [mailto:siman_74@xxxxxxxxxxxxxx]
Sent: Saturday, January 19, 2002 12:52 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Why 50 ohms
Hello SIexperts,=0D=0A I am a student, doing Engineering in Electri=
cal.=0D=0AWhile discussing with my friend, he told me that the PCB trace im=
pedance should be maintained as 50 Ohms (for differential=3D 100 and for so=
me RDRAM buses 27ohms).=0D=0AI tried to find the answer for this in some of=
the standard books but i could find the answer for this. Books also saying=
that we should maintain 50 ohms. But i couldn't find the reason for that.=
=0D=0A=0D=0ACould any body please through a light on me on this topic.This =
may be a newbie question. But i thought this group is the correct place to =
get my answer.=0D=0A=0D=0A=0D=0Athanks in advance,=0D=0ASiMan=0D=0A =0A
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