[SI-LIST] Re: 22.1 Ohm vs 50 Ohm Termination
- From: "Bill Reams" <breams@xxxxxxxxxxxxxx>
- To: <arageeb@xxxxxxxxx>
- Date: Mon, 9 Sep 2002 08:49:54 -0500
The reason for a value such as 22.1 ohms is not the precision of the =
actual resistance, it has to do with manufacturing costs. You are =
probably correct that one would not see much if any difference in =
performance between a 22.1 ohm, 21.5 ohm, or 22.6 ohm resistor (all =
standard E-96 values).=20
However, back in the "old" days when I started, 10% was pretty much the =
standard and 5% was not much more costly (20% were still quite =
available). From a cost perspective, 10% was the place to be if one =
could stand the tolerance. But times change and manufacturing improves =
(we hope). Presently, 1% resistors are about the same price as 5% and, =
with the exception of a few common values, 10% is actually demanding a =
premium cost. It's basic supply and demand.
So, following the technology curve, many of us are using 1% even when =
10% would be adequate. The reasons have to do with cost not performance =
- and the fact that I don't want to come back to requalify parts for a =
cost reduction ECO in a couple of years.
-----Original Message-----
From: Hora Abu [mailto:arageeb@xxxxxxxxx]
Sent: Friday, 06 September, 2002 10:30 PM
To: kevinlfitzgerald@xxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: 22.1 Ohm vs 50 Ohm Termination
{snip}
has a resistor of 27.8 Ohms. ( I can never understand the business of =
22.1 Ohms since I have not seen a CMOS driver that can gurantee its =
output impedance with accuracies that suggest 0.2 Ohms. Also typical =
PCB supplier can not guarantee a board impedance better than 10%)
Hora
{snip}
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