[SI-LIST] Re: 22.1 Ohm vs 50 Ohm Termination
- From: "Dunbar, Tony" <tony_dunbar@xxxxxxxxxxx>
- To: 'SI-List' <si-list@xxxxxxxxxxxxx>
- Date: Fri, 30 Aug 2002 12:45:37 -0500
Kevin,
There are two main aspects to the 'correct' choice of terminator value.
First - conveniently ignoring the fact that the output impedance of a buffer
is never really constant throughout its switching range and so there is no
one ideal value - the value is chosen to make the sum of the buffer output
impedance and the terminator value equal the Zo of the line being driven.
Second, there may be timing implications associated with the 'correct'
choice. There is a trade-off between the correct value to minimize or remove
under/overshoot and a value above which the timing requirements of the net,
perhaps relative to another group of nets as in the case of a clock, are
violated. The way I put this is that the real answer to how much does a
resistor cost may not be measured in cents - it may need to be measured in
nano-seconds. Terminating above 22ohms may introduce too much delay on the
net.
Regards,
Tony
-----Original Message-----
From: kevinlfitzgerald [mailto:kevinlfitzgerald@xxxxxxxx]
Sent: Friday, August 30, 2002 8:40 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] 22.1 Ohm vs 50 Ohm Termination
Hi,
I am wondering what the reason for the use of 22.1 Ohm series
termination on a 50 Ohm line, as opposed to matching it with 50 Ohm.
I have simulated signals with both, and both look reasonable. Is
there an electrical reason for using 22.2 Ohm or is it just lab
experience.
Thanks,
kevin
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