[SI-LIST] Re: 22.1 Ohm vs 50 Ohm Termination
- From: Hora Abu <arageeb@xxxxxxxxx>
- To: kevinlfitzgerald@xxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Fri, 6 Sep 2002 20:29:45 -0700 (PDT)
In order to match a 50 Ohms transmission line, the series termination resistor
+ the output buffer impedance must add up to 50 Ohms. So the total resistor
seem looking back into the driver which include the series termination resistor
+ the output resistor of the drive need to match the transmission line
impedance. In your case it must be assumed that your output driver 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
kevinlfitzgerald wrote:
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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arag abu deeb
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