[SI-LIST] Re: Series termination value
- From: "Sanchez, Louis" <louis.sanchez@xxxxxxxxx>
- To: "'pintu22@xxxxxxxxx'" <pintu22@xxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Fri, 22 Mar 2002 11:18:44 -0800
Hi AB...........
If the impedance of the traces is 71 ohms, then a series termination of 71
ohms will prevent reflections due to mismatch from occuring. The series
termination resistor is the sum of the source impedance of the driver, and
an external series resistor placed on the board. In this case, since the
source impedance is 20 ohms, then the external resistor is 51 ohms.
You are correct in stating that the total impedance seen by the driver is
140 ohms. If we assume (for example) that the drivers swing is from 0 to 3.3
volts, then the driver must be capable of supplying 3.3/140=23.6 mA. The
load current is inversely porportional to the total load impedance seen by
the driver, so I don't understand your concern about a "whopping 140 ohms."
Lou Sanchez
---Original Message-----
From: pintu22 [mailto:pintu22@xxxxxxxxx]
Sent: Friday, March 22, 2002 10:47 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Series termination value
If your impedence is 71 Ohms
and you are driving a 71 ohms + a resistor of 71 ohms the driver
incounters a whopping 140 ohms... YOu need to do an analysis of the
driver to see how much you can drive. This resistance might be too
much for the driver to drive as you need to jack up the buffer
strength
signal integrity wise series termination are used to take advantage
of reflection and if there is no termination on reciever u dont worry
about it
AB
--- In si-list@xxxx, "SEOW,ERWIN-SP (HP-Singapore,ex6)" <erwin-
sp_seow@xxxx> wrote:
>
>
> Hi all,
>
> I have a question on series resistors termination. I have a 4 layer
FR4
> board with standard PCB stackup (Trace width = 6 mils).
>
> We have calculated the Characteristic Impedence to be about 71 Ohm.
>
> With an output impedence of about 20 Ohm, the resistor value for
series
> termination is 71 Ohm - 20 Ohm = 50 Ohm. My question is this value
too big?
> Will it attenuate the signal? We are running at about 90MHz.
>
> Your advice is appreciated.
>
> Thanks!
>
> Erwin
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