[SI-LIST] Re: termination with ac coupling
- From: David Instone <dave.instone@xxxxxxxxxx>
- To: Stephen.Greenhalgh@xxxxxxxxxxx
- Date: Thu, 23 Mar 2006 16:13:57 +0000
Stephen,
It depends more on whether the driver can take the load resistors
going to ground, or where ever you decide to connect them. Do you have
control of the design of the driver so that if you decide to so connect
them it will work with any driver? I know of one major company that
thought the phrase 'receivers shall be AC coupled' didn't include the
terminating resistors, they DC coupled the terminators and then AC
coupled them to the receiver. The PECL (I think) driver objected! From
the fact that you say your proposed device terminates with 75 ohm I'd
take a bet that it's a Fibre Channel device, if so, and you're using
the corresponding FC driver it will not like the terminating resistor
being DC coupled.to its output. You might also find that if the
receiver is expecting to be AC coupled it also will probably not expect
to have it's input bias voltage being upset by you connecting a resistor
to ground. The upshot could be that you will have to use caps in series
with the line to keep the driver happy and and also have caps in series
with the external terminators to keep the receiver happy. In the case
where your terminator consists of a series resistor and cap put the cap
at the cold end, it better isolates the package parasitic capacitance.
On the other hand, practical experience tells me that for an internal
1Gbd Fibre channel link you can ignore the fact that you would be
terminating that 50 ohm line with 75 ohms and forget your external 150.
It pretty much works for 2 Gbd FC as well if you have control of both
ends of the link.
Regards
Dave
Stephen Greenhalgh wrote:
> When ac coupling into a receiving device at the end of a (differential)
> transmission line, I have generally put the termination resistors first, then
> through the capacitors into the IC. Nowadays many receiving devices have
> built-in resistors, so the terminations are after the coupling capacitors. I
> realise that larger capacitor values are then required to handle the lower
> frequencies. Apart from this, are there any other issues to consider? For
> example, as regards reflections, does the position of the termination (before
> or after the coupling capacitors) make any difference?
> In particular, I want to connect from a 100 ohm (differential) transmission
> line into a device having inputs each of which is characterized as 75 ohm. If
> I connect 150 ohm in parallel with each input, this should correctly
> terminate the line. But should I place these 150 ohm resistors before or
> after the coupling capacitors? Any opinions or insights are welcome.
>
> Thanks,
> Stephen Greenhalgh
>
>
-- Binary/unsupported file stripped by Ecartis --
-- Type: text/x-vcard
-- File: dave.instone.vcf
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
