[SI-LIST] Re: termination with ac coupling

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
>
>   


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