[SI-LIST] Re: [Help] How to do termination in differential signal with series termination ?
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: King Da <yardala@xxxxxxxxx>
- Date: Fri, 05 Dec 2008 09:20:43 -0500
Hi Yardala,
Just a generic clarification about the naming conventions of terminations.
With point-to-point one-directional interconnects, we can terminate at
either or both ends. For non-demanding
applications it may be enough to terminate one end only. For demanding
applications we usually have to terminate
both ends (reason: nothing is perfect, so whatever small reflection we
may have from the termination at one
end, will be further attenuated by the termination at the other end).
When we terminate
at the driver, we usually call it source (or near-end) termination.
When we terminate at the far end, we call it
receiver (or far-end) termination. These names refer to the location of
the termination. The other usual names,
series, or parallel, refers to the way how we connect the termination,
and it may refer to either source or
receiver terminations. When the source has low impedance compared to
the interconnect, we have to increase
it by adding a series impedance, hence the name series termination.
When the electronics has much higher
impedance compared to the interconnect impedance, we have to add
something in parallel to bring it down,
hence the name parallel termination. But eventually either source or
receive termination can be either series
or parallel, it just depends on the numbers. Low-impedance drivers
require series terminations. High-impedance
(current-mode) drivers require parallel termination. High impedance
receivers require parallel termination.
The above three are the typical scenarios. The fourth combination,
far-end series termination, is also possible,
though may be very rare. It requires a low-impedance receiver, like
feeding a negative-feedback operational
amplifier at its quasi-ground inverting input.
Regards,
Istvan Novak
SUN Microsystems
King Da wrote:
> Dear Steve,
> Thanks for your answer, but I don't really know what your mean "you've got
> your impedances backward" ? Could you explain more detail ?
> I use 100 Ohm for differential termination for diffrential signal and also
> want use series termination for each line for common mode signal
> termination. How to choose the series resistor value ?
>
>
> On 12/5/08, steve weir <weirsi@xxxxxxxxxx> wrote:
>
>> Parallel terminations absorb energy at the far end and is related only to
>> the channel, not the driver Z. Also, you've got your impedances backward.
>>
>> Steve.
>> King Da wrote:
>>
>>
>>> Hi,Folks,
>>> I have a differential pair which odd mode impedance is 48 Ohm and even
>>> mode
>>> impedance is 108 Ohm by TDR measurement.
>>> I would like use 100 Ohm for differential termination at far end, but how
>>> to
>>> choose series termination resistor for each trace at near end?
>>>
>>> I refered Bogatin's "Signal Integrity:Simplified" book that said parallel
>>> termination R2 equal Zeven, So, I should use 108 Ohm subtract 20 Ohm of my
>>> driver's Ro is 88 Ohm for my series termination at near end, am I right?
>>>
>>> The Rseries = 108 (Zeven) - 20 (Driver's Ro) = 88 Ohm, right ?
>>>
>>> Does the differential termination resistor is 100 Ohm better than 91Ohm ?
>>>
>>> Thanks,
>>>
>>> Yardala
>>>
>>>
>>>
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