# [SI-LIST] Re: [Help] How to do termination in differential signal with series termination ?

• From: steve weir <weirsi@xxxxxxxxxx>
• To: "Kiran Kumar Kalsani Narsaiah, TLS,Chennai" <Kirankumar.k@xxxxxx>
• Date: Thu, 04 Dec 2008 23:12:59 -0800

```CM refers to all in-phase energy between multiple lines.  It is most
often discussed in the context of a differential pair, but applies for
any number of signals greater than one.  An unsaturated CM choke
operating within its inductive or resistive regions concentrates most of
the flux from multiple lines into a small volume, creating an impedance
mismatch that blocks reflects most of the energy so that it does not
propagate forward.

Steve.

Kiran Kumar Kalsani Narsaiah, TLS,Chennai wrote:
> CM signal is unwanted only when we are talking about differential
> common mode signal by using common mode choke. How this thing works
> exactly? Can we use for all frequency or any particular range.
>
> Regards,
> Kiran
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of olaney@xxxxxxxx
> Sent: Friday, December 05, 2008 12:03 PM
> To: yardala@xxxxxxxxx
> Cc: weirsi@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: [Help] How to do termination in differential
> signal with series termination ?
>
> Series termination implies a round trip reflection, and that is exactly
> what you don't want to do with a common  mode signal unless the intent
> is
> termination at the receiving end to parallel terminate both modes, pi or
> Y configuration depending on relative impedances.  You can use just two
> resistors if Zcm = Zdiff/4, with each = Zcm/2 to ground.  Better yet,
> suppress the CM signal at the sending end with a CM choke.  I presume
> that any CM signal is unwanted, and not some sort of independent channel
> as is sometimes used in telephony, or for PoE (power over ethernet).
>
> Orin Laney
>
> On Fri, 5 Dec 2008 14:20:09 +0800 "King Da" <yardala@xxxxxxxxx> writes:
>
>> Dear Steve,
>> "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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>>>>
>>> --
>>> Steve Weir
>>> Teraspeed Consulting Group LLC 121 North River Drive Narragansett,
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--
Steve Weir
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882

California office
(707) 780-1951 Fax

Main office
(401) 284-1840 Fax

Oregon office
(503) 430-1285 Fax

http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property of
Teraspeed Consulting Group LLC
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```