[SI-LIST] Re: Why Termination at Both End ?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 23 Aug 2011 22:49:17 -0700
Sen, for electrically long lines, that topology introduces about 13% -
14% ISI. Arrange an electrical length that happens to be close to half
of one phase interval and the ISI will step right onto the incident edge
of your timing strobe which is boo hiss bad stuff.
I am not saying that there are never places where you would want to
compromise this way. If you don't have the amplitude to spare
termination at both ends then its something you can do, provided you do
the rest of the homework to make sure you don't get yourself into
trouble. In that spirit, I would recommend only applying a band-aid
like this where one either is absolutely certain from the back of the
envelope that ISI won't be a problem, or one takes the time to simulate.
Steve.
On 8/23/2011 4:07 PM, Sen Velmurugan wrote:
> Steve,
> Here the buffer is a current source, the termination(pulldown) is the
> load, the high input impedance receiver see the voltage across the load.
> The foot print at destination does offer some(pf) capacitance. These
> layouts are done in the absence of simulations and to clean up mismatches.
> Usually done for clock and analog video signals.
> Thanks
> Sen
>
>
>> Msg: #2 in digest
>> Date: Mon, 22 Aug 2011 16:55:47 -0700
>> From: steve weir<weirsi@xxxxxxxxxx>
>> Subject: [SI-LIST] Re: Why Termination at Both End ?
>>
>> Sen, are you suggesting this:
>>
>> Tx side: Isink || (Pull-up = 2*Zline) --> Tx Line ---> (Pull-up =
>> 2*Zline) Rx side
>>
>> In this case you are mismatched at both sides and you will have ISI due
>> to reflections.
>>
>> If you have an RLL signal stream, from an ISI point of view you would be
>> better off with:
>>
>> Tx side: Isink || (Pull-up = Zline) --> Tx Line ---> ( R=Zline --> C
>> --> Vss ) || Receiver
>>
>> Where RC> 5X the maximum run length.
>>
>> Steve
>>
>> On 8/22/2011 4:17 PM, Sen Velmurugan wrote:
>>
>>> Rajan, When you need Xohm termination to sink a current source, you can
>>> use 2Xohm termination each at source and destination.
>>> This way source see, 2X || 2X = X ohm sink and voltage at load is
>>> "cleaner". Basically, you have more freedom of choice to play with.
>>>
>>> ~Sen.Velmurugan@xxxxxxxxxx
>>>
>>>
>>>
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