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Design >> High-Speed I/O Design >> CML buffer driving off chip load
https://designers-guide.org/forum/YaBB.pl?num=1208454063

Message started by akki on Apr 17^th, 2008, 10:41am

Title: CML buffer driving off chip load
Post by akki on Apr 17^th, 2008, 10:41am

hii all,

I have some doubts regarding working of CML buffer with off chip load and shunt RC termination.
without Tx line i know its working is like: Current switches between two branches if with input voltage exceeding some limit, and the output at the two o/p nodes is Vdd & Vdd - IR, so we get the output voltage swing of I.R.

my question is how does it work with off chip load, cause my output do not match with and without off-chip load.

Title: Re: CML buffer driving off chip load
Post by loose-electron on Apr 17^th, 2008, 3:47pm

Frequently the structure you show is done with the resistors that are shown inside the chip actually placed outside the chip and at the far end of the termination line. That eliminates the need for the termination structures as shown.

A possible architecture to consider and there are other possibilities.

Jerry

Title: Re: CML buffer driving off chip load
Post by akki on Apr 17^th, 2008, 4:03pm

Thanks a lot sir for ur reply,
but my problem is a bit different,when i am designing a cml buffer for a op swing of 0.6v i only took care of back termination resistor, after adding transmission line my voltage swing reduces very much.

my question is for what voltage swing (at the differntial amp o/p )and hence current should i design cml buffer, so that i can get a Vpp of 0.6 v at the output of transmission line ?

thanks,
akki

Title: Re: CML buffer driving off chip load
Post by sugar on Apr 24^th, 2008, 12:23am

akki,

As you mentioned, voltage swing is equal to I*R
When you connect a transmission line to your driver, R is reduced. Actually in this case, R = RDN // Z0
therefore, you will see that you voltage swing is reduced a lot, usually reduce to half of your expectation.

--River

Title: Re: CML buffer driving off chip load
Post by akki on Apr 24^th, 2008, 1:23am

hello sir,
Thanx, it would really be helpful in further designing :)

-akki

Title: Re: CML buffer driving off chip load
Post by SATurn on May 30^th, 2008, 12:25pm

CML output drivers need a very careful design. The first question is that why your swing reduces. If it is because of load resistance in parallel to the termination, then you should double the current. The other way to avoid vasting that much power in the driver is removing the internal termination resistor and put some simple RC matching network. In this case, you will not increase the power by a factor of 2.

SATurn

Title: Re: CML buffer driving off chip load
Post by Vaibhav on May 8^th, 2009, 9:52am

Dear SATurn,
can u please elaborate on the method of using RC matching network and/or give some reference for it.

Thanks in advance
Vaibhav