[SI-LIST] Re: CML versus ECL/PECL

Hello Wolfgang
My two cents on the advantages of CML. The first being for high speed designs 
the fundamental limitation is the load capacitance, It is the the load 
capacitance that determines the switching speed. In the case of CML the load 
capacitances are charged or discharged depending on the signal swing,thus 
enabling faster switching. I assume your signal swings are extremely small at 
high speeds. The other advantage I found with CML is that 
 since we have a  NMOS current sink providing constant current  , switching 
noise is greatly reduced, thus helping with the speed of operation.
Regards
Srivats

--
P.Srivatsan

--
I have no special talents. I am only passionately curious. AE

----- Original Message ----
From: "wolfgang.maichen@xxxxxxxxxxxx" <wolfgang.maichen@xxxxxxxxxxxx>
To: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
Sent: Thursday, December 20, 2007 2:13:29 PM
Subject: [SI-LIST] CML versus ECL/PECL


Here is a question to all design experts: I have been wondering for a 
while what the design advantages (or disadvantages) of CML signaling
  vs. 
ECL/PECL signaling are. I can think of a few, but I may be wrong, and
 I'm 
sure there are more considerations:
- both schemes are typically differential, so no advantage here

- I noticed most high-speed drivers use CML; I guess this is because it
 
can easily provide double termination (50 Ohm internal pullup at driver
 
and 50 Ohm pullup at receiver) which is important for signal integrity
 at 
10+ Gb/s

- but I guess that combined 25 load means CML is typically more power 
hungry

- ECL/PECL needs a third voltage (Vcc - 2V) for end termination while
 CML 
can usually be terminated to Vcc. Of course I can use a approx. 150 Ohm
 
pulldown to GND for the driver (assuming 3.3V PECL) to get around this,
 
and add a series resistor at the driver and bridge termination at the 
receiver, but that makes for a smaller swing - not good.

- CML can driver larger swings, while ECL/PECL is normally restricted
 to 
standard values (0.4V or 0.8V swing, respectively).

I'm looking forward to more insight on this.

Regards,

Wolfgang


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