Ecl logic termination

[jovin555]

Why do ECL logic output needs 50 ohm termination?

 

[kenambo]

Hi,

ECL gate's output is taken from the Emitter... So there is no Pull down for ECl.. this has to be done externally..

Because internal termination causes more power if we done this in IC level..
And it also used for high speed applications .. so to reduce "Reflections" and ringing we have connect resistors externally

It may between 25-100 ohms... But generally 50 ohm is used..

Thanks..

 

[jovin555]

How do we design that resistor value?

 

[kenambo]

Hi,

Where are you using this circuit...

If you are designing this circuit in Chip level .. I think the termination should be done outside of the chip..

If you are designing it in series of logic gates then terminate the output internally and connect it to the next stage of the gate....

Thanks..

 

[dick_freebird]

ECL was designed with the presumption of 50-ohm
transmission line interconnects, so it wants that
termination. Why the kooky -2V-ish termination,
well, -5.2V would have meant a way bigger output
driver and supply current; a compromise was made
and now you get to respect it.

People know how to do 50-ohm traces, a good PCB
designer "just does it naturally, when told". Demand
a 125-ohm system and enjoy the befuddled stare.

Internal vs external, you want to look at the effects
of the parasitic inductances and where these are best
placed - between driver pads and between PCB traces,
will differ in the details and you may want to spend
some time modeling this well, if you're after ultimate
signal integrity. It could get complex; above 1GHz,
bet on it being a real good idea.

 

[FvM]

You'll understand the need for termination when you study the transistor level circuits.
Once you know the principle topology and chip operation conditions, you should be able to modify the circuits according to your specfic design requirements, e.g. speed, power, supply levels, interconnect.

 

[jovin555]

I am using MC100EP196 to produce different delayed pulse.the output is connected to a d flipflop.MC100EP31.and am using LVPECL logic.So what should be the termination resistance?Also i like to implement single ended thevenin resistance so i can avoid extra power supply 1.3V.So what would be the value of the resistance used in the resistor divider?