[SI-LIST] Re: Why 220ohms at driver end in PECL Terminations
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 7 Oct 2002 09:25:30 -0400
> The recommended Termination for PECL has a 220ohms to ground on each
> leg of
> the differential pair at the driver end. Can anyone explain what is
> the
> purpose of 220ohms. If it is to damp the reflections from the receiver
> why
> 220ohms ?. Is it good to design without the 220ohms.
ECL/PECL are similar to open-drain or open-collector outputs, in that
the driver only pulls one way. Open-drain outputs only pull down, and
you must have an external pull-up resistor. ECL/PECL outputs only pull
up, and you must have an external pull-down resistor. Otherwise, you
don't get any signal.
With ECL (and PECL is essentially the same), there were two standard
recommended "terminations".
(1) Wires longer than "several" inches (depending on rise/fall times)
require true terminations to control overshoot and ringing, typically
about 50-75 ohms (to match the impedance of your wires). The resistor
should be at the receiver end, and goes to Vtt. It serves the dual
purpose of terminating the transmission line, and providing the current
path to keep the output transistor conducting. (One could use a
two-resistor combination that was electrically identical to 50 ohms to
Vtt, without requiring a Vtt supply.)
(2) Shorter wires can have just a pulldown resistor to Vee (=ground when
using PECL). The resistor could be anywhere, driver or receiver end.
Typically 220 or 330 ohms to Vee works pretty well. The advantage is no
need for a Vtt supply, or less power dissipation than the two-resistor
Thevenin equivalent.
Other variations are possible, including pulldowns plus series
termination resistors. No matter what you do, you need to provide a DC
path from the output pin to a more negative voltage (Vtt or Vee), or it
doesn't work.
If you have AC (capacitor) coupling between one stage and the next, you
need to put a pulldown resistor before the capacitor (i.e., somewhere
near the driver), even if the line is long and there is also a matched
termination at the receiver end. The pulldown does not terminate or
damp reflections. However, in this case there is an extra thing to
consider. The pulldown resistor is needed not only to keep the
transistor conducting, but also because the current to the AC coupled
load, flows both ways. So the pulldown resistor has to be small enough
to keep the transistor conducting a little, even when the output
switches from high to low and the resistor sinks the transient load
current too.
Andy
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