[SI-LIST] Re: Why 220ohms at driver end in PECL Terminations
- From: sdb@xxxxxxxxxx (Stuart Brorson)
- To: JineshwariB@xxxxxxxxxxxxxxx
- Date: Sat, 5 Oct 2002 09:03:04 -0400 (EDT)
ECL = emitter coupled logic. This means that the output emitters of
the transistors are tied to the output pins of the transmitting
package. A common practice is to use "pull down" resistors *directly*
at the output of the transmitting part in order to provide a DC path
for the emitter bias current to ground (to keep the transistors out of
cut-off and turned on). Typical pull down values are in the 220 --
330 ohm range. You can use the lower values to turn the transistor on
more and get somewhat faster rise/fall times at the expense of higher
power consumption.
The pull down has nothing to do with terminating the line. Also, you
*must* have some DC path to ground in order to turn the transistor
on. Don't remove the pull down unless you have a different way to
hold the transistor on. For example, another common practice is to
provide a totem pole term at the receiver, i.e. resistors between VCC
and the input pin and between teh input pin and GND. The resistors
are choosen to set the voltage at the reciver to be VTT = VCC - 2V,
and present a 50 ohm Thevenin equivalent resistance to the line.
Stuart
> 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.
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