Hi. I am designing a board which uses MC10EL11, ECL fanout chips. After reading their data sheet and AND8020/D, I got a bit confused. Can anyone confirm that my termination plan will work?
I will have 100 ohm resistor across pins 6 and 7. Also, I will have 220 ohm resistors across 5 and 1, 5 and 2, 5 and 3, and 5 and 4. Ideally I want to use 235 ohm ones, but I couldn't find such things.
Thank you for your help.
The 100 ohm differential termination on the input (pins 6 and 7) will only properly terminate a fully differential signal (not a single-ended signal), but only if the device driving this input can effectively drive this differential load, and the input traces are either 50-ohm lines or a 100-ohm differential pair. So, without knowing what is driving this fanout buffer, we can't answer this first part properly.
The pull-down resistors on the two sets of differential outputs will adequately bias the emitter follower outputs. However, they will not help to terminate a long PCB trace or transmission line. If the output traces are short (an inch or less), then you'll probably be OK. However, if the lines are longer than this, then you'll have to consider a different termination scheme to precent reflection problems. Very good results can be obtained by putting proper terminations at the far end of the lines (opposite end from the driver). Proper ECL terminations will present a 50ohm load that is terminated to a VCC-2V potential. This properly terminates the 50-ohm transmission line (to avoid reflections), and also presents a proper bias to the emitter follower outputs of the driver. A split termination can be used on each line which presents the thevenin equivalent of 50ohm to VCC-2V. For typical 5V ECL/PECL applications, a split termination is made by putting an 82ohm resistor from VCC to the line, and then a 130ohm resistor from the line to VEE. If you are routing the signals differentially, then other delta or Y style terminations can be used which don't draw standing power like the spit termination does.
Lots of good information on proper handling of ECL/PECL circuits can be found on Freescale Semicondutor's website (formally On Semiconductor, formally Motorola Semiconductor).
Thank you. I will drive this device with fully differential ECL signals and 100 ohm differential pair input traces.
OK. You can save the standing current of the split terminations by using either a 'delta' or 'wye' differential termination. These terminations are to be placed at the receiving end of the line, not the driven end. Thus, you'll have a termination at pins 5&6, and then at the ends of the lines driven by 1&2, and 3&4.
Assuming VCC-VEE=5V: The delta termination consists of a 120-ohm resistor connected between the differential lines, and a 270-ohm resistor connected from each line to VEE.
The wye termination consists of two 50-ohm resistors in series, connected between the differential lines, and a 110-ohm resistor connected from the junction of the two 50-ohm resistors to VEE.
Each of these terminations does a nice job of approximating the ideal
50-ohm to VCC-2V termination that ECL likes to see. NOTE - if your power supply (VCC-VEE) does not equal 5.2V, then you'll have to recalculate the resistors slightly.I hope this helps!
Correction: The values shown are valid for VCC-VEE=5.2V (not 5V).
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