How important are the specific termination values used for PoE pairs (pins 4&5, 7&8)?
Below is the recommended Ethernet front end for Micrel’s KSZ8841/42.
http://www.micrel.com/_PDF/Ethernet/app-notes/an-139.pdf
Would it be acceptable to use 49.9 ohm resistors in place of the 51ohm resistors? Why did they specify 49.9 on one side, and 51 ohms on the other? I’d like to make them the same in order to reduce the number of component types. This would not only help drive down the price of the 49.9 resistors but also mean there is one less component type to manage in inventory and load into the pick n place machine.
There must be a good reason for specifying slightly different resistor. I'm sure they didn't do it just for fun. How critical are the terminations of the PoE pairs? In my mind they are not critical because there is no data being transmitted over them, but I’m not sure how much their termination influences the characteristics of the cable for the other conductors.
Best Answer
Yes it would be totally acceptable to use 49.9 ohms instead of 51. Theoretically the value should be 50 ohms I think so 49.9 is even closer to that.
The terminations are there to terminate the transmission line which in this case is the ethernet cable. By terminating it properly you make sure any signals that are on the line (cable) dissipate in the resistor instead of reflecting back into the cable and distorting the signal.
For unused ethernet pairs it is probably not that critical but it can be that due to capacitive coupling there could be some signals on the unused pairs. By aso terminating these properly also these signals cannot reflect and disturb the other signals. Also it's just good practice.