Electronic – preferred placement of termination resistor for a differential clock line


Looking at a reference design board from Xilinx, we noticed the placement of the termination resistor for a differential clock line (~300 MHz):

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The resistor is placed not at the closest possible location, which would be right under the vias which receive the clock (there appears to be a lot of space), rather, it is placed "far away", to the left of the vias. We are wondering if there is a good reason to place a differential termination resistor that way. Any ideas?

Best Answer

Termination resistors sit at one of two places: As close to the driver as possible (for source termination), or as close to the end of the trace as possible (for the kind of termination that you have).

Notice that I said "as close to the end of the trace as possible", and not "as close to the receiver as possible"? There is a difference!

Most people think of the signal chain for end termination being like this: Driver --> Long Trace --> Termination --> Receiver. But that is incorrect. The termination should be at the end of the signal. Like this: Driver --> Long Trace --> Receiver --> Termination.

If you have several receivers, then you get this: Driver --> trace --> receiver1 --> trace -> receiver2 --> termination.

The reason for this is that the termination prevents the signal from reflecting back to the receiver(s) and corrupting their signal. In theory you could have a super long trace from the last receiver to the terminator and that would work just fine!

That is why your termination resistor is not near the driver-- because it doesn't have to be. The resistor is at the end of the line, right where it should be. There are other reasons why the resistor is placed that far away, but that isn't too important.

As for the little squiggles, that is because in a differental signal the lengths of the two traces should be identical in length. The Squiggles are done to adjust the length of one trace to make it match the other trace.