Allegro PCB Design Tutorial

Routing Differential Pair

Allegro provides a way to define the differential pairs in its Constraint Manager, so that you can route these signals as differential pair. The steps involved are

1. Goto Setup -> Constraints -> Spacing -> . It launches the Spacing Contraint Manager.

2. Click Objects -> Create -> Spacing C-set. Name the Spacing C-Set name such as DIFF-100 or anything. Assign the trace width and spacing that will make the impedance of the differential traces - the desired impedance. You can use the differential impedance Calculator to find the differential impedance.

3. Click on Net -> All Layers -> Line under the Spacing Constraint Set. Now select the two nets that you want to declare differential. Then Right click -> Create-> Differential pair. This will create the differential pair. Against the two rows select the required Referenced Spacing Set, you created in step 2.

4. This pair is now a differential pair. When you route them it will be routed as differential pair.

5. If during the routing, you wish to route it as individual traces, just click on the trace -> right click -> Select Single Trace Mode.

This youtube captures the complete sequence.