Cadence PCB Best Practices

Using Differential Pairs in Allegro PCB Editor February 9, 2012 13 Product Version 16.5 Example - Differential Pair Spacing For maintaining a larger line-to-line spacing between differential pair objects to other nets as compared to differential pairs to differential pairs, a spacing constraint could be created and assigned. This example shows how to set up differential pairs in Constraint Manager so they are spaced 20 mils from other nets. Specific line-to-line spacing is maintained between objects of differential pairs. Assume that you have a requirement that differential pairs (DP) must have 20 mils spacing between each DP object and any other non-DP net while the spacing between all non-DP nets is 5 mils. To accomplish this task, refer database. It can be found at 'Attachments' sections below the PDF. This pdf can be searched with title on https://support.cadence.com. The dp_16_space1.brd has been already setup with the required constraints. The board dp_16_stripped1.brd is the same board, but does not have the constraints. This board can be used with the following information. Procedure to follow (assuming DPs are already set up): 1. Create an ECSet which will contain the DP settings. a. In the ELECTRICAL domain of Constraint Manager, select the Electrical Constraint Set > Routing > Differential Pair worksheet. b. Right-click on the design name object and choose Create > Electrical CSet from the menu. c. Enter a meaningful name (DP_10, for example) and click OK. d. Enter the DP electrical constraints (you may need to click on the "+" to expand the rows). Note: Repeat steps b-d for additional constraints with different values. 2. Assign the ECSet to the appropriate DPs. a. In the ELECTRICAL domain of ConstraintManager, select the Net > Routing > Differential Pair worksheet. b. Click in the Referenced Electrical CSet Constraint Values Used: ======================= ELECTRICAL Domain DP_10 ECS 3.0 Min Line Spacing (must be set to avoid L-L DRCs of the DP) 10.0 Primary Gap 8.0 Primary Width 4.0 Neck Gap 4.0 Neck Width 1.0 (+) Tolerance 1.0 (-) Tolerance SPACING Domain DEFAULT SCS 5.0 All Constraints DP_SP SCS 20.0 Line to Line 5.0 Remaining constraints column for the DP row(s) and select the DP_10 ECSet from the pull-down.

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