Lesson 9: Differential Pairs

Differential pairs are groups of two opposing signal nets routed together to create effective noise cancellation and a robust signal path. This walk-through demonstrates how to define differential pairs in the schematic in OrCAD X Capture 25.1. By defining differential pairs, traces will be automatically routed together, and specific rules can be defined for the PCB layout including maximum uncoupled length, primary gap, neck gap, and more. Differential pairs can also be created during the PCB layout. After you complete the topic, you will be able to:

• Define differential pair manually
• Define differential pairs automatically

To follow along, continue with the design from the last topic or use the provided design files. If the design files were not downloaded in the beginning of the Capture Walk-Through, the design files for this lesson can be found under the Materials tab.

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Creating Differential Pairs Manually

Step 1: Select capture_tutorial.dsn in the project hierarchy.

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Creating Differential Pairs in the Schematic with OrCAD X Capture

Step 2: Select Tools > Create Differential Pair from the menu.

Step 3: Select net D+ from the net list.

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Selecting Differential Pair Nets in OrCAD X Capture

Step 4: Select the > arrow to move the net to the Selections list.

Step 5: Select net D- from the list.

Step 6: Select the > arrow to move the net.

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Naming a Differential Pair

Step 7: Enter DPD into the Diff Pair Name field.

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Creating a Differential Pair in OrCAD X Capture

Step 8: Click Create to create the differential pair.

Note: By defining a differential pair, the nets of the differential pair will be routed together in the PCB.

Creating Differential Pairs Automatically

Note: In this walk-through, we will delete the differential pair only to show the automatic process of creating a differential pair. For typical designs, use either manual or automatic creation.

Step 9: Select Delete to clear the differential pair definition.

Step 10: Select Auto Setup to open the Automatic Setup window.

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Defining a Differential Pair Prefix

Step 11: Automatic setup automatically identifies nets with the defined suffixes and assigns differential pairs based on:

• Defined prefix
• Positive suffix filter
• Negative suffix filter

Enter DP into the Prefix field.

Step 12: Enter + into the + Filter field.

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Defining the Differential Pair Suffixes

Step 13: Enter – into the – Filter field.

Note: Use the TAB key to quickly jump to the next field.

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Generating a Differential Pair in OrCAD X Capture

Step 14: Click the Diff Pair window to generate the differential pair.

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Automatically Creating a Differential Pair in OrCAD X Capture

Step 15: Click Create to create the differential pair.

Step 16: Click Close to close the Automatic Setup window.

Step 17: Click Close to close the Create Differential Pair window.

This completes the lesson on differential pair creation. In the next lesson, you will learn how to configure and perform a design rule check of the schematic.