Differential Pairs (High-Speed)
Definition: Rules for routing differential pairs, which are pairs of traces that carry equal and opposite signals.
Differential pairs showing signal propagation with Static Phase vs. Dynamic Phase.
This blog on our website explains two important concepts for differential pairs, inter pair skew and intra pair skew:
"Ideally, synchronized signals need to arrive at their destination simultaneously to ensure there is no loss of data arising from
timing issues. In reality, however, differences (even minute) in the propagation paths mean no two signals will ever arrive at
the same time. For greater ease of use, components are forgiving and allow for some timing mismatch – known as clock skew
– within tolerable limits.
Timing mismatches are most heavily reliant on the length of the traces, resulting in two variations:
f
Intra pair skew between two lines of a differential pair, which indicates the timing difference between the positive and
negative signal lines. Keeping the mismatch low between the two differential signals allows for more headroom and better
noise protection in balanced lines.
f
Inter pair skew indicates the timing difference between signals used in data formats that do not have an embedded clock
signal. Inter-pair skew can cover both single-ended and differential pairs within a data format, making some signals doubly
constrained by skew."
Example: For this example, we will use Constraint manager to ensure differential pairs are routed together with consistent
spacing to maintain signal integrity for dynamic signals and for overall length matching.
1. Within the Constraint Manager, go to the Electrical > Electrical Constraint Set > Routing > Differential Pair worksheet.
2. Create a constraint set (e.g. ECS1).
3. Fill in values for Static Phase Tolerance and Dynamic Phase Tolerance as shown below.
68 www.cadence.com
OrCAD X High-Speed Digital Design Guide
