AN 672: Transceiver Link Design Guidelines for High-Gbps Data Rate Transmission

ID 683624
Date 1/29/2020
Public Loose vs. Tight Coupled Traces

The decision to use loose vs. tight coupling is mainly a trade-off between routing density and impedance control.

Table 4.  Loosely vs. Tightly Coupled Trace Routing
Routing Advantage Disadvantage
Loosely Coupled
  • Thinner dielectrics required for the same trace width
  • Less sensitivity to trace-to-trace variations provides better impedance control
Consumes more area vs. tight coupling
Tightly Coupled
  • Higher routing density
  • Smaller trace width for the same trace impedance
  • Better common mode noise rejection
Impedance control highly sensitive to trace-to-trace variations

For example, differential pair length matching typically requires serpentining of one leg of the differential pair to maintain P to N skew. For loosely coupled traces, the serpentining does not drastically alter the differential impedance of the trace. However, for tightly coupled traces, the change in the trace-to-trace separation can significantly change the nominal differential impedance of the pair beyond the ±10% tolerance. When applying serpentine routing, it is best to deskew after each bend or node that causes the trace lengths to be mismatched. Doing so helps reduce common mode noise incurred along the signal path.

Figure 16. Differential Pair Length Matching

Differential Pair Length Matching

Table 5.   Loosely vs. Tightly Coupled Routing Impedance Control
Routing Topology Dielectric Constant (Er) Trace Width (W) Trace Separation (S) Height above reference plane (H) Zdiff (Ω)
Loosely coupled microstrip 3.7 6 mils 12 mils 4 mils 100
Loosely coupled microstrip 3.7 6 mils 18 mils 4 mils 102
Tightly coupled microstrip 3.7 6 mils 6 mils 4.8 mils 100
Tightly coupled microstrip 3.7 6 mils 12 mils 4.8 mils 112
Note: Loosely coupled traces are easier to route and maintain impedance control but take up more routing area. Tightly coupled traces saves routing space but can be difficult to control impedance.