22.214.171.124. Loose vs. Tight Coupled Traces
The decision to use loose vs. tight coupling is mainly a trade-off between routing density and impedance control.
||Consumes more area vs. tight coupling|
||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.
|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|