Digital Signal Processing Reference
In-Depth Information
An explanation and example violations for each rule appear in the following
sections.
13.6.1 Obtaining the Proper Differential Impedance
Differential pairs are so common in modern designs that nearly all circuit board
fabrication shops now have experience in creating them. This is especially true
for edge-coupled 100
diff-pairs that are wider than the minimum allowed trace
width. For instance, most shops can reliably create an edge coupled 100
Ω
diff-pair
using 8-mil-wide (or wider) traces. High-end shops can reliably form 100
Ω
Ω
diff-
pairs with traces less than 5 mils wide.
It is important not to overspecify when discussing the requirements with the
circuit board shop. For instance, it is proper to specify that the traces are to be a
100
differential pair using traces 8 mils in width and that the traces are to be
spaced as close together as possible. This gives the shop freedom to adjust the trace
width (bias the trace), spacing, and laminate thickness based on their experience
manufacturing similar boards. However, it is undesirable to tell the shop that the
traces are to be 8 mils wide and have a 50
Ω
odd-mode impedance when spaced
8 mils apart. Although technically correct, this terminology not widely used by
circuit board manufacturers and is so restrictive that it might increase the board
manufacturing cost.
The second aspect of maintaining proper differential impedance involves trace
routing. Once the manufacturer has specified the trace separation necessary to ob-
tain the desired differential impedance, make sure the layout designer adheres to
this everywhere.
One example of how this rule is violated is shown in Figure 13.13.
In this example a diff-pair jogs around a via, which for a short distance increase
the trace-to-trace spacing. This encourages the traces to couple to the via rather
than to each other, altering the impedance.
The third aspect of maintaining proper differential impedance involves trace
width. It is sometimes necessary to intentionally make a trace narrow so it can be
routed through a connector or BGA pin field. This is especially likely with long
traces because they are often made very wide to reduce signal loss. Such wide
Ω
DO
+
DO
−
Figure 13.13
Top view of diff-pair traces jogging around a via and its antipad (shown as a broken
circle). This routing increases the differential impedance.
