Digital Signal Processing Reference
In-Depth Information
DO
+
DO
−
(a)
DO
+
DO
−
(b)
Figure 13.16
(a) Bends in diff-pairs cause
DO
+ to be longer than
DO
. (b) Back-to-back bends can
−
readjust this skew.
matched, and the lengths of the two stripline traces should be matched. This ensures
that the delay and impedance of each segment is the same.
13.6.10 Ensure That Noise Is Equally Coupled to Both Traces
Subtle mechanisms can cause unequal noise coupling on the diff-pair traces. An ex-
ample showing the effects on a broadside-coupled pair placed between two return
planes appears in Figure 13.17.
Noise present on plane
L
1 will preferentially couple onto the
L
2 trace, and
noise on
L
4 will couple onto
L
3. The traces forming the diff-pair are subjected to
different noise unless the planes have a low impedance between them and have the
same noise voltages. If both
L
1 and
L
4 are ground planes, placing numerous vias
between the planes (plane stitching) can lower the impedance between the planes.
Alternatively, if one is a power plane, decoupling capacitors can be used to provide
an AC connection between the planes. In this way the same noise appears on both
planes, making the noise coupled from
L
1 to
L
2 identical to the noise coupled from
L
4 to
L
3.
Another effect that can occur with edge coupled diff-pairs (either stripline or
microstrip, but not with broadside pairs) is shown in Figure 13.18.
