HD74AC Datasheet, PDF (40/52 Pages) Hitachi Semiconductor – HD74AC Series Common Information

Design Considerations
4.1 Crosstalk
The problem of crosstalk and how to deal with it is becoming more important as system performance and
board densities increase. Crosstalk is the capacitive coupling of signals from one line to another. The
amplitude of the noise generated on the inactive line is directly related to the edge rates of the signal on the
active line, the proximity of the two lines and the distance that the two lines are adjacent.
Crosstalk has two basic causes. Forward crosstalk, figure 11, is caused by the wavefront propagating down
the printed circuit trace at two different velocities. This difference in velocities is due to the difference in
the dielectric constants of air (Er = 1.0) and epoxy glass (Er = 4.7). As the wave propagates down the trace,
this difference in velocities will cause one edge to reach the end before the other. this delay is the cause of
forward crosstalk; it increases with longer trace length, so consequently the magnitude of forward crosstalk
will increase with distance.
Reverse crosstalk, figure 12, is caused by the mutual inductance and capacitance between the lines which is
a transformer action. Reverse crosstalk increases linearly with distance up to a critical length. This critical
length is the distance that the signal can travel during its rise or fall time.
Although crosstalk cannot be totally eliminated, there are some design techniques that can reduce system
problems resulting from crosstalk. FACT’s industry-leading noise margins make systems immune to
crosstalk-related problems easier to design. FACT’s AC noise margins, shown in figures 13 and 14,
exemplify the outstanding immunity to everyday noise which can effect system reliability.
0.0 V
Time (ns) (5.0 ns/div)
Key
Vertical Scale Horizontal Scale
Active Driver
1.0 V/Div
50 ns/Div
Forward Crosstalk 0.2 V/Div
5.0 ns/Div
Active Receiver 1.0 V/Div
5.0 ns/Div
This figure shows traces taken on a test fixture designed to
exaggerate the amplitude of crosstalk pulses.
Figure 11 Forward Crosstalk on PCB Traces
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