HD74AC datasheet(40/52 Pages) HITACHI

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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.

Time (ns) (5.0 ns/div)

Key

Active Driver

Forward Crosstalk

Active Receiver

This figure shows traces taken on a test fixture designed to

exaggerate the amplitude of crosstalk pulses.

Vertical Scale

1.0 V/Div

0.2 V/Div

1.0 V/Div

Horizontal Scale

50 ns/Div

5.0 ns/Div

0.0 V

Figure 11 Forward Crosstalk on PCB Traces

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