HD74AC Datasheet, PDF (42/52 Pages) Hitachi Semiconductor – HD74AC Series Common Information

Design Considerations
Volts
V_CC ³
V_CC = 5.0 V
Gnd
70%
HAZARD
2
SAFE
1
0
0 5 10 15 20 25 30 35 40 45 50
Pulse Width (ns)
Figure 14 Low Noise Margin
With over 2.0 V of noise margins, the FACT family offers better noise rejection than any other comparable
technology.
In any design, the distance that lines run adjacent to each other should be kept as short as possible. The
best situation is when the lines are perpendicular to each other. For those situations where lines must run
parallel, the effects of crosstalk can be minimized by line termination. Terminating a line in its
characteristic impedance reduces the amplitude of an initial crosstalk pulse by 50%. Terminating the line
will also reduce the amount of ringing (figure 15). Crosstalk problems can also be reduced by moving lines
further apart or by inserting ground lines or planes between them.
Signal on Active Line
Passive Line
Noise Amplitude at A
Versus R
R
V _R+ZO
A
Z_O
R
Vn = Coupled Signal
V_T
100% : N_O R
67% : R = 2Z_O
60% : R = 1.5Z_O
50% : R = Z_O
Figure 15 Effects of Termination on Crosstalk
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