HD74AC datasheet(42/52 Pages) HITACHI

42 / 52 page

Design Considerations

Gnd

VCC

70%

5 10 15 20 25

Pulse Width (ns)

30 35 40 45 50

Volts

VCC = 5.0 V

HAZARD

SAFE

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

Noise Amplitude at A

Versus R

Passive Line

Vn = Coupled Signal

100% : NO R

R+ZO

67% : R = 2ZO

60% : R = 1.5ZO

50% : R = ZO

Figure 15 Effects of Termination on Crosstalk

HD74AC Datasheet(PDF) 42 Page - Hitachi Semiconductor