HD74AC Datasheet, PDF (37/52 Pages) Hitachi Semiconductor – HD74AC Series Common Information

Design Considerations
a: No Termination
b: Series Termination
c: Parallel Termination
d: AC Parallel Termination
e: Thevenin Termination
Figure 8 Termination Schemes
There are several termination schemes which may be used (figure 8). Included are series, parallel, AC
parallel and Thevenin terminations. AC parallel and series terminations are the most useful for low power
applications since they do not consume any DC power. Parallel and Thevenin terminations cause high DC
power consumption.
2.1 Series Terminations
Series terminations are most useful in high-speed applications where most of the loads are at the far end of
the line. Loads that are between the driver and the end of the line will receive a two-step waveform. The
first wave will be the incident wave. The amplitude is dependent upon the output impedance of the driver,
the value of the series resistor and the impedance of the line according to the formula
V_W = V_CC · Z_oe/(Z_oe + R_S + Z_S)
The amplitude will be one-half the voltage swing if R_S (the series resistor) plus the output impedance (Z_S)
of the driver is equal to the line impedance. The second step of the waveform is the reflection from the end
of the line and will have an amplitude equal to that of the first step. All devices on the line will receive a
valid level only after the wave has propagated down the line and returned to the driver. Therefore, all
inputs will see the full voltage swing within two times the delay of the line.
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