SG1625 National Semiconductor Corporation, SG1625 Datasheet - Page 7

• Manufacturer Part Number: SG1625
• Description: DS0026 - 5 MHZ Two Phase MOS Clock Driver Life-time Buy , Package: Soic Narrow, Pin Nb=8
• Manufacturer: National Semiconductor Corporation

Application Hints
Lastly, the clock lines must be considered as noise genera-
tors. Figure 5 shows a clock coupled through a parasitic
coupling capacitor, C
by a 7404. A parasitic lumped line inductance, L, is also
shown. Let us assume, for the sake of argument, that C
1 pF and that the rise time of the clock is high enough to
completely isolate the clock transient from the 7404 because
of the inductance, L.
With a clock transition of 20V the magnitude of the voltage
generated across C
FIGURE 5. Clock Coupling
L
C
is:
, to eight data input lines being driven
(Continued)
00585320
C
is
7
This has been a hypothetical example to emphasize that
with 20V low rise/fall time transitions, parasitic elements can
not be neglected. In this example, 1 pF of parasitic capaci-
tance could cause system malfunction, because a 7404
without a pull up resistor has typically only 0.3V of noise
margin in the “1” state at 25˚C. Of course it is stretching
things to assume that the inductance, L, completely isolates
the clock transient from the 7404. However, it does point out
the need to minimize inductance in input/output as well as
clock lines.
The output is current, so it is more meaningful to examine
the current that is coupled through a 1 pF parasitic capaci-
tance. The current would be:
This exceeds the total output current swing so it is obviously
significant.
Clock coupling to inputs and outputs can be minimized by
using multilayer printed circuit boards, as mentioned previ-
ously, physically isolating clock lines and/or running clock
lines at right angles to input/output lines. All of these tech-
niques tend to minimize parasitic coupling capacitance from
the clocks to the signals in question.
In considering clock coupling it is also important to have a
detailed knowledge of the functional characteristics of the
device being used. As an example, for the MM5262, cou-
pling noise from the 2 clock to the address lines is of no
particular consequence. On the other hand the address
inputs will be sensitive to noise coupled from 1 clock.
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