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 MIL-HDBK-419A
d.
Should the tape channels receive data in parallel with other single-ended channels (such as arise in
A/D systems), take special care to minimize the effects of inherent loops. The optimum method of recording
with two single-ended devices in parallel from the same data channel is to use an amplifier with isolated
outputs. (In this way, the inherent loop is broken and noise is minimized.)
3.5.2.3.2 Strip Chart Recorders.
Strip chart recorders are mostly single-ended and should be grounded as described in Section
a.
3.5.2.3.1 for magnetic tape devices.
Since the strip chart recorder is a nulling device, its input impedance will change as it deflects from
b.
one position to another. This impedance change and the accompanying voltage feedback can be coupled directly
from the strip chart input over to the input of a paralleled device such as an A/D converter. Gross error can
result in the A/D channel. This difficulty can be resolved by using resistive isolation as shown in Figure 3-32 or
by employing dual amplifier outputs, one for each channel, as described in Section 3.5.2.3.1 for analog tape
systems.
3.5.2.3.3 X-Y Plotters. X-Y plotters are available in either digital or analog input configurations. The
digital type plotters are usually connected as peripheral devices to computers or A/D systems and should be
grounded in accordance with the recommended digital practices. Analog type X-Y plotters are normally
single-ended and should be grounded and connected in the same manner as described for strip chart recorders.
3.6 EMP CONSIDERATIONS. Those general design practices which are effective for the control of
electromagnetic interference generally are applicable to protection against EMP. The two factors of the EMP
threat to be given particular emphasis are (1) the strong magnetic field component and (2) the overall high level
of the incident field (see Volume I, Chapter 10).
Because of the magnetic field component, design practices which minimize magnetic pickup are
a.
most important. Practices such as minimizing loop pickup area by twisting signal conductors with their returns,
twisting of power supply conductors, routing conductors close to ground planes, and utilizing minimum length
conductors must be emphasized.
EMP shielding requires the use of materials which provide high attenuations at frequencies up to
b.
200 MHz. Because of saturation effects, the thickness necessary for complete shielding can lead to rather
heavy and bulky enclosures. Seams, joints, and apertures must be given the same careful attention and control
required in the construction of shields for buildings and structures. For these reasons, it is generally more
practical to shield the building or structure in which the equipment is located than it is to shield individual
pieces of equipment.
In a shielded area or facility, critical equipment should be located as far from corners,
c.
discontinuities, openings and penetrations as is practical. The most susceptible equipment should occupy the
center locations with the least susceptible equipment closer to the shield.
3-42
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