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 MIL-HDBK-419A
d.
Connect amplifier output guard shield to data system ground bus.
e.
If a permanent unavoidable instrumentation ground exists at test area as well as at the data system,
use isolated differential amplifiers to break the ground loop.
3.5.2.2 Digital Data Systems. A digital circuit operates by recognizing the state of a two-level voltage or
current signal. The speed of the system is determined by the speed at which the levels can be changed.
Because of the capacitive and magnetic coupling effects resulting from the very fast rise and fall times of
digital pulses, take every precaution to minimize the effects.
Twist clock lines with their return leads to minimize the magnetic field near such lines.
a.
b.
Use point-to-point wiring as much as possible to reduce capacitive coupling.
Provide multiple paths in the ground wiring to distribute the ground current among several wires.
c.
Use electrostatic shielding with care to avoid excessive loading of data lines.
d.
Wire all digital circuits using shortest wire length possible.
e.
All ground wires must converge to system common ground point.
f.
g.
Maintain maximum distance between digital circuits and low level analog circuits.
In a system where both analog and digital circuits must be housed in the same equipment cabinet,
h.
keep as much physical separation between them as possible, e.g., at opposite ends of the cabinet. A common
ground plate for the system can be located in the center of the cabinet or two ground plates can be utilized, one
for analog ground and one for digital ground. These two ground plates must then be tied together with a low
inductance bus and then tied to the system ground bus line.
3.5.2.3 Recording Devices.
3.5.2.3.1 Magnetic Tape Recorders.
Magnetic tape recording systems are mostly single-ended and should therefore conform to much the
a.
same grounding practices as recommended for digital systems.
Earth ground should be made at the test area with a single 1/0 cable extending to an isolated
b.
grounding plate within the tape cabinet.
When using only one recording device for a given channel, the channel may be connected directly
c.
into the recorder as shown in Figure 3-31.
3-40
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