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 MIL-HDBK-419A
m. Use internal walls and compartments to limit propagation of interference into and out of equipment
subassemblies.
n.
Separate high level sources from sensitive receptors with internal shields such as panels or
partitions.
3.5 COMMON-MODE NOISE CONTROL AND INSTRUMENTATION GROUNDING.
3.5.1 Common-Mode Noise Control.
Primarily, common-mode noise is reduced by either (1) controlling its magnitude in the vicinity of the
susceptible circuit, network, or device, or (2) employing those practices which minimize the coupling to the
existing voltages, currents, or fields. The first alternative is accomplished by physically separating the source
from the susceptible circuits, shielding the source, reducing the current or voltage level at the source, and
minimizing any impedance shared in common by the desired and undesired currents. The second alternative
involves grounding only one side of a noise voltage source, balancing of signal lines, operating in a differential
mode, striving for a zero-resistance reference plane, shielding of signal lines, using a less susceptible mode of
signal transmission, minimizing signal loop pickup area, or combinations thereof. (See Volume I, Section 6.4 for
additional information on these minimization techniques.)
In some situations, one of these techniques may be sufficient (if properly implemented) to reduce the noise level
to an acceptable level. In general, however, combinations of elements of both alternatives will be necessary.
To decide which combinations offer the greatest advantages for least cost, the equipment or system must be
carefully analyzed to determine its specific operational properties (or its requirements); the anticipated
electromagnetic environment should be carefully examined in order to ascertain the specific type of protection
that will probably be required; then these two sets of factors must be compared with each other to determine
the steps to be taken to achieve effective and reliable equipment or system operation.
Common-mode noise control is primarily the application of proper grounding, bonding, and shielding practices in
combination with careful circuit and equipment functional design to maximize the signal to noise ratio within
the overall system. In particular, the lower frequency practices of Section 3.2.1.1 should be emphasized.
Instrumentation Grounding. (3-5)
3.5.2
Many, if not most, data instrumentation systems are concerned with the measurement or detection of physical
phenomena (or changes in them) that require periods of observation or measurement that range from a few
milliseconds to several minutes or longer. Because of the relatively slow nature of the event, the fundamental
frequency of the transducer output may range from zero (dc) to a `few hundred hertz. Power distribution
systems, electromechanical switches, and atmospheric noise produce extraneous voltages whose energy content
is strongly concentrated within this lower frequency region. Because of this overlap of signals, special
techniques are generally required to keep the voltages or currents produced by the extraneous sources from
obscuring the transducer outputs.
Data instrumentation systems may employ either analog or digital signals or a combination of both. The
methods of grounding analog and digital systems are considered separately although the physical principles of
noise reduction for both are basically the same.
3-34
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