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 MIL-HDBK-419A
3.5.2.1 Analog Systems. Since the analog signals are primarily lower frequency in nature, a basic single-point
ground should be implemented. The signal return line should be grounded at one end only or not at all (i.e., it
should be balanced). Similarly, shields around signal lines should be grounded at one end only.
3.5.2.1.1 Grounded Transducers. The bonded (grounded) thermocouple, illustrated in Figure 3-26, is used with
a single-ended data amplifier whose output drives recording devices, such as oscillographs, strip-chart
recorders, and magnetic tape recorders.
The shield which surrounds the transducer signal leads should be grounded at the same point as the
a.
transducer to ensure that the shield and signal leads are at virtually the same potential.
b.
When single-ended amplifiers must be used, the recorder should be left grounded.
When the bonded thermocouple is connected to an isolated differential amplifier as shown in
c.
Figure 3-27, the shield of the input cable should be connected to the amplifier internal guard shield to continue
the signal shield within the amplifier. Notice that a grounding bus is shown connected between the data system
signal reference and earth ground of the test area. This ground bus is necessary in any instrumentation system
which uses isolated differential amplifiers in order to (1) provide the earth reference for the signal circuitry
within the recording system to reduce high voltage hazards, and (2) minimize the common-mode potentials that
otherwise exist between the amplifier's input and output if the data recording system was grounded to a
separate earth or facility ground. Notice, also, that the amplifier case and output shield are connected to the
data system (or load end) ground.
d.
Grounded bridge transducers should be excited with a balanced dc source. By balancing the dc
excitation supply relative to ground as shown in Figure 3-28 the entire bridge will be balanced with respect to
ground and the unbalanced impedance presented to the amplifier input will be due only to the leg resistances in
the bridge. Although a ground loop still exists, its effect is greatly reduced by a balanced excitation supply.
e.
Wherever possible, use an isolated amplifier in the manner illustrated in Figure 3-29 with bridge
transducers. With this configuration, both the transducer and the amplifier can be grounded without degrading
system performance.
f.
Ensure a low resistance earth ground connection (see Section 1.2).
Provide a single common signal ground reference point for all grounded transducers at the test area.
g.
h.
Connect the instrumentation cable shield of each data channel as close to the transducer ground
connection as possible.
3-35
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