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 MIL-HDBK-419A
i.
Use twisted shielded transducer extension wires.
j.
Use a floating load on the output of a single-ended data amplifier when the amplifier input is a
grounded transducer.
Connect guard shield of data amplifier to input cable shield.
k.
Always use insulated shielded cables.
Uninsulated shields should never be used in data
l.
instrumentation systems.
3.5.2.1.2 Ungrounded Transducers.
Figure 3-30 illustrates the grounding techniques recommended for ungrounded transducers. The
a.
metallic enclosure of the transducer is connected to the cable shield and both the enclosure and the shield are
grounded at the transducer. If the load on the cable signal line is a single-ended amplifier as shown in Figure
3-30(a), the shield of the input cable should not be connected to the amplifier. The case of the amplifier should
be grounded at the load.
b.
Figure 3-30(b) shows the recommended way of grounding the system when using an isolated
amplifier. (Certain types of non-isolated differential amplifiers require that a transducer ground path be
provided for proper amplifier operation. The instructions supplied by the amplifier manufacturer should be
consulted for correct procedures.)
Provide a single common ground reference point for all cable shields.
c.
d.
Ground all input cable shields at the transducer.
Provide a continuous overall shield for signal wires from the transducer case to the input of the data
e.
amplifier.
f.
Connect isolated amplifier guard shield to input cable shield.
Do not allow more than one ground connection in each input cable shield.
g.
3.5.2.1.3 Amplifiers.
a.
Single-ended amplifiers can be used in digital data acquisition systems if channel-to-channel
isolation is provided (e.g., through the use of floating loads).
b.
Single-ended amplifiers should not be used with grounded (bonded) transducers in order to avoid
channel-to- channel ground loops.
c.
Single-ended amplifiers should not be used with grounded bridges to avoid short circuiting one leg of
the bridge.
3-38
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