handbook of modern sensors

5.9 Noise in Sensors and Circuits 217
Fig. 5.53. Receiver’s loop is formed by long conductors.
arrangement to work, none of the current can be shared with another conductor,
(e.g., a ground plane, which may result in ground loops).
• Use a shielded cable with the high-level source circuit’s return current carried by
the shield (Fig. 5.52A). If the shield current i 2 is equal and opposite to that of
the center conductor i 1 , the center conductor field and the shield field will cancel,
producing a zero net field. This case seems to be a violation of the rule “no shield
currents” for the receiver’s circuit; however, the shielded cable here is not used
to electrostatically shield the center conductor. Instead, the geometry produces a
cancellation of the magnetic field which is generated by a current supplied to a
“current-hungry” device (an electric motor in this example)
• Because magnetically induced noise depends on the area of the receiver loop, the
induced voltage due to magnetic coupling can be reduced by making the loop’s
area smaller.
What is the receiver’s loop Figure 5.53 shows a sensor which is connected to
the load circuit via two conductors having length L and separated by distance D.
The rectangular circuit forms a loop area a = LD. The voltage induced in series with
the loop is proportional to the area and the cosine of its angle to the field. Thus, to
minimize noise, the loop should be oriented at right angles to the field and its area
should be minimized.
The area can be decreased by reducing the length of the conductors and/or decreasing
the distance between the conductors. This is easily accomplished with a twisted
pair, or at least with a tightly cabled pair of conductors. It is good practice to pair the
conductors so that the circuit wire and its return path will always be together. This
requirement must not be overlooked. For instance, if wires are correctly positioned
by a designer, a service technician may reposition them during the repair work. A new
wire location may create a disastrous noise level. Hence, a general rule is: Know the
area and orientation of the wires and permanently secure the wiring.
Magnetic fields are much more difficult to shield against than electric fields because
they can penetrate conductive materials.
5.9.6 Mechanical Noise
Vibration and acceleration effects are also sources of transmitted noise in sensors
which otherwise should be immune to them. These effects may alter transfer charac-