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Figure 6.37 shows such a shield-to-ground arrangement for a grounded voltage source connected to
a floating ground measuring device.
A common source of electrical fields is an AC power supply transformer. A capacitance
coupling between the 60- or 50-Hz power supply wires and the signal wires is set up. For example, a
1-pF capacitance superimposes a 40-mV interference on a 1-mV signal.
A different source of noise is from a magnetic field. When a lead wire moves within a magnetic
field, a voltage is induced in the signal path. The same effect occurs with a lead wire in proximity to
an operating motor. The best prevention is to separate the signal lead wires from such sources. Also,
twisting the lead wires together tends to cancel any induced voltage, as the currents through the two
wires are in opposite directions; the more twists per meter, the better. A final recourse is a magnetic
shield made from a material having a high ferromagnetic permeability.
Connecting Wires
There are several types of wire available. Single cable refers to a single length of common wire or
wire strand. The conductor is coated for electrical insulation. The wire is readily available and
cheap, but should not be used for low-level signal (millivolt level) connections or for connections of
more than a few wires. Flat cable is similar but consists of multiple conductors arranged in parallel
strips, usually in groups of 4, 9, or 25. Flat cable is commonly used for short connections between
adjacent electrical boards, but in such applications the signals are on the order of 1 V or more or are
TTL signals. Neither of these two types of wire offer any shielding.
Twisted pairs of wires consist of two electrically insulated conductors twisted about each other
along their lengths. Twisted pairs are widely used to interconnect transducers and equipment. The
intertwining of the wires offers some immunity to noise with that effectiveness increasing with the
number of twists per meter. Cables containing several twisted pairs are available; for example, CAT5
network cable (such as Internet cable) consists of four twisted pairs of wires. Shielded twisted pairs
wrap the twisted pairs within a metallic foil shield. CAT6 network cable consists of four twisted
pairs with a shield separating its wire pairs. Shielded twisted pairs are one of the best choices for
applications requiring low-level signals or high data transfer rates.
Coaxial cable consists of an electrically insulated inner single conductor surrounded within an
outer conductor made of stranded wire and a metal foil shield. In general, current flows in one
direction along the inner wire and the opposite direction along the outer wire. Any electromagnetic
fields generated will cancel. Coaxial cable is often the wire of choice for low-level, high-frequency
signals. Signals can be sent over very long distances with little loss. A variation of coaxial cable is
triaxial cable, which contains two inner conductors. It is used in applications as with twisted pairs
but offers superior noise immunity.
Optical cable is widely used to transmit low-level signals over long distances. This cable may
contain one or more fiber-optic ribbons within a polystyrene shell. A transmitter converts the lowlevel voltage signal into infrared light. The light is transmitted through the cable to a receiver, which
converts it back to a low-level voltage signal. The cable is virtually noise-free from magnetic fields
and harsh environments and is incredibly light and compact compared to its metal wire counterpart.
6.10 SUMMARY
This chapter has focused on classic but basic analog electrical measurement and signal conditioning
devices. Devices that are sensitive to current, resistance, and voltage were presented. Signal
252 Chapter 6 Analog Electrical Devices and Measurements
11:55:7 Page 252
Figure 6.37 shows such a shield-to-ground arrangement for a grounded voltage source connected to
a floating ground measuring device.
A common source of electrical fields is an AC power supply transformer. A capacitance
coupling between the 60- or 50-Hz power supply wires and the signal wires is set up. For example, a
1-pF capacitance superimposes a 40-mV interference on a 1-mV signal.
A different source of noise is from a magnetic field. When a lead wire moves within a magnetic
field, a voltage is induced in the signal path. The same effect occurs with a lead wire in proximity to
an operating motor. The best prevention is to separate the signal lead wires from such sources. Also,
twisting the lead wires together tends to cancel any induced voltage, as the currents through the two
wires are in opposite directions; the more twists per meter, the better. A final recourse is a magnetic
shield made from a material having a high ferromagnetic permeability.
Connecting Wires
There are several types of wire available. Single cable refers to a single length of common wire or
wire strand. The conductor is coated for electrical insulation. The wire is readily available and
cheap, but should not be used for low-level signal (millivolt level) connections or for connections of
more than a few wires. Flat cable is similar but consists of multiple conductors arranged in parallel
strips, usually in groups of 4, 9, or 25. Flat cable is commonly used for short connections between
adjacent electrical boards, but in such applications the signals are on the order of 1 V or more or are
TTL signals. Neither of these two types of wire offer any shielding.
Twisted pairs of wires consist of two electrically insulated conductors twisted about each other
along their lengths. Twisted pairs are widely used to interconnect transducers and equipment. The
intertwining of the wires offers some immunity to noise with that effectiveness increasing with the
number of twists per meter. Cables containing several twisted pairs are available; for example, CAT5
network cable (such as Internet cable) consists of four twisted pairs of wires. Shielded twisted pairs
wrap the twisted pairs within a metallic foil shield. CAT6 network cable consists of four twisted
pairs with a shield separating its wire pairs. Shielded twisted pairs are one of the best choices for
applications requiring low-level signals or high data transfer rates.
Coaxial cable consists of an electrically insulated inner single conductor surrounded within an
outer conductor made of stranded wire and a metal foil shield. In general, current flows in one
direction along the inner wire and the opposite direction along the outer wire. Any electromagnetic
fields generated will cancel. Coaxial cable is often the wire of choice for low-level, high-frequency
signals. Signals can be sent over very long distances with little loss. A variation of coaxial cable is
triaxial cable, which contains two inner conductors. It is used in applications as with twisted pairs
but offers superior noise immunity.
Optical cable is widely used to transmit low-level signals over long distances. This cable may
contain one or more fiber-optic ribbons within a polystyrene shell. A transmitter converts the lowlevel voltage signal into infrared light. The light is transmitted through the cable to a receiver, which
converts it back to a low-level voltage signal. The cable is virtually noise-free from magnetic fields
and harsh environments and is incredibly light and compact compared to its metal wire counterpart.
6.10 SUMMARY
This chapter has focused on classic but basic analog electrical measurement and signal conditioning
devices. Devices that are sensitive to current, resistance, and voltage were presented. Signal
252 Chapter 6 Analog Electrical Devices and Measurements