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Source ground Measurement system ground
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Grounded Ground-referenced signal source measurement system
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Figure 158 Ground loop in ground-referenced measurement system
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A differential measurement system is often a way to avoid ground loop problems, because the signal source and measurement system grounds are not connected to each other, and especially because the signal low input of the measuring instrument is not connected to either instrument case ground The connection of a grounded signal source and a differential measurement system is depicted in Figure 159
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R1 R2
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Grounded signal source
Differential (nonreferenced) measurement system
Floating signal source
Figure 159 Differential (nonreferenced) measurement system
Differential Floating Ground-referenced measurement (nonreferenced) signal system source measurement system (a) (b)
Figure 1510 Measuring signals from a oating source: (a) differential input; (b) single-ended input
If the signal source connected to the differential measurement system is oating, as shown in Figure 1510, it is often a recommended procedure to reference the signal to the instrument ground by means of two identical resistors that can provide a return path to ground for any currents present at the instrument An example of such input currents would be the input bias currents inevitably present at the input of an operational or instrumentation ampli er The simple concepts illustrated in the preceding paragraphs and gures can assist the user and designer of instrumentation systems in making the best possible wiring connections for a given measurement Noise Sources and Coupling Mechanisms Noise meaning any undesirable signal interfering with a measurement is an unavoidable element of all measurements Figure 1511 depicts a block diagram of the three essential stages of a noisy measurement: a noise source, a noise coupling mechanism, and a sensor or associated signal-conditioning circuit Noise sources are always present, and are often impossible to eliminate completely; typical sources of noise in practical measurements are the electromagnetic elds caused by uorescent light xtures, video monitors, power supplies, switching circuits, and high-voltage (or current) circuits Many other sources exist, of course, but often the simple sources in our everyday environment are the most dif cult to defeat
Coupling mechanism
Noise source AC power systems High-voltage or high-current circuits Switching circuits
Sensor or signal conditioning circuit
Conductive coupling Capacitive coupling Inductive coupling Radiative coupling
Figure 1511 Noise sources and coupling mechanisms
Figure 1511 also indicates that various coupling mechanisms can exist between a noise source and an instrument Noise coupling can be conductive; that
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Electronic Instrumentation and Measurements
+V i Power supply
Load
vo +V Sensor b
The ground loop created by the load circuit can cause a different ground potential between a and b
+V i Power supply
Load
a vo
Sensor
Separate ground returns for the load and the sensor circuit eliminate the ground loop
Figure 1512 Conductive coupling: ground loop and separate ground returns
VN Noise source
is, noise currents may actually be conducted from the noise source to the instrument by physical wires Noise can also be coupled capacitively, inductively, and radiatively Figure 1512 illustrates how interference can be conductively coupled by way of a ground loop In the gure, a power supply is connected to both a load and a sensor We shall assume that the load may be switched on and off, and that it carries substantial currents The top circuit contains a ground loop: the current i from the supply divides between the load and sensor; since the wire resistance is nonzero, a large current owing through the load may cause the ground potential at point a to differ from the potential at point b In this case, the measured sensor output is no longer vo , but it is now equal to vo + vba , where vba is the potential difference from point b to point a Now, if the load is switched on and off and its current is therefore subject to large, abrupt changes, these changes will be manifested in the voltage vba and will appear as noise on the sensor output This problem can be cured simply and effectively by providing separate ground returns for the load and sensor, thus eliminating the ground loop The mechanism of capacitive coupling is rooted in electric elds that may be caused by sources of interference The detailed electromagnetic analysis can be quite complex, but to understand the principle, refer to Figure 1513(a), where a noise source is shown to generate an electric eld If a noise source conductor is suf ciently close to a conductor that is part of the measurement system, the two conductors (separated by air, a dielectric) will form a capacitor, through which any time-varying currents can ow Figure 1513(b) depicts an equivalent circuit in which the noise voltage VN couples to the measurement circuit through an imaginary capacitor, representing the actual capacitance of the noise path The dual of capacitive coupling is inductive coupling This form of noise coupling is due to the magnetic eld generated by current owing through a conductor If the current is large, the magnetic elds can be signi cant, and the mutual inductance (see s 5 and 16) between the noise source and the measurement circuit causes the noise to couple to the measurement circuit Thus, inductive coupling, as shown in Figure 1514, results when undesired (unplanned) magnetic coupling ties the noise source to the measurement circuit
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