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Figure 7 illustrates that a single-channel NRSE measurement system is the same as a single-channel differential measurement system.
Now that we have identified the different signal source type and measurement systems, we can discuss the proper measurement system for each type of signal source. Back to Top 2. Measuring Grounded Signal Sources A grounded signal source is best measured with a differential or nonreferenced measurement system.
Figure 8 shows the pitfall of using a ground-referenced measurement system to measure a grounded signal source. In this case, the measured voltage, Vm, is the sum of the signal voltage, Vs, and the potential difference, DVg, that exists between the signal source ground and the measurement system ground.
This potential difference is generally not a DC level; thus, the result is a noisy measurement system often showing power-line frequency 60 Hz components in the readings. Ground-loop introduced noise may have both AC and DC components, thus introducing offset errors as well as noise in the measurements. The potential difference between the two grounds causes a current to flow in the interconnection. This current is called ground-loop current.
Figure 8. A Grounded Signal Source Measured with a Ground-Referenced System Introduces Ground Loop A ground-referenced system can still be used if the signal voltage levels are high and the interconnection wiring between the source and the measurement device has a low impedance.
In this case, the signal voltage measurement is degraded by ground loop, but the degradation may be tolerable. The polarity of a grounded signal source must be carefully observed before connecting it to a ground-referenced measurement system because the signal source can be shorted to ground, thus possibly damaging the signal source. Wiring considerations are discussed in more detail later in this application note.
With either of these configurations, any potential difference between references of the source and the measuring device appears as common-mode voltage to the measurement system and is subtracted from the measured signal.
This is illustrated in Figure 9. Figure 9. Measuring Floating Nonreferenced Sources Floating signal sources can be measured with both differential and single-ended measurement systems. In the case of the differential measurement system, however, care should be taken to ensure that the common-mode voltage level of the signal with respect to the measurement system ground remains in the common-mode input range of the measurement device.
A variety of phenomena—for example, the instrumentation amplifier input bias currents—can move the voltage level of the floating source out of the valid range of the input stage of a data acquisition device. To anchor this voltage level to some reference, resistors are used as illustrated in Figure These resistors, called bias resistors, provide a DC path from the instrumentation amplifier inputs to the instrumentation amplifier ground.
These resistors should be of a large enough value to allow the source to float with respect to the measurement reference AI GND in the previously described measurement system and not load the signal source, but small enough to keep the voltage in the range of the input stage of the device.
These bias resistors are connected between each lead and the measurement system ground. Warning: Failure to use these resistors will result in erratic or saturated positive full-scale or negative full-scale readings. If the input signal is DC-coupled, only one resistor connected from the — input to the measurement system ground is required to satisfy the bias current path requirement, but this leads to an unbalanced system if the source impedance of the signal source is relatively high.
Balanced systems are desirable from a noise immunity point of view.
A single bias resistor is sufficient for low-impedance DC-coupled sources such as thermocouples. Balanced circuits are discussed further later in this application note. If the input signal is AC-coupled, two bias resistors are required to satisfy the bias current path requirement of the instrumentation amplifier.
Only R2 is required for DC-coupled signal sources. Figure Floating Source and Differential Input Configuration If the single-ended input mode is to be used, a RSE input system Figure 11a can be used for a floating signal source.
No ground loop is created in this case. The NRSE input system Figure 11b can also be used and is preferable from a noise pickup point of view. Table 1. Failure to do so will result in erratic or saturated positive full-scale or negative full-scale readings.
In general, a differential measurement system is preferable because it rejects not only ground loop-induced errors, but also the noise picked up in the environment to a certain degree.
The single-ended configurations, on the other hand, provide twice as many measurement channels but are justified only if the magnitude of the induced errors is smaller than the required accuracy of the data.
Single-ended input connections can be used when all input signals meet the following criteria.
Input signals are high level greater than 1 V Signal cabling is short and travels through a noise-free environment or is properly shielded All input signals can share a common reference signal at the source Differential connections should be used when any of the above criteria are violated. Back to Top 4. Minimizing Noise Coupling in the Interconnects Even when a measurement setup avoids ground loops or analog input stage saturation by following the above guidelines, the measured signal will almost inevitably include some amount of noise or unwanted signal "picked up" from the environment.
This is especially true for low-level analog signals that are amplified using the onboard amplifier that is available in many data acquisition devices.
Consequently, any activity on these digital signals provided by or to the data acquisition board that travels across some length in close proximity to the low-level analog signals in the interconnecting cable itself can be a source of noise in the amplified signal. In order to minimize noise coupling from this and other extraneous sources, a proper cabling and shielding scheme may be necessary. Before proceeding with a discussion of proper cabling and shielding, an understanding of the nature of the interference noise-coupling problem is required.
There is no single solution to the noise-coupling problem. Moreover, an inappropriate solution might make the problem worse. An interference or noise-coupling problem is shown in Figure Noise-Coupling Problem Block Diagram As shown in Figure 12, there are four principal noise "pick up" or coupling mechanisms—conductive, capacitive, inductive, and radiative.
Conductive coupling results from sharing currents from different circuits in a common impedance. Capacitive coupling results from time-varying electric fields in the vicinity of the signal path. Inductive or magnetically coupled noise results from time-varying magnetic fields in the area enclosed by the signal circuit. If the electromagnetic field source is far from the signal circuit, the electric and magnetic field coupling are considered combined electromagnetic or radiative coupling.
Conductively Coupled Noise Conductively coupled noise exists because wiring conductors have finite impedance. The effect of these wiring impedances must be taken into account in designing a wiring scheme.
Conductive coupling can be eliminated or minimized by breaking ground loops if any and providing separated ground returns for both low-level and high-level, high-power signals. A series ground-connection scheme resulting in conductive coupling is illustrated in Figure 13a. It's easier to figure out tough problems faster using Chegg Study. Unlike static PDF Noise Reduction Techniques in Electronic Systems solution manuals or printed answer keys, our experts show you how to solve each problem step-by-step.
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Tsaliovich, A. More than you ever care to know about cables and cable shielding. A good reference book on cables. If you are interested in Signal Integrity this is the second book you should download the Johnson and Graham book is first. This book starts where the Johnson and Graham book ends. The two books compliment each other very well. This book could have been subtitled, "Noise versus Grounding versus Safety.
The second Johnson and Graham book. This is an advanced level text on Signal Integrity, with virtually no overlap with the material in the first book. A lot of good, hard to find information.
Bogatin, E. One hundred pages of down-to-earth practical advice on EMC, without any equations or mathematics, by two authors who work regularly in the EMC trenches.
An excellant, down to earth, practical guide to EMC troubleshooting and precompliance measurements.