in materials and workmanship, for a period of 90 days from date of shipment, as evidenced by The NI Developer Zone tutorial, Field Wiring and Noise. Field Wiring and Noise Considerations for Analog Signals. Publish Date: Aug 17, | Ratings | out of 5 | Print implementation of an 8-channel differential measurement system used in a typical device from National Instruments. How To Measure Voltage. Publish Date: maj 11, CompactDAQ Chassis with an NI Analog Input Module. Figure 2 shows the Learn more about Field Wiring and Noise Considerations for Analog Signals.
Field Wiring and Noise Considerations for Analog Signals - National Instruments
Each individual cell may generate approximately 1 V, but a stack of cells may produce several kilovolts or more. To accurately measure the voltage of a single 1 V cell, the measurement device must be able to reject the high common-mode voltages generated by the rest of the stack.
Back to Top 2. Most sources of common-mode voltage contain an AC component in addition to a DC offset. Noise is inevitably coupled onto a measured signal from the surrounding electromagnetic environment. This is particularly troublesome for low-level analog signals passing through the instrumentation amplifier on a DAQ device.
Sources of AC noise may be broadly classified by their coupling mechanisms — capacitive, inductive, or radiative.
Capacitive coupling results from time-varying electric fields, such as those created by nearby relays or other measurement signals. Inductive or magnetically coupled noise results from time-varying magnetic fields, such as those created by nearby machinery or motors.
If the electromagnetic field source is far from the measurement circuit, such as with fluorescent lighting, the electric and magnetic field coupling is considered combined electromagnetic or radiative coupling. In all cases, a time-varying common-mode voltage is coupled onto the signal of interest, most often in the range of Hz power-line frequency.
An ideal measurement circuit has a perfectly balanced path to both the positive and negative terminals of an instrumentation amplifier. Such a system would completely reject any AC-coupled noise.
Five Tips to Reduce Measurement Noise
A practical device, however, specifies the degree to which it can reject common-mode voltage with a common-mode rejection ratio CMRR. The CMRR is the ratio of the measured signal gain to the common-mode gain applied by the amplifier, as noted by the following equation: This is equivalent to a 10 times better attenuation of 60 Hz noise.
Any application may benefit from rejecting 60 Hz noise. However, those with large rotating machinery or motors require noise immunity at higher frequencies. At 1 kHz, NI devices reject noise times better than NI devices, making them ideal for industrial applications. Back to Top 3. Break Ground Loops Ground loops are arguably the most common source of noise in data acquisition systems. Proper grounding is essential for accurate measurements, yet it is a frequently misunderstood concept.
A ground loop forms when two connected terminals in a circuit are at different ground potentials. This difference causes a current to flow in the interconnection, which can produce offset errors.
Further complicating matters, the voltage potential between signal source ground and DAQ device ground is generally not a DC level.
This results in a signal that reveals power-line frequency components in the readings. Consider the simple thermocouple application in Figure 4. A differential thermocouple measurement with a grounded signal source can create a ground loop. Here, an otherwise straightforward temperature measurement is complicated by the device under test DUT being at a different ground potential than that of the DAQ device.
Though both devices share the same building ground, the difference in ground potential could be mV or more if the power distribution circuits are not properly connected. The difference appears as a common-mode voltage with an AC component in the resulting measurement.
Recall that isolation is a means of electrically separating signal source ground from the instrumentation amplifier ground reference see Figure 5. Isolation eliminates ground loops by separating earth ground from the amplifier ground reference. Because current cannot flow across the isolation barrier, the amplifier ground reference can be at a higher or lower potential than earth ground. You cannot inadvertently create a ground loop with this circuit.
Using an isolated measurement device removes the ambiguity of properly grounding a measurement system, ensuring more accurate results. Back to Top 4. Use mA Current Loops Long cable lengths and the presence of noise in industrial or electrically harsh environments can make accurate voltage measurements difficult.
As a result, industrial transducers that sense pressure, flow, proximity, and so on often emit current signals instead of voltage. A mA current loop is a common method of sending sensor information over long distances in many process-monitoring applications, as shown in Figure 6. An instrumentation amplifier uses a shunt resistor to convert process current signals into voltage.
Each of these current loops contains three components — a sensor, a power source, and one or more DAQ devices. The current signal from the sensor is typically between 4 and 20 mA, with 4 mA representing the lowest signal value and 20 mA representing the maximum.
This transmission scheme has the advantage of using 0 mA to indicate an open circuit or bad connection. Power supplies are typically in the range of 24 to 30 VDC, depending on the total amount of voltage dropped along the circuit. Finally, the DAQ device uses a high-precision shunt resistor between the leads of the instrumentation amplifier to convert the current signal into a voltage measurement. 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. If the resistance of the common return lead from A to B is 0. The circuit of Figure 13b provides separate ground returns; thus, the measured temperature sensor output does not vary as the current in the heavy load circuit is turned on and off.
For an intuitive and qualitative understanding of these coupling channels, however, lumped circuit equivalents can be used. Figures 14 and 15 show the lumped circuit equivalent of electric and magnetic field coupling. Inductive Coupling between the Noise Source and Signal Circuit, Modeled by the Mutual Inductance M in the Equivalent Circuit Introduction of lumped circuit equivalent models in the noise equivalent circuit handles a violation of the two underlying assumptions of electrical circuit analysis; that is, all electric fields are confined to the interior of capacitors, and all magnetic fields are confined to the interior of inductors.
Capacitive Coupling The utility of the lumped circuit equivalent of coupling channels can be seen now. An electric field coupling is modeled as a capacitance between the two circuits. The equivalent capacitance Cef is directly proportional to the area of overlap and inversely proportional to the distance between the two circuits. Thus, increasing the separation or minimizing the overlap will minimize Cef and hence the capacitive coupling from the noise circuit to the signal circuit.
Other characteristics of capacitive coupling can be derived from the model as well. For example, the level of capacitive coupling is directly proportional to the frequency and amplitude of the noise source and to the impedance of the receiver circuit.
Thus, capacitive coupling can be reduced by reducing noise source voltage or frequency or reducing the signal circuit impedance. The equivalent capacitance Cef can also be reduced by employing capacitive shielding.
Capacitive shielding works by bypassing or providing another path for the induced current so it is not carried in the signal circuit. Proper capacitive shielding requires attention to both the shield location and the shield connection.
The shield must be placed between the capacitively coupled conductors and connected to ground only at the source end. Significant ground currents will be carried in the shield if it is grounded at both ends. For example, a potential difference of 1 V between grounds can force 2 A of ground current in the shield if it has a resistance of 0. Potential differences on the order of 1 V can exist between grounds.
The effect of this potentially large ground current will be explored further in the discussion of inductively coupled noise. As a general rule, conductive metal or conductive material in the vicinity of the signal path should not be left electrically floating either, because capacitively coupled noise may be increased. Proper Shield Termination—No Ground or Signal Current Flows through the Shield Inductive Coupling As described earlier, inductive coupling results from time-varying magnetic fields in the area enclosed by the signal circuit loop.
These magnetic fields are generated by currents in nearby noise circuits.
Field Wiring and Noise Considerations for Analog Signals
The induced voltage Vn in the signal circuit is given by the formula: The lumped circuit equivalent model of inductive coupling is the mutual inductance M as shown in Figure 15 b.
In terms of the mutual inductance M, Vn is given by the formula: Because M is directly proportional to the area of the receiver circuit loop and inversely proportional to the distance between the noise source circuit and the signal circuit, increasing the separation or minimizing the signal loop area will minimize the inductive coupling between the two circuits.
Reducing the current In in the noise circuit or reducing its frequency can also reduce the inductive coupling. The flux density B from the noise circuit can also be reduced by twisting the noise source wires. Finally, magnetic shielding can be applied either to noise source or signal circuit to minimize the coupling. Shielding against low-frequency magnetic fields is not as easy as shielding against electric fields.
The effectiveness of magnetic shielding depends on the type of material—its permeability, its thickness, and the frequencies involved. Due to its high relative permeability, steel is much more effective than aluminum and copper as a shield for low-frequency roughly below kHz magnetic fields. At higher frequencies, however, aluminum and copper can be used as well.
Absorption loss of copper and steel for two thicknesses is shown in Figure The magnetic shielding properties of these metals are quite ineffective at low frequencies such as those of the power line 50 to 60 Hzwhich are the principal low-frequency, magnetically-coupled noise sources in most environments. Better magnetic shields such as Mumetal can be found for low-frequency magnetic shielding, but Mumetal is very fragile and can have severe degradation of its permeability, and hence, degradation of its effectiveness as a magnetic shield by mechanical shocks.
Absorption Loss as a Function of Frequency from Reference 1 Because of the lack of control over the noise circuit parameters and the relative difficulty of achieving magnetic shielding, reducing the signal circuit loop area is an effective way to minimize inductive coupling.
Twisted-pair wiring is beneficial because it reduces both the loop area in the signal circuit and cancels induced errors. Formula 2 determines the effect of carrying ground-loop currents in the shield for the circuit in Figure The effectiveness of the data acquisition system is thus reduced roughly to that of a bit acquisition system.
When using an E Series device with a shielded cable in differential mode, the signal circuit loop area is minimized because each pair of signal leads is configured as a twisted pair. This is not true for the single-ended mode with the same device and cable because loop areas of different sizes may be formed with different channels. Current signal sources are more immune to this type of noise than voltage signal sources because the magnetically induced voltage appears in series with the source, as shown in Figure V21 and V22 are inductively coupled noise sources, and Vc is a capacitively coupled noise source.
The level of both inductive and capacitive coupling depends on the noise amplitude and the proximity of the noise source and the signal circuit. Thus, increasing separation from interfering circuits and reducing the noise source amplitude are beneficial. Conductive coupling results from direct contact; thus, increasing the physical separation from the noise circuit is not useful.
Radiative Coupling Radiative coupling from radiation sources such as radio and TV broadcast stations and communication channels would not normally be considered interference sources for the low-frequency less than kHz bandwidth measurement systems. But high-frequency noise can be rectified and introduced into low-frequency circuits through a process called audio rectification.
This process results from the nonlinear junctions in ICs acting as rectifiers. Simple passive R-C lowpass filters at the receiver end of long cabling can reduce audio rectification. The ubiquitous computer terminal is a source of electric and magnetic field interference in nearby sensitive circuits. This is illustrated in Figure 20, which shows the graphs of data obtained with a data acquisition device using a gain of with the onboard programmable gain amplifier. The input signal is a short circuit at the termination block.
For differential signal connection, the channel high and channel low inputs were tied together and to the analog system ground. For the single-ended connection, the channel input was tied to the analog system ground. Computer Monitor Miscellaneous Noise Sources Whenever motion of the interconnect cable is involved, such as in a vibrational environment, attention must be paid to the triboelectric effect, as well as to induced voltage due to the changing magnetic flux in the signal circuit loop.
The triboelectric effect is caused by the charge generated on the dielectric within the cable if it does not maintain contact with the cable conductors. Changing magnetic flux can result from a change in the signal circuit loop area caused by motion of one or both of the conductors—just another manifestation of inductive coupling.
The solution is to avoid dangling wires and to clamp the cabling. In measurement circuits dealing with very low-level circuits, attention must be paid to yet another source of measurement error—the inadvertent thermocouples formed across the junctions of dissimilar metals.
Errors due to thermocouple effects do not constitute interference type errors but are worth mentioning because they can be the cause of mysterious offsets between channels in low-level signal measurements. Back to Top 5. Balanced Systems In describing the differential measurement system, it was mentioned that the CMRR is optimized in a balanced circuit. A balanced circuit is one that meets the following three criteria: The source is balanced—both terminals of the source signal high and signal common have equal impedance to ground.
The cable is balanced—both conductors have equal impedance to ground. The receiver is balanced—both terminals of the measurement end have equal impedance to ground. Capacitive pickup is minimized in a balanced circuit because the noise voltage induced is the same on both conductors due to their equal impedances to ground and to the noise source. If the circuit model of Figure 21 represented a balanced system, the following conditions would apply: The higher the imbalance in the system or mismatch of impedances to ground and the capacitive coupling noise source, the higher the differential component of the capacitively coupled noise will be.
A differential connection presents a balanced receiver on the data acquisition device side of the cabling, but the circuit is not balanced if either the source or the cabling is not balanced.JUNO: Inside The Unknown - 2016 Space Documentary
This is illustrated in Figure The data acquisition device is configured for differential input mode at a gain of The common-mode rejection is better for the circuit in Figure 22b than for Figure 22a.
Figure 22c and 22d are time-domain plots of the data obtained from configurations 22a and 22b respectively.
Notice the absence of noise-frequency components with the balanced source configuration. The noise source in this setup was the computer monitor.