DaqScribe Solutions P.O Box 958 Carson City NV Author: Sam Herceg A basic understanding of grounding and ground loops

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1 DaqScribe Solutions P.O Box 958 Carson City NV Author: Sam Herceg A basic understanding of grounding and ground loops In the world of data acquisition if there is one thing which causes more anguish that anything else, it is grounding! Some sensor manufactures and most instrument providers include a provision for a third wire either as part of the signal output or the signal input path. The third wire has a variety of labels, shield, guard, common, ground, signal ground etc. Often, the signal wires are shielded and contain a drain wire adding to the connection complexity. This article attempts to provide a simple (if ground loops could ever be called simple) explanation of this phenomenon. Drain wire

2 Common Mode/Normal Mode Any discussion of grounding and ground loops must contain at least a basic discussion and some understaning of terms like single ended inputs, differential inputs, common mode voltages and normal mode voltages. The following definitions are included to at a minimun provide a very simple definition of these terms. Single Ended Inputs. Single ended inputs are referenced to the module s power supply common. The singnel being measures is on the positive conductor and the negative conductor is tied to the supply common. Single Ended input + Differential inputs. Differential inpus are two single ended inputs referenced to a common potential. The signal being measured is the difference between these two single ended inputs. Two Wire Differential input Typically these wires are twisted Common Mode Voltage: When referenced to the local common or ground, a commonmode signal appears on both lines of a 2-wire cable, in-phase and with equal amplitudes. Common mode voltages are induced by (A) Radiated signals coupled equally to both wires. (B) An offset from the signal common created in the driver circuit. (C) A ground differential between the transmitting and receiving locations.(d) Magnetic Influcenses, things like large motors and transformers can radiate magnetic fields that can induce signels into the wires. Normal Mode Voltage: A normal mode voltage is any type of signal that appears between a pair of wires, or on a single wire referenced to the earth/ground.

3 The Third wire and the Sensor The third wire, generally the shield wire represents the common mode potential of the signal. If not taken into account properly there will likely be an erroneous normal mode signal generated. The center or electrical balance point of the signal probably will not have identical impedance to each of the two signal wires. If a potential exist between the shield and the signal, and unequal voltage will appear in series with each signal lead caused by the current flowing from the shield through the unequal impedance of the signal source. In this way, the voltage that exists between the sensor case and either of the input leads will generated a false normal mode voltage. Balancing of the input leads is crucial to the reduction of these common mode voltages. For this reason differential input systems are generally preferred over single ended systems. If the sensor is of a design that uses an internal high frequency voltage (Excitation) to enhance the signal generating ability of the sensor, the unbalance effect will be greatly exaggerated with the additional likelihood of cross modulation products of the shield voltage frequency and sensor carried frequency appearing in the output signal. In most applications, the sensor case mounting determines the common mode potential of the sensor. Many transducers isolate the case from the signal path. Unfortunately at higher frequencies insulating the case does little to assure signal noise integrity as the capacitive reactance at higher frequencies significantly affects the case potential. There are methods to address this problem but it is not the intent of this article to address high frequency noise applications ( >100 Hz). Signal Receiver The third wire is used to present the common mode (CM) voltage existing at the sensor to the receiving device. The difference between the average signal voltage and input circuit reference voltage (common mode voltage) will generate an error in all receiving devices. Receiving devices may be signal conditioning amplifiers, data acquisition digitizers, recorders or data display devices (DMM, DVM, Scopes etc). Whether or not this error is significant depends upon the amplitude and frequency of the common mode voltage and the immunity of particular receiving devices. It is not unusual for two identical receiving devices to exhibit entirely different apparent zero offset values as the signal wires are moved from one device to the other because of differing and insufficient CM immunity. In general, the very best devices will have common mode rejection ratios (CMRR) of about 1,000,000/1 at low frequencies (<100 Hz). As the CM frequency increases, both the CMRR and the maximum allowable CM voltage decrease at about 6dB/octave. At higher CM frequencies (>100 khz), more error is caused by slew limiting in the input amplifier. The error appears as a DC offset. It is therefore not coherent with the CM voltage and the term CMRR is meaningless. The onset of slew error is sudden and serious and is easily recognizable.

4 Accurate receiving devices have input filters to help limit the CM voltage presented to the input circuit and a method of driving the receiving device input circuit common. The filter must also remove any signal components that might alias the sampling signal. System Considerations The overall data acquisition system design must not allow the receiving device to be presented with the normal or CM signals that have voltage levels or slewing rates that are incompatible with the device s input circuit limitations. The system design must also provide a method to connect floating signals ohmically to the receiving device circuit common. This is necessary to provide a pump-out current source and to limit the CM signal voltage at the device input. Many systems, utilizing devices that exhibit good test bench accuracy fail when long signal leads, (typical of many test environments), are used. Before an installed system is used with certainty, checks must be made to assure that the total system is not being exposed to and responding to unknown and undesired signal sources. Grounding (earth) This term is purposely avoided in the above discussion as almost never is an instrument or sensor ever tied directly to ground. In most cases when the term ground or grounding is used it refers to a method of connecting to a location which at some point is connected to a piece of metal which is actually tied to an earth connection somewhere down the line. It may be a facility grounding bus tied to a copper stake driven 6 into the earth out side the building, or tied to a U fer ground connection in the building foundation, or it may be tied to the facility AC ground connection in the buildings electrical panel. The electrical panels ground is tied to the power company s earth connection (and who knows where that is). The earth connection may be at the power pole located where the power line enters the facility or may be located some distance away. This in itself produces a potential difference in the ground reference. All instruments containing electronics are sensitive to higher frequencies. Usually the higher the frequency, the more effect. The signal presented to an instrument is recognized by the instrument relative to its reference (ground), not necessarily the system designer s designated ground. It is therefore incumbent upon the system designer to consider all signals, both wanted and unwanted, and their effect upon the individual instrument as well as the total system accuracy. A good wiring practice for a voltage-input channel is shown in figure 1

5 Sensor Isolated Sensor Ground Connections + _ Figure 1 In this case, the sensor is not grounded and the cable shield is connected to the midpoint of the sensor as well as to the ground connection at the data acquisition system. This shield connection also meets the requirements of most instrumentation front-ends. (THERE MUST BE A RETURN PATH TO THE INSTRUMENTATION COMMON SO THAT THE INPUT CURRENT (as low as it might be)will NOT CAUSE THE INPUT PAIR TO FLOAT OUTSIDE THE COMMON-MODE RANGE) Failure to have this return path is a common cause of common mode problems. The situation is more complex if the sensor is grounded. The customary recommendation in this case is to connect the shield at the sensor and not at the DAQ system input to avoid a ground loop. Refer to Figure 2. Sensor-end shield grounded configuration Sensor Figure 2 If adequate CMMR is not achieved with this method another method to be tried is to leave the shield unconnected at the sensor end and connect it at the receiving end, as is shown in the following diagram. Refer to Figure 3 Amplifier/DAQ end shield connection configuration Sensor Figure 3

6 For many applications, the circuit shown in the following diagram may produce the best results, Note the shields are connected at both ends, even though this creates a ground loop. This is an example of a good ground loop. Sensor and Signal Conditioning shield connection configuration Sensor Figure 4 In many cases, this double grounding has been shown to give superior performance over configurations with the shield open at either end. How can it be that a ground loop substantially improves performance? A ground loop is generally bad if it involves a signal carrying conductor. An example of this was shown in Figure 2, where the voltage produced by the ground current translated one to one into a normal mode voltage because the shield is a return for the signal. A ground loop is often good if it does not involve a signal carrying conductor. High frequency hash that enters the system through reduced common mode rejection and nonlinearities in the instrumentation amplifier at high frequencies may or may not be of concern. Often filtering is provided in the amplifier to eliminate this wide band noise. The cable capacitance and other factors reduce the transmission of high frequency noise when the shield is connected to the circuit elements at both ends. Indeed, with a connection to ground at both ends, current flows through the shield, particularly at power line frequencies, and the signal effect of this transformer action is greatly reduced because these voltages cancel out in the differential input instrumentation amplifier. Note: The double grounding cannot be applied if there is a substantial potential difference between the two circuit commons. For this case, an isolated input channel, as shown in Figure 5 should be used.

7 Power Source Isolated data acquisition input circuit Shunt No Connection Figure 5 Isolation Amplifier If the data system uses more than one equipment rack, the racks should be bonded to each other. The rack sub-systems should be connected to a good ground. Often the electrical conduct is used as the tie point. Care must be taken to assure the conduit and all its connections are sufficient to achieve good continuity to the facility power ground. Often loose connections, or poor connections of the screws at the conduit coupling produce high impedance junctions, resulting in higher CM potentials. Ideally the impedance should be 0 Ohms. A better ground connection would be attaching directly to the ground wire (green wire) inside the conduit. In some facilities the electrician may have used the conduit as the ground instead of pulling a third wire (green wire). If the impedance of the ground connection is too high, a better direct connection must be found. If there are wire ducts or conduit carrying the signal cables, the recommendation is that these be bonded to the ground reference for the sensors and the racks that contain the data front-end. Note that this creates another ground loop, usually the good kind. This approach is controversial when the sensors are in a rather hostile electrical environment. The concern often expressed is that ground bonding at both ends will cause the electrical noise at the sensor to be transmitted to the data system ground and create more interference. Generally, the noise reduction resulting from the sensors and front end being at nearly the same potential will far outweigh the introduction of noise by the ground loop. An additional benefit of ground bonding is that it will reduce the chance of damage to the electronics in the presence of lighting or other voltage transients. The lower the electrical impedance between the various parts of circuit, the lower the potential difference in the event of large voltage transients induced into the instrumentation elements. If the sensors are not connected to any part of an electrical circuit or ground, as shown in Figure 6 the preferred technique is to bond the grounds at the sensors and the DAQ front end using the input cable conduit, This will minimize the effect of any electrostatic coupling of noise to the sensors. Again, this is to keep the entire data acquisition front end, including the sensors, in as uni-potential an environment as possible. A similar approach involves the use of double shielded cables, where the inner shield is connected as in figure 6 and the outer shield is connected to ground as the sensors and to the front

8 end equipment chassis ground. The guiding principle is to keep all parts of the analog subsystem at the same potential. Sensor Shielded Twisted Pair Figure 6 Other approaches can be used if the primary common mode (signal to ground) interference is primarily high frequency in nature. One configuration uses a trifilar transformer, which is a tightly coupled three-winding transformer, with one winding in series with each of the two signal conductors and the third winding in series with the guard connection. This is quite effective in enhancing common mode rejection at high frequencies. Another approach is to use a capacitor to connect the circuit ground to the shield at the instrumentation front end. This provides a high frequency ground while reducing the current caused by power line frequencies. The effectiveness of the capacitor depends upon the particular situation. Also, some front ends provide a guard signal for connection to the shield. The guard voltage is derived from a special instrumentation amplifier output that monitors the common mode voltage and produces a signal to cancel it. Often asked is what are the correct cabling and ground connections when the signal source is an amplifier output instead of a passive sensor. Properly wired, this output circuit can be single-ended and still produce a good signal to noise ratio. If the amplifier output circuit drives a two contact and a shield type connector. The connections are shown in figure 7. This configuration provide a good balance because the output impedance of an instrumentation or operational amplifier is generally less than 1 ohm at low frequencies Driving a differential input from an unbalanced source Figure 7 Many signal conditioning chassis contain BNC single contact connectors at their output and are intended to be used with coaxial cable. This can present problems in correctly wiring a coaxial cable to a differential input front end. If the output shield connection on the signal conditioning unit is isolated from ground or has a resistance path to ground of 1000 ohms or greater, then the connections shown in Figure 8 should be used. If the

9 shield conductor is grounded at the source, then the diagram shown in Figure 9 can be employed to prevent a ground loop in a signal carrying conductor. This approach may be satisfactory unless the data acquisition system contains a high frequency filter. Also it is desirable the ground frames of the chassis associated with the signal transmitter and receiver are mounted in the same rack or nearby racks and are electrically bonded together so that the ground noise between them is minimized. A good wiring alternative, particularly if the two units are not in the same rack, is to convert the cable to a shielded twisted pair type as close to the source as possible as shown in Figure 10 R GND Floating source Connection BNC Input Connection + _ Figure 8 Grounded source Connection BNC Input Connection + _ This bond is very important Figure 9 This connection may or may not be required depending upon the difference in the ground potentials + _ Twin-Ax Cable configuration Figure 10 Conclusion

10 As we have discussed there are many considerations when it comes to connecting a sensor to a DAQ, but the main objective should always be getting the ground reference at the sensor to be at the same level as the ground reference at the receiving end. The use of the third wire become somewhat of an art, but certainly an art that can be if not mastered at least controllable by most instrumentation engineers. The afore mentioned and describe connection diagrams will generally provide an acceptable grounding solution for most input configurations.

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