Simcenter Testing Solutions Functional Grounding in Data Acquisition
2023-02-09T20:09:19.000-0500
Simcenter SCADAS
Summary
Details
Besides its role in safety, grounding also enables shielding against electromagnetic interferences or noise.
Although there are many active and passive factors impacting how susceptible a measurement setup is to electromagnetic interference, grounding is often overlooked or underestimated.
Various scenarios and types of electromagnetic interference are shown in Figure 1.
This article covers functional grounding and cabling against interferences and ground loops in data acquisition:
1. Terminology
2. Motivation
3. Interference Couplings
4. Ground
4.1 Definition
4.2 Types
4.3 Star Ground
4.4 Objective
4.4.1 Functional Grounding
4.4.2 Protective Grounding
5. Cables
5.1 Coax versus STP
5.2 Cabling Good Practices
6. Grounding The Setup
6.1 DAQ and DUT
6.2 Simcenter SCADAS Hardware
6.3 Lab versus Portable Scenarios
7. DAQ Inputs
7.1 Ground loops – Break the loop!
7.2 Grounded and Ground Loop Free Setups
7.2.1 DUT – Sensor Case not in Contact: Star Ground Compliant
7.2.2 Sensor Case Isolated
7.2.3 Floating DAQ inputs
7.3 Additional Notes
8. DAQ Outputs
8.1 PC Connection
8.2 DAC and Shakers
8.3 External Power Supplies
9. Practical Examples
9.1 Does My Measurement Setup Suffer from Interferences?
9.2 Do-it-yourself in your office desk: Impact of an ungrounded setup
10. Quick Guide
1. Terminology
You just connected your sensors to your data acquisition hardware (DAQ or SCADAS front-end). They are properly configured in the hardware and software. You check the signals on the channels, and have a green light to do the measurement. But after one acquisition, the result is the red curve in Figure 2:
Figure 2: The red curve shows “The Hum” (50 or 60 Hertz spikes due to interference) in the channel input. The blue curve is the desired signal to measure without that interference, and it was addressed by taking care of grounding.
In the electric ecosystem, everything is and needs to be inter-connected. While they are many choices for devices that can be used, they are limited choices for proper grounding of a measurement system. The playground possibilities is then reduced to the connection setup of the measurement system and peripherals (like transducers), where best practices on functional grounding and cabling come into play.
Therefore, every single case in the field presenting problems related to grounding is unique and difficult to analyze. If those best practices can be in place from the beginning, the problems can be minimized that would otherwise be encountered a posteriori.
3. Interference Couplings
Interferences come from any source of remote electrical current, especially AC (alternating) currents. These currents can couple to the measurement setup and desired signals to measure, introducing mirrors of the original currents. And they couple via four open gates to the setup. These four gates are illustrated in Figure 4:
Each of the four different coupling gates requires a different manner of addressing it:
What does “grounding” mean then? What is a “ground loop”? Let’s investigate how good practices on functional grounding and cabling can help us to close the four doors of interferences.
4. Ground
Grounding is the action of connecting a floating conductor or metal part, to a known potential* conductor like the ground conductor. Regarding functional grounding, and especially against the Electric coupling, all metals should have a controlled, known potential. This includes all metal parts, both in the measurement setup, and those surrounding it. If they are floating (i.e., unknown potential), they should be grounded. This will avoid Electric coupling on those conductors and help with magnetic coupling.
But what’s ground exactly?
4.1 Definition
Ground is a conductor with capacity to source or sink indeterminate amounts of current and present no charge at same time. That's a lot to require from ground!
But there’s practical and acceptable candidate, even though it’s not per se a good conductor: Planet Earth, or the soil (Figure 5).
Figure 5: Planet Earth fits quite well to the definition for Ground.
We can add charge to the planet Earth, but it will get diffused and scattered as there’s a lot of space for it. Earth can be considered to have a constant overall charge, and specifically, to have no charge (i.e., neutral**). This means that it’s potential, proportional to the charge, can also be considered zero Volts.
(*) Voltage(V) is a difference between 2 potentials. There’s a Potential(U) at a given point if there’s Charge(C) in the nearby; and will be proportional to it and inversely proportional to its distance. We normally talk about voltage instead of potential, thanks to taking as reference the potential of the ground conductor: 0V.
(**) Planet Earth is actually understood to be negatively charged. As engineers however, we usually tend to approximate if there are no drawbacks to our specific application. In our case, we can just bring this reference to 0V, for our own practical convenience.
4.2 Types
In electronics especially, how many of us use words like reference, common, negative, 0V, housing, shielding, mass, earth, etc. to refer to ground? Ground is a generic term, so let’s set a coherent language first (Figure 6):
Three different types of grounds include:
4.3 Star Ground
Besides specific applications and special signaling (i.e., floating), all ground types should generally be connected, and ultimately to Earth ground. We could do it arbitrarily, but a fractal-like structure (or star-like) is the best approach.
Figure 7: Star grounding: All grounds get referred to a unique Earth ground point, following similar paths.
This way, all circuits on the top get (almost) the same voltage, near zero Volts, compared to Earth ground.
4.4 Objective
What does our measurement setup earn if it’s (properly) connected to ground? It brings at least two advantages: safety and shielding against interferences.
4.4.1 Functional grounding – Shielding (Faraday Cage)
In the end, we want to protect all the wires, all the cables, and all the interconnected devices like the sensor and our DAQ against interferences. The Faraday Cage concept (Figure 8) enables a metal conductor enclosure to act as a shield if grounded. If a metal is left ungrounded, it will act as an amplifier of interferences.
Figure 8: Faraday Cage configuration. A metal enclosure shields devices inside.
[https://backyardbrains.com/experiments/faraday]
The enclosure or housing, continuous or not, covers our susceptible devices or signals. For flexibility, a mesh is often used instead of a solid material. Like the braid in a cable, the gaps in the braid should be smaller than the wavelength of the interferences we want to protect us against.
NOTE: There are also Faraday Cages configurations against magnetic fields, with different materials and principles.
4.4.2 Protective grounding – User safety
Another function of grounding is to protect the device and any person who comes in contact with the equipment. See Figure 9 for description of protective grounding:
Figure 9: Other applications of grounding, including safety. Use 3-prong pin plugs, rather than 2-prong plugs.
Description of these shield types:
5.2 Cabling Good Practices
There are several good practices when it comes to cabling (Figure 11):
6. Ground The Setup
Bring ground to all metallic parts: DAQ, cable braids, sensor cases, and the Device Under Test (DUT). Spoiler alert again: ground them only from one point to avoid a loop:
6.1 DAQ & DUT
In Figure 12 below, the sensor is grounded only from one point, the cable shield, and not from the DUT contact thanks to the isolator:
Especially important is not to forget to ground the DUT. You don’t want cables to potentially couple with the big metal DUT (the closest thing to the sensors) and thereby transfer interferences into the measurement signals and sensors.
And how do you ground your setup? In short, you could say just connect things to the Planet... Well of course we probably have some more comfortable connection provisions, both in your devices and your lab or vehicle.
6.2 Simcenter SCADAS Hardware
Grounding options and single-ended versus differential options for SCADAS Mobile/Recorder and SCADAS RS are shown in Figure 13.
Figure 13: Top: Simcenter SCADAS Lab(L)/Mobile (M)/Recorder(R) (top left) and SCADAS RS (bottom left) offer analog input modules and units like seen in this table. Bottom: Grounding kit (bottom left) and Siemens AC/DC adapter (bottom right).
Figure 14: In the lab (left), Earth ground may be found in some building electrical panels, or in the mains socket. In a portable application (right), Earth ground isn't available, so the vehicle chassis can be used as ground.
In a lab, some electrical panels in the building have a specific place where the ground can be taken, normally via the typical yellow and green cable. These typically go to Earth ground.
And what if there is no access to the Earth, like in a portable application? Instead of Earth, the largest available metallic component in the portable test article should be used as the ground instead. For example, a metallic vehicle chassis could be used as the ground because of its large size and the fact that it is a good conductor (i.e., has low impedance). Individual non-metallic components like a seat or carpet would not be good conductors and are not suitable for grounding.
7. DAQ Inputs
So, let’s ground everything ad libitum! Shouldn’t we? Remember that we should ground all components of the measurement chain (cables, DAQs, DUTs, sensors, etc). But there should only be one path to ground from each component. Otherwise, we would have created a ground loop.
7.1 Ground loops – Break the loop!
A physical ground loop is accidentally created when a ground conductor has more than one path to Earth ground. The only enabler for a ground loop in our measurement setup, considering only the elements in Figure 15, is the contact between the sensor case and the DUT. This does not mean that we will automatically have a ground loop if that contact exists. Anyway, this contact is necessary for a ground loop.
Figure 15: Ground loops need electrical contact between sensor case and DUT. In this figure, a single ended (SE) signaling example
7.2 Grounded and Ground Loop Free Setups
First of all, the DAQ and the DUT are supposed to be already grounded by default. They may have their own ground (in the Figures 16-18, DAQ has a purple ground and DUT has a blueish ground, colored for clarity).
We will picture the three most straightforward and practical options. All of them are valid, grounded and ground loop free.
7.2.1 DUT – Sensor Case Not in Contact: Star Ground Compliant
Figure 16: Preferred grounded and ground loop free setup. Flexible and star ground compliant.
Use a plastic feed, tape, or isolator (or other) to separate the DUT and sensor. The cable shield should normally be connected at both sides. This is the only way to follow the star ground principle: see how the sensor at the far end gets beautifully purple-grounded from the DAQ. No floating* inputs are needed, and it’s independent of the sensor case being isolated or not.
On top of that, we shouldn’t forget that our setup is normally made of more than one sensor, sometimes lots of them. So, it will be enough for one, and only one of them to present a ground loop, to spoil the whole setup for those sensors not being isolated from the DUT in the first place.
NOTE: Bridge based devices, like strain gauges, have the particularity that they don’t have any case. So, for strain gages, we are always doing good, given that cable braid should shield the wires up to the strain gage end.
(*) For simplicity, consider a floating input as having its signal ground detached from other inputs and from DAQ’s chassis ground via a capacitor. In other words, the signal ground path dies in a floating input. This floating condition will degrade with frequency.
7.2.2 Sensor Case Isolated
When sensor-DUT contact can’t be avoided, then we should start thinking about exceptions as shown in Figure 17.
If we are lucky and the sensor is case-isolated, or there can be a version of the same sensor with case-isolation (meaning we will probably need to pay more for it), a cable whose braid isn’t connected at the sensor’s side will be enough. If this cable is coaxial, a case isolated sensor might take care of this in its BNC connector itself, not touching the case.
7.2.3 Floating DAQ Inputs
Finally, if we still can’t avoid contact between sensor-DUT, and we only have a normal-regular case sensor, we should turn to the use of a floating input and avoid cable braid contact to DAQ’s side as shown in Figure 18.
We have been considering single ended signaling by default. If a differential input or a floating input is used, the ground loop won’t affect our input signal around DC. For a differential input the ground loop doesn’t practically affect our plus/minus signals; for a floating input the ground loop is de facto broken for DC. However, those inputs won’t help much for higher frequency ground loops, i.e., already around 1kHz. Both Common Mode Rejection (CMR) in differential inputs and parasitic capacitances in floating inputs degrade their shielding function against high frequencies.
So-called semi-differential inputs could also do the job thanks to their almost-floating characteristic.
We should then not forget that high frequency ground loops will start hurting for this option.
7.3 Additional Notes
When all channels in a device are floating, and the device offers independent sensor supplies per channel, it’s said that the DAQ is a channel-to-channel isolated device. There is an associated parameter to a floating input, called isolation voltage, or that voltage that would short this floating characteristic with an arc flash. In this document we are focusing on functional grounding, being +/-100V isolation a decent value. When grounding for safety becomes relevant (for example when our sensors need to be placed in some specific parts of an electric vehicle), we can talk about kV of isolation voltage.
There may be other combinations, but too limiting in terms of flexibility. For simplicity, we didn’t picture any sensor supply coming from the DAQ, and will consider Single-Ended signaling (i.e., ICP, analog tacho, 2-wire signaling sensors). For example, we could think of an ICP accelerometer, or a MEMS accelerometer connection. The same reasoning will apply for differential signaling (i.e., bridges, current loops, LVDT, potentiometers, 3-wire signaling sensors).
See the knowledge article: Single Ended vs Differential Inputs
8. DAQ Outputs
Up to this point we have just considered two main actors in our setup and their cables: Our DAQ and the DUT. But what about any other external connected devices (i.e., PC, shakers, etc.)? They may be considered as another factor multiplying the probability of ground loops: We should always break the loop between our setup and these external devices, to be sure they won’t spoil all our efforts until this point.
8.1 PC connection
Never connect cable braid between DAQ and PC!
A good practice is to already break the loop at the cable braid on the PC side.
8.2 DAC and Shakers
If our DAQ has also an output to a shaker, it should also be free of ground loops, because it’s analog and we know analog signals are the first to be protected against interferences.
Several shakers are hydraulic systems, others electrodynamic, and they can draw very high power. Some shaker amplifiers can be configured as differential or floating inputs. Most amplifier builders are aware of ground loop issues, so they normally take a smart design.
In problematic situations, a ground isolator can be used (sometimes space test laboratories have them).
8.3 External Power Supplies
If we need to use external power supplies for our sensors, and wanted to get the excellence in grounding, here’s the star ground concept back as a good practice. The principle is bringing all our ground plugs to the same point, like a grounding fractal tree.
9. Practical Examples
At this point, the idea should be clear. Grounding is not only a best practice; it is needed for our applications. It enables shielding against electric fields that can, and will present a threat for our delicate, accurate sensors and measurement signals.
9.1 Does My Measurement Setup Suffer from Interferences?
We might find an existing measurement setup with persisting noise problems, or we might just be seeing unexpected data and wondering if our setup suffers from interferences. Especially, knowing that magnetic coupling is difficult to eliminate completely.
This example can be taken as a proof that there’s an interference coupling problem in our setup if we measure unforeseen signals. Thanks to the fact that Simcenter SCADAS hardware is fully protected at device level, we can also remove the DAQ itself from the equation: just short the input of the channel. There won’t be any trace of interferences, as no external coupling is possible, including the magnetic.
Otherwise stated: If you wanted to consciously couple interferences present in your setup and surroundings, the easiest way is to leave the cable connected to the DAQ without a sensor in the far end. The effect can be amplified if we don’t follow this document good practices.
9.2 Do-It-Yourself in Your Office Desk: Impact of an Ungrounded Setup
Here’s a good example about the consequences of leaving conductors ungrounded, both in our setup (DUT: load cell body) and surrounding it (ourselves). Although not a perfect one, we are a floating conductor in a soup of interferences even in your office desk.
Just connect a load cell to your grounded DAQ but don’t ground the DUT. The load cell body is the closest thing to your strain gages and is left ungrounded. The strain gage will see everything the body wants: Because of not being grounded, the body couples interferences present in our office. They come from fluorescent lights, power supplies, power grid… And also come from ourselves, as we act as a pick-up antenna and radiate them amplified back to the surroundings.
You should then try to move, walk, or even wave your hands around the DUT, and you will see several dB of difference in the pick-up noise from the DUT right in your channel input.
10. Quick Guide
Figure 23 is a summary of the contents of this article. If instrumenting from scratch, this can be used as a guide for a well grounded measurement setup.
Following the quick guide should result in a ground loop free setup that is robust against interferences. It can also be used as a troubleshooting guide for a problematic setup.
NOTE: The third option for triaxial accelerometers with an early-split cable is made for simplicity. The optimal connection for that very specific setup requires a mix of solutions.
Questions? Email aleix.riera@siemens.com.
Related Links:
Although there are many active and passive factors impacting how susceptible a measurement setup is to electromagnetic interference, grounding is often overlooked or underestimated.
Various scenarios and types of electromagnetic interference are shown in Figure 1.
Figure 1: If a measurement setup is not properly grounded, it will be left at the mercy of interferences.
This article covers functional grounding and cabling against interferences and ground loops in data acquisition:
1. Terminology
2. Motivation
3. Interference Couplings
4. Ground
4.1 Definition
4.2 Types
4.3 Star Ground
4.4 Objective
4.4.1 Functional Grounding
4.4.2 Protective Grounding
5. Cables
5.1 Coax versus STP
5.2 Cabling Good Practices
6. Grounding The Setup
6.1 DAQ and DUT
6.2 Simcenter SCADAS Hardware
6.3 Lab versus Portable Scenarios
7. DAQ Inputs
7.1 Ground loops – Break the loop!
7.2 Grounded and Ground Loop Free Setups
7.2.1 DUT – Sensor Case not in Contact: Star Ground Compliant
7.2.2 Sensor Case Isolated
7.2.3 Floating DAQ inputs
7.3 Additional Notes
8. DAQ Outputs
8.1 PC Connection
8.2 DAC and Shakers
8.3 External Power Supplies
9. Practical Examples
9.1 Does My Measurement Setup Suffer from Interferences?
9.2 Do-it-yourself in your office desk: Impact of an ungrounded setup
10. Quick Guide
1. Terminology
- AC: Alternating Current
- BNC: Bayonet Neill–Concelman cable
- C: Charge
- Coaxial: Single ended cable configuration, consisting of inner conductor surrounding by concentric conducting shield, BNC cable is example coaxial cable
- DAQ: Data Acquisition System, generic term for a data collection equipment like Simcenter SCADAS
- DC: Direct Current
- DIFF: Differential Inputs carry signals on two wires
- DUT: Device Under Test
- EMC: Electromagnetic Compatibility
- EMI: Electromagnetic Interference
- IT: Isolated to Earth - I is Insulatum (Latin for Isolated), T is Terra (Latin for Earth)
- U: Potential Voltage
- SCADAS: Signal Conditioning and Data Acquisition System as in Simcenter SCADAS
- SE: Single Ended carry signals on a single wire
- STP: Shielded Twisted Pairs cable
- TN-S, TN-C, TN-C-S: T is Terra, N is Neutral, C is Combined, S is Separated
- TT: Direct to Earth - T is Terra (Latin for Earth)
You just connected your sensors to your data acquisition hardware (DAQ or SCADAS front-end). They are properly configured in the hardware and software. You check the signals on the channels, and have a green light to do the measurement. But after one acquisition, the result is the red curve in Figure 2:
Figure 2: The red curve shows “The Hum” (50 or 60 Hertz spikes due to interference) in the channel input. The blue curve is the desired signal to measure without that interference, and it was addressed by taking care of grounding.
There are 50 or 60 Hertz spikes in the data (red curve). These are interference and not real measurement data, rather the spikes are due to the line frequencies of the power mains. The interference must be dealt with and eliminated.
Unfortunately, there is not just one, but several types of interference couplings. Unless there’s a proper grounding and cabling strategy in place, interferences will come right into measurement inputs, mask the signals, and reduce measurement accuracy. Grounding issues can demonstrate themselves in the data and may lead to chasing the wrong path – we may think that the issue could be a structural resonance or a system dynamics issue when in fact it could be a grounding related issue.
Moreover, there is no magic formula for grounding. There’s always a compromise: If the setup isn’t grounded, it will easily pick up interferences; but if it’s “too much” grounded, most Earth-coupled interferences from our power grids (otherwise called "The Hum") can find a bigger entry gate to the measurement channel inputs, and even get amplified i.e., by ground loops.
Further, there are also rules and standards for grounding, but they don’t specify any optimal single solution, and there are reasons for that:
Unfortunately, there is not just one, but several types of interference couplings. Unless there’s a proper grounding and cabling strategy in place, interferences will come right into measurement inputs, mask the signals, and reduce measurement accuracy. Grounding issues can demonstrate themselves in the data and may lead to chasing the wrong path – we may think that the issue could be a structural resonance or a system dynamics issue when in fact it could be a grounding related issue.
Moreover, there is no magic formula for grounding. There’s always a compromise: If the setup isn’t grounded, it will easily pick up interferences; but if it’s “too much” grounded, most Earth-coupled interferences from our power grids (otherwise called "The Hum") can find a bigger entry gate to the measurement channel inputs, and even get amplified i.e., by ground loops.
Further, there are also rules and standards for grounding, but they don’t specify any optimal single solution, and there are reasons for that:
- The application scope normally stops:
- Above device level: Electromagnetic Compatibility (EMC)
- Below the building electrical installation: Earthing System
- Depend on the Earthing System type adopted in the area:
- Isolated (IT)
- Local direct Earth (TT)
- Network distributed Separated, Combined or Separated+Combined Earth (TN-S, TN-C, TN-C-S)
- Depend on other regulations of each country.
Figure 3: Electric ecosystem in which a measurement setup exists.
In the electric ecosystem, everything is and needs to be inter-connected. While they are many choices for devices that can be used, they are limited choices for proper grounding of a measurement system. The playground possibilities is then reduced to the connection setup of the measurement system and peripherals (like transducers), where best practices on functional grounding and cabling come into play.
Therefore, every single case in the field presenting problems related to grounding is unique and difficult to analyze. If those best practices can be in place from the beginning, the problems can be minimized that would otherwise be encountered a posteriori.
3. Interference Couplings
Interferences come from any source of remote electrical current, especially AC (alternating) currents. These currents can couple to the measurement setup and desired signals to measure, introducing mirrors of the original currents. And they couple via four open gates to the setup. These four gates are illustrated in Figure 4:
Figure 4: Four coupling gates for interferences, principles, and how to address them.
Each of the four different coupling gates requires a different manner of addressing it:
- Radiated coupling: High frequency effect. Occurs when the conductor lengths (i.e., cables) are comparable to the wavelength related to those frequencies of the interferences. For 5MHz interferences for example, any wire longer than 10m will act as a perfect antenna: they can’t be considered as equipotential conductors anymore, but as transmission lines; meaning that we will find different voltages along the conductor length, instead of the same.
- Electric coupling: Capacitive effect. It will couple variations of voltage from other circuits to ours (Gauss’s law).
- Magnetic coupling: Inductive effect, enemy #1. It will couple variations of current from other circuits to ours (Faraday’s law). It’s difficult to be totally protected, as mostly everything in our devices (and cables) are circuits.
- Galvanic coupling: Conductive effect. If we have a common conductor used by different circuits (most shared conductor is, guess what, the ground conductor), we can “see” undesired conducted currents from other circuits. We will later discuss the ground loops: they enable this coupling, and because it’s a loop, it will even worsen the effects of Magnetic coupling.
What does “grounding” mean then? What is a “ground loop”? Let’s investigate how good practices on functional grounding and cabling can help us to close the four doors of interferences.
4. Ground
Grounding is the action of connecting a floating conductor or metal part, to a known potential* conductor like the ground conductor. Regarding functional grounding, and especially against the Electric coupling, all metals should have a controlled, known potential. This includes all metal parts, both in the measurement setup, and those surrounding it. If they are floating (i.e., unknown potential), they should be grounded. This will avoid Electric coupling on those conductors and help with magnetic coupling.
But what’s ground exactly?
4.1 Definition
Ground is a conductor with capacity to source or sink indeterminate amounts of current and present no charge at same time. That's a lot to require from ground!
But there’s practical and acceptable candidate, even though it’s not per se a good conductor: Planet Earth, or the soil (Figure 5).
Figure 5: Planet Earth fits quite well to the definition for Ground.
We can add charge to the planet Earth, but it will get diffused and scattered as there’s a lot of space for it. Earth can be considered to have a constant overall charge, and specifically, to have no charge (i.e., neutral**). This means that it’s potential, proportional to the charge, can also be considered zero Volts.
(*) Voltage(V) is a difference between 2 potentials. There’s a Potential(U) at a given point if there’s Charge(C) in the nearby; and will be proportional to it and inversely proportional to its distance. We normally talk about voltage instead of potential, thanks to taking as reference the potential of the ground conductor: 0V.
(**) Planet Earth is actually understood to be negatively charged. As engineers however, we usually tend to approximate if there are no drawbacks to our specific application. In our case, we can just bring this reference to 0V, for our own practical convenience.
4.2 Types
In electronics especially, how many of us use words like reference, common, negative, 0V, housing, shielding, mass, earth, etc. to refer to ground? Ground is a generic term, so let’s set a coherent language first (Figure 6):
Figure 6: In electronics, different symbols are used to refer to a specific ground types.
Three different types of grounds include:
- Signal ground: a local ground in a device circuit. That’s what sensors and inputs (measurement amplifiers in our DAQ) talk with. Sets a needed common reference, a “language” between a transmitter and receiver, i.e., between a sensor and the DAQ.
- Chassis ground: the metal enclosure, a priori not a functional conductor. It serves to mechanically facilitate ground interconnection between signal ground inside and Earth ground outside. Additionally, it gives more surface and volume to its conducting area, lowering its impedance.
- Earth ground: Planet Earth, the soil. Should be the global and unique reference to all circuit points. Think of its sense of mass; if the setup is grounded, it becomes rooted to Earth and gain its inertia. It somehow becomes stronger against anything else willing to mess around with it, like interferences.
4.3 Star Ground
Besides specific applications and special signaling (i.e., floating), all ground types should generally be connected, and ultimately to Earth ground. We could do it arbitrarily, but a fractal-like structure (or star-like) is the best approach.
Figure 7: Star grounding: All grounds get referred to a unique Earth ground point, following similar paths.
This way, all circuits on the top get (almost) the same voltage, near zero Volts, compared to Earth ground.
4.4 Objective
What does our measurement setup earn if it’s (properly) connected to ground? It brings at least two advantages: safety and shielding against interferences.
4.4.1 Functional grounding – Shielding (Faraday Cage)
In the end, we want to protect all the wires, all the cables, and all the interconnected devices like the sensor and our DAQ against interferences. The Faraday Cage concept (Figure 8) enables a metal conductor enclosure to act as a shield if grounded. If a metal is left ungrounded, it will act as an amplifier of interferences.
Figure 8: Faraday Cage configuration. A metal enclosure shields devices inside.
[https://backyardbrains.com/experiments/faraday]
The enclosure or housing, continuous or not, covers our susceptible devices or signals. For flexibility, a mesh is often used instead of a solid material. Like the braid in a cable, the gaps in the braid should be smaller than the wavelength of the interferences we want to protect us against.
NOTE: There are also Faraday Cages configurations against magnetic fields, with different materials and principles.
4.4.2 Protective grounding – User safety
Another function of grounding is to protect the device and any person who comes in contact with the equipment. See Figure 9 for description of protective grounding:
Figure 9: Other applications of grounding, including safety. Use 3-prong pin plugs, rather than 2-prong plugs.
If there’s a faulty high voltage device (meaning more than 50 Volts AC, which is not the case of our current Simcenter SCADAS), that by accident gets a voltage line to the chassis, and it happens that the chassis is NOT grounded, the current will follow the only path it can encounter to the ground, i.e., a person could get electrocuted. So, ground your devices, always.
And don’t cheat the plug. Just take the ground terminal from your mains socket as normal. The problems we will want to solve related to this practice are addressed wisely in this document.
5. Cables
Devices (i.e., sensors and DAQ) need cables to be interconnected. Considering interferences, a cable can be a problem, connecting sources of problems.
First of all, the metallic cable braid (otherwise called mesh, or shield) should be configured as a Faraday Cage and used as shield to protect inner wires. Cable shielding is enabled by ensuring its braid is grounded.
5.1 Coax vs. STP
Regarding cable types, although there are a large number of options (sensor connector type, DAQ sockets, availability of mating cables, etc.), there are two fundamental shield types:
And don’t cheat the plug. Just take the ground terminal from your mains socket as normal. The problems we will want to solve related to this practice are addressed wisely in this document.
5. Cables
Devices (i.e., sensors and DAQ) need cables to be interconnected. Considering interferences, a cable can be a problem, connecting sources of problems.
First of all, the metallic cable braid (otherwise called mesh, or shield) should be configured as a Faraday Cage and used as shield to protect inner wires. Cable shielding is enabled by ensuring its braid is grounded.
5.1 Coax vs. STP
Regarding cable types, although there are a large number of options (sensor connector type, DAQ sockets, availability of mating cables, etc.), there are two fundamental shield types:
Figure 10: Coaxial cable (left) vs Shielded Twisted Pairs (STP, right). SE = Single-ended, DIFF = Differential
Description of these shield types:
- Coaxial: Its outer conductor is the “braid” and a signal: the signal ground. Signal and chassis ground are grounded automatically by the connector. The fact that the current flows in the outer conductor makes the core wire most protected against magnetic fields. Perfect for Single Ended (SE) signaling, reaching larger distances and higher bandwidths, as the outer conductor contacts cases by construction, so it gets automatically grounded (if the device is also grounded of course)
- Shielded Twisted Pairs (STP): Just have it shielded with a mesh, and only one mesh, and not with a continuous foil because of a so-called skin effect. They are normally thinner and cheaper, and they allow differential signaling, as the number of wires isn’t limited to one like the coaxial.
There are several good practices when it comes to cabling (Figure 11):
Figure 11: Several common and specific good practices for a smart cabling regarding susceptible and noisy devices.
In all cases, regarding general good practices in cabling:
- Short cables
- Larger cross-section (higher cross-sectional area) and low impedance cables
- Cables of same nature to be bundled together
- Susceptible bundles to be kept separated from the noisy ones, so we achieve the smallest possible surface of loops that may and will couple magnetically
- Perpendicular crossings, if they need to, so magnetic field lines from ones won’t “see” the others
- Balanced wires of same sizes and spacing
- Minimized bending
- Twisted pairs, both signals and supply wires
6. Ground The Setup
Bring ground to all metallic parts: DAQ, cable braids, sensor cases, and the Device Under Test (DUT). Spoiler alert again: ground them only from one point to avoid a loop:
6.1 DAQ & DUT
In Figure 12 below, the sensor is grounded only from one point, the cable shield, and not from the DUT contact thanks to the isolator:
Figure 12: Our setup properly grounded if we can ensure isolation between sensor case and Device Under Test (DUT).
Especially important is not to forget to ground the DUT. You don’t want cables to potentially couple with the big metal DUT (the closest thing to the sensors) and thereby transfer interferences into the measurement signals and sensors.
And how do you ground your setup? In short, you could say just connect things to the Planet... Well of course we probably have some more comfortable connection provisions, both in your devices and your lab or vehicle.
6.2 Simcenter SCADAS Hardware
Grounding options and single-ended versus differential options for SCADAS Mobile/Recorder and SCADAS RS are shown in Figure 13.
Figure 13: Top: Simcenter SCADAS Lab(L)/Mobile (M)/Recorder(R) (top left) and SCADAS RS (bottom left) offer analog input modules and units like seen in this table. Bottom: Grounding kit (bottom left) and Siemens AC/DC adapter (bottom right).
Regarding grounding provision, it’s important to remark that Simcenter SCADAS Lab (SCL) /Mobile (SCM) /Recorder (SCR) get grounded via their frames. The frames are grounded by the mains of the AC/DC adapter or via the grounding kit.
On the other hand, Simcenter SCADAS RS does not get grounded via the AC/DC power supply nor via the mounting stack mechanism. At least one of the Simcenter SCADAS RS units must be grounded using the grounding kit available as an accessory, for example. All other units interconnected via a daisy chain cable (of 10m or less) will get ground from the daisy chain cable.
6.3 Laboratory versus Portable Scenarios
Where to get ground from? Some examples of grounding options in a laboratory setting versus a portable setting are shown in Figure 14.
On the other hand, Simcenter SCADAS RS does not get grounded via the AC/DC power supply nor via the mounting stack mechanism. At least one of the Simcenter SCADAS RS units must be grounded using the grounding kit available as an accessory, for example. All other units interconnected via a daisy chain cable (of 10m or less) will get ground from the daisy chain cable.
6.3 Laboratory versus Portable Scenarios
Where to get ground from? Some examples of grounding options in a laboratory setting versus a portable setting are shown in Figure 14.
Figure 14: In the lab (left), Earth ground may be found in some building electrical panels, or in the mains socket. In a portable application (right), Earth ground isn't available, so the vehicle chassis can be used as ground.
In a lab, some electrical panels in the building have a specific place where the ground can be taken, normally via the typical yellow and green cable. These typically go to Earth ground.
And what if there is no access to the Earth, like in a portable application? Instead of Earth, the largest available metallic component in the portable test article should be used as the ground instead. For example, a metallic vehicle chassis could be used as the ground because of its large size and the fact that it is a good conductor (i.e., has low impedance). Individual non-metallic components like a seat or carpet would not be good conductors and are not suitable for grounding.
7. DAQ Inputs
So, let’s ground everything ad libitum! Shouldn’t we? Remember that we should ground all components of the measurement chain (cables, DAQs, DUTs, sensors, etc). But there should only be one path to ground from each component. Otherwise, we would have created a ground loop.
7.1 Ground loops – Break the loop!
A physical ground loop is accidentally created when a ground conductor has more than one path to Earth ground. The only enabler for a ground loop in our measurement setup, considering only the elements in Figure 15, is the contact between the sensor case and the DUT. This does not mean that we will automatically have a ground loop if that contact exists. Anyway, this contact is necessary for a ground loop.
Figure 15: Ground loops need electrical contact between sensor case and DUT. In this figure, a single ended (SE) signaling example
We are especially interested in those potential loops involving our channel signal ground: the one that will be harmful to our measurement data.
The loop per se is harmless in an ideal world. The problem comes when considering interferences: a ground loop has not one, but two big, wide-open doors both to galvanic and magnetic couplings:
The loop per se is harmless in an ideal world. The problem comes when considering interferences: a ground loop has not one, but two big, wide-open doors both to galvanic and magnetic couplings:
- Ground Loops Couple Galvanically: Different paths to ground will probably mean different ground potentials; they will probably present different lengths and/or different connections to different grounding points. If there’s a potential difference in a loop, there’s a voltage and a circuit, so there will be a current. Seen otherwise: The sensor will see a different signal ground value than the one DAQ is seeing. Reasons may be:
- Separation between those points by a long distance, like it may happen between the sensor and our DAQ. This distance is by the way modelled by that resistance in the Earth line
- Current that may already be flowing in the Earth line because of other devices plugged into the same electrical supply line, like laptops, engines, ac/dc adaptors, big machinery, etc.
- Ground Loops Couple Magnetically: Actually, everything with circuits or loops will couple magnetically, and we do want to avoid creating more of them with a wide area (considering that sensor and DAQ are normally relatively far from each other).
7.2 Grounded and Ground Loop Free Setups
First of all, the DAQ and the DUT are supposed to be already grounded by default. They may have their own ground (in the Figures 16-18, DAQ has a purple ground and DUT has a blueish ground, colored for clarity).
We will picture the three most straightforward and practical options. All of them are valid, grounded and ground loop free.
7.2.1 DUT – Sensor Case Not in Contact: Star Ground Compliant
This configuration is the ideal one, the one to go by default: Do not make electrical contact between the sensor case and the DUT as shown in Figure 16.
Figure 16: Preferred grounded and ground loop free setup. Flexible and star ground compliant.
Use a plastic feed, tape, or isolator (or other) to separate the DUT and sensor. The cable shield should normally be connected at both sides. This is the only way to follow the star ground principle: see how the sensor at the far end gets beautifully purple-grounded from the DAQ. No floating* inputs are needed, and it’s independent of the sensor case being isolated or not.
On top of that, we shouldn’t forget that our setup is normally made of more than one sensor, sometimes lots of them. So, it will be enough for one, and only one of them to present a ground loop, to spoil the whole setup for those sensors not being isolated from the DUT in the first place.
NOTE: Bridge based devices, like strain gauges, have the particularity that they don’t have any case. So, for strain gages, we are always doing good, given that cable braid should shield the wires up to the strain gage end.
(*) For simplicity, consider a floating input as having its signal ground detached from other inputs and from DAQ’s chassis ground via a capacitor. In other words, the signal ground path dies in a floating input. This floating condition will degrade with frequency.
7.2.2 Sensor Case Isolated
When sensor-DUT contact can’t be avoided, then we should start thinking about exceptions as shown in Figure 17.
Figure 17: Grounded and ground loop free setup if sensor case contacts DUT: Use sensor case isolated sensor, and avoid its cable braid contact.
If we are lucky and the sensor is case-isolated, or there can be a version of the same sensor with case-isolation (meaning we will probably need to pay more for it), a cable whose braid isn’t connected at the sensor’s side will be enough. If this cable is coaxial, a case isolated sensor might take care of this in its BNC connector itself, not touching the case.
7.2.3 Floating DAQ Inputs
Finally, if we still can’t avoid contact between sensor-DUT, and we only have a normal-regular case sensor, we should turn to the use of a floating input and avoid cable braid contact to DAQ’s side as shown in Figure 18.
Figure 18: Grounded and ground loop free setup if sensor case contacts DUT and it’s a regular sensor case: Use floating input and avoid its cable braid contact.
We have been considering single ended signaling by default. If a differential input or a floating input is used, the ground loop won’t affect our input signal around DC. For a differential input the ground loop doesn’t practically affect our plus/minus signals; for a floating input the ground loop is de facto broken for DC. However, those inputs won’t help much for higher frequency ground loops, i.e., already around 1kHz. Both Common Mode Rejection (CMR) in differential inputs and parasitic capacitances in floating inputs degrade their shielding function against high frequencies.
So-called semi-differential inputs could also do the job thanks to their almost-floating characteristic.
We should then not forget that high frequency ground loops will start hurting for this option.
7.3 Additional Notes
When all channels in a device are floating, and the device offers independent sensor supplies per channel, it’s said that the DAQ is a channel-to-channel isolated device. There is an associated parameter to a floating input, called isolation voltage, or that voltage that would short this floating characteristic with an arc flash. In this document we are focusing on functional grounding, being +/-100V isolation a decent value. When grounding for safety becomes relevant (for example when our sensors need to be placed in some specific parts of an electric vehicle), we can talk about kV of isolation voltage.
There may be other combinations, but too limiting in terms of flexibility. For simplicity, we didn’t picture any sensor supply coming from the DAQ, and will consider Single-Ended signaling (i.e., ICP, analog tacho, 2-wire signaling sensors). For example, we could think of an ICP accelerometer, or a MEMS accelerometer connection. The same reasoning will apply for differential signaling (i.e., bridges, current loops, LVDT, potentiometers, 3-wire signaling sensors).
See the knowledge article: Single Ended vs Differential Inputs
8. DAQ Outputs
Up to this point we have just considered two main actors in our setup and their cables: Our DAQ and the DUT. But what about any other external connected devices (i.e., PC, shakers, etc.)? They may be considered as another factor multiplying the probability of ground loops: We should always break the loop between our setup and these external devices, to be sure they won’t spoil all our efforts until this point.
8.1 PC connection
Never connect cable braid between DAQ and PC!
Figure 19: Never connect the cable braid at the PC side.
A good practice is to already break the loop at the cable braid on the PC side.
8.2 DAC and Shakers
If our DAQ has also an output to a shaker, it should also be free of ground loops, because it’s analog and we know analog signals are the first to be protected against interferences.
Several shakers are hydraulic systems, others electrodynamic, and they can draw very high power. Some shaker amplifiers can be configured as differential or floating inputs. Most amplifier builders are aware of ground loop issues, so they normally take a smart design.
In problematic situations, a ground isolator can be used (sometimes space test laboratories have them).
8.3 External Power Supplies
If we need to use external power supplies for our sensors, and wanted to get the excellence in grounding, here’s the star ground concept back as a good practice. The principle is bringing all our ground plugs to the same point, like a grounding fractal tree.
Figure 20: Star ground concept for external power supplies.
So, avoid plugging an external sensor supply next to the sensor, and rather connect it to the same mains socket of the DAQ.
9. Practical Examples
At this point, the idea should be clear. Grounding is not only a best practice; it is needed for our applications. It enables shielding against electric fields that can, and will present a threat for our delicate, accurate sensors and measurement signals.
9.1 Does My Measurement Setup Suffer from Interferences?
We might find an existing measurement setup with persisting noise problems, or we might just be seeing unexpected data and wondering if our setup suffers from interferences. Especially, knowing that magnetic coupling is difficult to eliminate completely.
Figure 21: By short-circuiting your input at the DAQ side (connect input signal pin to GND pin, i.e., with a paper clip), we get rid of any external factor impacting the measurement, other than your DAQ internal noise level.
This example can be taken as a proof that there’s an interference coupling problem in our setup if we measure unforeseen signals. Thanks to the fact that Simcenter SCADAS hardware is fully protected at device level, we can also remove the DAQ itself from the equation: just short the input of the channel. There won’t be any trace of interferences, as no external coupling is possible, including the magnetic.
Otherwise stated: If you wanted to consciously couple interferences present in your setup and surroundings, the easiest way is to leave the cable connected to the DAQ without a sensor in the far end. The effect can be amplified if we don’t follow this document good practices.
9.2 Do-It-Yourself in Your Office Desk: Impact of an Ungrounded Setup
Here’s a good example about the consequences of leaving conductors ungrounded, both in our setup (DUT: load cell body) and surrounding it (ourselves). Although not a perfect one, we are a floating conductor in a soup of interferences even in your office desk.
Figure 22: Leaving DUT ungrounded has a major impact in interferences coupling.
Just connect a load cell to your grounded DAQ but don’t ground the DUT. The load cell body is the closest thing to your strain gages and is left ungrounded. The strain gage will see everything the body wants: Because of not being grounded, the body couples interferences present in our office. They come from fluorescent lights, power supplies, power grid… And also come from ourselves, as we act as a pick-up antenna and radiate them amplified back to the surroundings.
You should then try to move, walk, or even wave your hands around the DUT, and you will see several dB of difference in the pick-up noise from the DUT right in your channel input.
10. Quick Guide
Figure 23 is a summary of the contents of this article. If instrumenting from scratch, this can be used as a guide for a well grounded measurement setup.
Figure 23: Quick guide for grounded and ground loop free setups.
Following the quick guide should result in a ground loop free setup that is robust against interferences. It can also be used as a troubleshooting guide for a problematic setup.
NOTE: The third option for triaxial accelerometers with an early-split cable is made for simplicity. The optimal connection for that very specific setup requires a mix of solutions.
Questions? Email aleix.riera@siemens.com.
Related Links:
- Introduction to Filters: FIR versus IIR
- Data Acquisition: Anti-Aliasing Filters
- Simcenter SCADAS hardware
- Simcenter SCADAS Mobile and SCADAS Recorder
- Simcenter SCADAS RS hardware
- The Gibbs Phenomenon
- AC versus DC Coupling - What's the difference?
- Single Ended vs Differential Inputs
- Overloads
- Gain, Range, and Quantization
- Tell Me What Filter You Use, and I’ll Tell You Who You Are!