Monday, May 10, 2021 · 0 min read
The Importance of Isolation in Data Acquisition Systems
In this article we will learn about the importance of isolation in Data Acquisition (DAQ) systems, describing it with enough detail that you will:
See what electrical isolation means
Learn about the different ways that isolation is achieved
Understand the importance of isolation in the DAQ process and its measurements
Are you ready to get started? Let’s go!
What is electrical isolation?
Sometimes also referred to as galvanic isolation, electrical isolation is the separation of a circuit from other sources of electrical potential.
Why is isolation needed?
Interfering potentials can be both AC and DC in nature. For example, when a sensor is placed directly on an article under test, (e.g. a power supply) which has a potential above ground (i.e., greater than 0 Volts), this can impose a DC offset on the signal. Electrical interference or noise can also take the form of AC signals created by other electrical components in the signal path or in the environment around the test.
Isolation is especially important with respect to the analog input signals that we want to measure. So many of these signals exist at relatively low levels, and external electrical potentials can influence the signal greatly, resulting in wrong readings. Imagine the output of a thermocouple, which is just a few thousands of a volt, and how easily it could be overwhelmed with electrical interference.
Even the regular line power in our buildings generates an electrical field at 50 or 60 Hz, depending on your country. This is why the best data acquisition systems have isolated inputs - to preserve the integrity of the signal chain and ensure that what the sensor outputs is truly what has been read.
There are also high voltages that, if allowed to cross-connect by a non-isolated system, can damage or destroy expensive equipment. In the worst case, it can bring physical harm or even death to the test operator. Voltages that are dangerous to people are generally considered to be those that are greater than 30 Vrms, 42.4 VAC, or 60 VDC.
In the world of test and measurement, avoiding or eliminating ground loops and common-mode voltage overloads is critical to making accurate measurements, protecting test equipment and objects under test, and most importantly protecting human beings from dangerous voltage potentials.
Before our signals go past the amplifier and are sent into the analog to digital converters, we must ensure their integrity, and the best way to do that is with isolation.
Check out Dewesoft's fully channel-to-channel isolated modern, digital data acquisition systems
When is isolation needed?
An easier question might be “when is isolation NOT needed?” Ask yourself these questions when considering if your application would require isolated inputs:
Are dangerous high voltages nearby? (High tension wires outside? Power generators?)
Are there large motors, turbines, welding machines, or any machines using heavy current in the same building or on the same power network?
Does your power system’s ground potential fluctuate or change?
Is your power system ever subject to electrical spikes or transients? Are you in a high-lightening potential area?
Are you making very millivolt level signal measurements directly on components or structures that may exist at a different voltage potential?
If one or more of these apply to you, then isolated inputs are probably warranted.
Let’s take a look at the measuring environment in key DAQ applications and their possible sources of signal interference:
|High voltages, power generators||Large motors, turbines, welding machines||Fluctuating ground potentials||Electrical spikes or transients||Millivolt level signals being measured|
|Jet engine plant||YesPower generatorsInverters||Yes||Possible||Possible||YesThermocouplesStrain gagesCharge accelerometers|
|Power plant||Yes Always!||Yes MotorsTurbines||Possible||YesSwitching relayBreaker transients||Yes|
|Test tracks||No||No||Yes(vehicle DC bus)||YesLightningBattery change-overs||YesThermocouplesStrain gages|
|Flight Test Center||Yes||Possible||Yespower switchingAC/DC buses||YesLightening||YesThermocouplesCharge accelerometers Strain gages|
|Structural Tests (laboratory)||Rarely||Rarely||No||Possible||YesStrain gagesCharge accelerometers|
|Structural Tests (outside)||Possible||Rarely||Possible||YesLightening||YesStrain gagesCharge accelerometers|
It’s clear that there is basically no major application that is not subject to interferences from the natural or man-made environment that isolated inputs can mitigate or eliminate completely.
Measurement systems that do not offer isolated inputs are less expensive than those with it. However, what is the point of a measuring system if not to make accurate, noise-free measurements?
Common-mode voltage problems and the solution
Common mode voltages are unwanted signals that get into the measurement chain, usually from the cable connecting a sensor to the measuring system. Sometimes referred to as “noise” these voltages distort the real signal that we’re trying to measure. Depending on their amplitude, they can range from being a “minor annoyance” to completely obscuring the real signal and destroying the measurement.
The most basic approach to eliminating common-mode signals is to use a differential amplifier. This amplifier has two inputs: a positive one and a negative one. The amplifier measures only the difference between the two inputs.
Electrical noise riding along on our sensor cable should be present on both lines - the signal positive line and the ground (or signal-negative) line. The differential amplifier will reject the signals common to both lines, and only the signal will be passed through, as shown in the graphic below:
This works great, but there are limits to how much common-mode voltage (CMV) the amplifier can reject. When the CMV present on the signal lines exceeds the differential amplifier’s maximum CMV input range, it will “clip.” The result is a distorted, unusable output signal, as shown below:
So in these cases, we need an additional layer of protection against CMV and electrical noise in general (as well as ground looping, which will be discussed in the next section) - isolation.
An isolated amplifier’s inputs “float” above the common-mode voltage. They are designed with an isolation barrier with a breakdown voltage of 1000 volts or more. This allows it to reject very high CMV noise and eliminate ground loops.
Isolated amplifiers create this isolation barrier by using tiny transformers to decouple (“float”) the input from the output, or by small optocouplers, or by capacitive coupling. The last two methods typically provide the best bandwidth performance.
What is common-mode voltage rejection ratio - CMRR
Common mode rejection ratio (CMRR) of a differential amplifier (or other devices) is a metric used to quantify the ability of the device to reject common-mode signals, i.e. those that appear simultaneously and in phase on both inputs.
An ideal differential amplifier would have infinite CMRR. However, this is not achievable in practice. A high CMRR is required when a differential signal must be amplified in the presence of a possibly large common-mode input, such as strong electromagnetic interference (EMI).
Ground loop problems and the solution
Unless they are prevented, ground loops can be a serious problem for measuring systems. Sometimes called “noise,” a ground loop is caused by inadvertently referencing electrical equipment to more than one path to the ground - any difference of potential at these grounding points can induce a current loop, which can lead to distortions in the signal. If the amplitude of these distortions is high enough, it can ruin the measurement.
In the picture below, the measurement amplifier is connected to the ground (GND 1) on one side. An asymmetrical shielded cable is used to connect the sensor, whose metallic housing is placed on a conductive surface at GND 2. Due to the length of the cable, there is a difference in potential between GND1 and GND 2. This potential difference acts like a voltage source, coupling with the electromagnetic noise from the environment.
If the sensor could be decoupled from GND2, it might solve the problem. But sometimes this is not possible. Furthermore, sometimes the referencing of a cable shield is required by safety rules, and therefore should not be removed.
The best solution is to use a differential amplifier within the signal conditioner which is isolated. With this one change, the problem is solved.
Ground loops can also come from the instrument itself, via its own power supply. Keeping in mind that our measuring system is plugged into power, which has a ground reference. It is critical, therefore to decouple this reference from the signal-handling components of the instrument to ensure that ground loops cannot be created within the instrument.
This scenario can become dangerous if there is a wiring fault. Looking at the high current path from the power supply, what will happen if the return line is broken? All of the energy will be routed through the signal conditioning part of the DAQ system. This could result in the entire system being damaged or destroyed, and even dangerous potentials for the human operator of the instrument.
By isolating the signal path completely from the power supply, it is not possible for the scenario above to occur.
There are two basic domains in which isolation can be achieved:
Analog domain isolation
Analog domain isolation is used with the outputs of analog sensors. This isolation takes place in the analog domain, i.e., before the ADC subsystem.
In any analog isolation system, it is critical that the gain and offset accuracy be quite high because we don’t want to digitize wrong signals.
Digital domain isolation
When our signals are digital, to begin with, we can employ digital isolation techniques to protect our signals, system, and human operators.
In this case, an isolation barrier separates the outside signal from the re-creation on the inside of the circuit. The isolated digital signal is then available to be routed to microprocessors, FPGAs, gate drivers, etc.
Now let’s look at the three basic types of isolation techniques that are used in both analog and digital isolation.
Three basic isolation techniques
There are several approaches for creating an isolation barrier between a signal source and the rest of the system:
Let’s take a look at each of them in this section.
Optical isolation is one of the most popular and effective methods of isolating a signal from the rest of the system and the outside world. An electrical signal is input to an LED, which transmits it across a dielectric isolation barrier to a photodiode, which converts it back to an electrical signal.
By converting an electrical signal to light and then back to electricity, it is completely decoupled from the outside world. Light is not susceptible to electromagnetic (EMI) or radio frequency (RFI) interference, the inherent benefit of this approach.
However, optocouplers are not as fast as light itself - they are limited by the LED’s switching speed. They are generally slower than inductive or capacitive isolators. In addition, the intensity of the LED light will degrade over time, requiring recalibration or replacement.
Engineers know that electrical current creates a magnetic field. By sending a signal into a winding and positioning it near to and in parallel with an identical winding, a representation of the signal will be induced, or “coupled” into the second winding.
In inductive coupling isolation, an electrical insulation barrier is placed between the windings so that the only signals that pass from the first winding to the second are those that have been magnetically induced - and there is no direct contact across the barrier. Inductive couplers have very high bandwidth and are extremely reliable, but they can be affected by nearby magnetic fields.
Capacitive isolators couple a signal across an isolation barrier, usually made from silicon dioxide. They cannot pass DC signals, which makes them very adept at blocking common-mode signals. The signal is converted to digital and then replicated on the other side of the barrier using capacitive coupling.
Unlike inductive isolation, capacitive isolation is not susceptible to magnetic interference. High data rates and long-life operation are hallmarks of these isolators. Capacitive isolators are available with different ratings to provide the right level of safety against failure and possible short circuits.
Isolation techniques comparison
Here is a high-level comparison of our three basic isolation techniques:
|Data rates||Medium(limited by LED switching speed)||Fast~100 Mb/s||Fast~100 Mb/s|
|Dielectric strength||Good~100 Vrms/µm||Better~300 Vrms/µm||Best~500 Vrms/µm|
|Magnetic interference||None||Can be affected||None|
Key isolation terms
Given all of the information above, it seems clear that our measuring systems should have isolated analog inputs. But when you’re reviewing the isolation specifications of various measurement systems and signal conditioners, you may find it specified with terms like “channel-to-ground” and “channel-to-channel.” What do these terms mean, and how do they relate to each other?
Channel-to-ground isolation defines the maximum voltage there can be between a channel’s input and instrument ground. Normally an instrument’s ground is referenced to the ground of the power supply. By isolating signal ground from chassis ground we can eliminate most ground loop problems.
Sometimes this is also referred to as input-to-output isolation. All of the channels share a common ground, which is isolated from the ground or earth potential of the instrument. This would not be a limitation if only one signal source were connected to the system. But when additional signals are connected, each with ground potential differences, it can lead to noise across all signals and common mode problems.
If two or more channels share a common ground, then they are not galvanically isolated. Take care when an instrument mentions only input-to-output or channel-to-ground isolation.
Channel-to-channel isolation defines the maximum voltage there can be between a channel and any other channel. Channels cannot share a ground bus, for example. Each channel must also be isolated from the rest of the system, e.g. the system’s power supply, chassis ground, and so on. If all channels are isolated from each other, then they are necessarily also isolated from the ground, so channel-to-ground isolation is included within channel-to-channel isolation.
So, if a system has channel-to-ground isolation, it does not necessarily mean that it has channel-to-channel isolation. But, if a system has channel-to-channel isolation, then it must also have channel-to-ground isolation.
SIRIUS DAQ systems from Dewesoft provide both channel-to-channel and channel-to-ground isolation, as shown in this short video:
Dielectric strength is the maximum voltage level at which an isolation barrier can prevent the signal from crossing. Various insulation materials have different dielectric strengths, measured in Vrms/µm. An air gap itself is typically rated at 1 Vrms/µm, whereas epoxies can be 20 times better, and the Silicon Dioxide found in many capacitive isolation barriers is roughly 500 Vrms/µm. There are other materials commonly used in barriers, including the polyimides found in capacitive isolators, and silica-filled epoxy molding compounds often found in optical isolators.
Dewesoft isolated data acquisition systems
SIRIUS data acquisition systems
SIRIUS high-speed DAQ systems are available in a wide array of physical configurations:
SIRIUS modular slices: connect to your computer via USB or EtherCAT
R3: 19'' rack-mounting DAQ system
R1/R2: stand-alone DAQ systems that include a built-in computer and optional display and batteries.
R4: stand-alone DAQ systems that include a built-in computer.
R8: high-channel-count stand-alone DAQ systems that include a built-in computer and optional display and batteries.
If you take a look at Dewesoft’s SIRIUS DualCore and SIRIUS HS (High-Speed) signal conditioners, you will see that all of these modules provide channel-to-channel and channel-to-ground isolation voltage of 1000V. SIRIUS HD (High Density) amplifiers are isolated ±500V in pairs.
The video below shows SIRIUS DAQ isolation in practice, in a real-world scenario:
In the real world of data acquisition, there are often more than just the signal inputs - signal conditioners often provide excitation voltage or current to power the sensors. Strain gages, RTDs, LVDTs, and IEPE accelerometers are all good examples of sensors that require power.
Sometimes overlooked by DAQ system makers, it is important that these excitation lines be isolated, which is why Dewesoft provides isolation and/or differential inputs and over-voltage protection with direct short-to-ground capability across its product line, and protects its instruments and human operators from ground loops.
KRYPTON and KRYPTON ONE data acquisition systems
KRYPTON is the most ruggedized range of products available from Dewesoft. Built to withstand extreme temperature and shock and vibration conditions, KRYPTON is rated to IP67, protecting them against water, dust, and more. They connect to any Windows computer via EtherCAT and can be separated by up to 100 meters (328 feet), allowing you to locate them near the signal source. Like SIRIUS, they run the most powerful DAQ software on the market, DewesoftX.
These extremely rugged systems are also available in single-channel modules called KRYPTON ONE. Both the multi-channel and single-channel KRYPTON modules provide the same level of performance and environmental robustness.
In terms of isolation performance, KRYPTON and KRYPTON-1 provide:
KRYPTON multi-channel modules
|Type||Strain / voltage||Thermocouple||RTD||IEPE / Voltage||Low Voltage||Low Amperes||Digital I/O|
|Isolation voltage||Differential||1000 V peak||1000 V peak||Differential||1000 V peak||1000 V peak||250 V|
KRYPTON ONE single-channel modules
|Isolation voltage||N/A||Galv.||Galv.||125 Vrms||125 Vrms||125 Vrms||1000V CAT II600 V CAT III||1000V CAT II600 V CAT III||N/A|
In the table above, Diff. means Differential, and Galv. refers to galvanic isolation.
IOLITE DAQ systems
IOLITE is a unique product that combines the essential capabilities of a real-time industrial control system with a powerful DAQ system. With IOLITE, hundreds of analog and digital channels can be recorded at full speed while simultaneously sending real-time data to any third-party EtherCAT master controller.
In terms of isolation performance, IOLITE provides:
IOLITE multi-channel input modules
|Type||Strain / V||Thermo||Dig Input||Dig Output||RTD||Low Voltage|
|Isolation Voltage||Differential||1000 V||1000 V||1000 V||1000 V||1000 V|