Proper ground configuration is essential for a good data acquisition system. Most measurement systems such as data acquisition devices allow for many different types of ground configurations depending on the type of signal being acquired or measured. This flexibility is the source of confusion when deciding which configuration to use in each specific situation.
National Instruments tutorial Ground Loops and Returns teaches you to select the right configuration to use. Here is the material from the document related to ground loop problems in condensed format:
A grounded signal source is one in which the voltage signals are referenced to a system ground, such as earth or building ground. The most common examples of grounded signal sources are devices, such as power supplies, oscilloscopes, and signal generators that plug into the building ground through a wall outlet. The difference in ground potential between two instruments connected to the same building ground system is typically 10mV to 200mV, or even more (up to several volts at normal use and tens of volts during short circuit surges).
Single ended is the “default” configuration for most data acquisition devices, modular instruments, and stand-alone devices. Single-ended systems are very susceptible to ground loops. There are essentially two main types of Single-Ended measurement systems: Ground Referenced Single Ended (GRSE) and Non-Referenced Single-Ended (NRSE).
An ideal differential measurement system reads only the potential difference between the positive and negative terminals of the amplifier and thus it completely rejects common-mode voltages. However, practical devices are limited in their ability to reject common-mode voltage.
A grounded signal source is best measured with a differential or non-referenced measurement system.
The pitfall of using a ground-referenced measurement system to measure a grounded signal source is that grounding potential difference between signal source and measuring system causes a current called ground loop current to flow in the interconnection which can greatly affect measurements causing offset errors, especially when measuring low level signals from sensors.
Image source: http://zone.ni.com/devzone/cda/tut/p/id/3394
15 Comments
Sam Freed says:
Hey Tomi – glad you found our tutorial on ground loops and returns useful… and nice article explaining the concepts!
We have a couple of closely related topics that your visitors might find helpful in their applications:
Field Wiring and Noise Considerations for Analog signals: http://zone.ni.com/devzone/cda/tut/p/id/3344
Five Tips to Reduce Measurement Noise: http://zone.ni.com/devzone/cda/pub/p/id/262
-Sam Freed, National Instruments
tomi says:
Thank you for your feedback.
The related articles you mentioned are really worth to read.
Matthew C. Kriner says:
Great information! Thanks!
data acquisition systems says:
I really get a lot out of reading web sites about the topic of data acquisition systems programs. There are so many programs. Which one have you found the most flexible? I don’t comment on many web sites but had to on yours.
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Jon Brink says:
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Tomi Engdahl says:
Q: do you have any suggestions for shield grounding on cables on machinery that interconnect (sometimes to 2 or 3 cables) from sensor to controller to avoid ground loops.
A: The classical “ground loop” is really a low-frequency phenomenon (<50 kHz, or so) and is usually typified by 60/120 Hz buzzing in audio systems through common-impedance coupling. However, I understand your question regarding shielding at one end or both ends. For systems with highly distributed signal references, where the potential difference between the main controller digital return and and various sensor returns can be quite different. The result would be noise currents flowing in the shield. In this case, it might be best to connect just the one end.
In the aerospace world, where we might construct an umbilical cable 300 feet long for a missile, NASA, ESA, and other like agencies specify all cables be constructed using the “Spacewire” standard (http://www.spacewire.esa.int/). This standard dictates that cable shields be connected at the source end only, but optionally may use the “hybrid” grounding scheme where a series capacitor is used to connect the non-source end of the shield. Refer to that standard (free download) for typical wiring diagrams.
For sensor technologies, it’s also common to use various means to “break” any noise currents in the shielded twisted pair by using opto-isolators, differential pairs, common-mode chokes, and the like.
Source: http://www.edn.com/electronics-blogs/the-emc-blog/4438390/EMC-questions-answered–part-6-?_mc=NL_EDN_EDT_EDN_today_20150121&cid=NL_EDN_EDT_EDN_today_20150121&elq=76d9d8ab506e46bdaddd00a63e5bc411&elqCampaignId=21259
Tomi Engdahl says:
4-Wire Current-Loop Sensor Transmitters
http://www.planetanalog.com/author.asp?section_id=3117
The 4-wire sensor transmitter is probably the least well-known of the current-loop sensor transmitter circuit types. These transmitters fit market needs for applications that require additional transmitter isolation options that aren’t possible with 2- and 3-wire transmitters.
Unlike the 2- and 3-wire transmitter representations shown in Figure 2, the 4-wire circuit has separate paths for the power current and signal current. Also, the 4-wire receiver does not share a common return (GND) with the power supply. This allows for several new isolation schemes, including fully isolated, power-isolated and output-isolated transmitters that expand on the input-isolated and non-isolated topologies we described for 2-wire and 3-wire transmitters.
While non-isolated and input-isolated systems exist for 2- and 3-wire sensor transmitters, these isolation schemes are not possible when designing with 4-wire transmitters. This is because non-isolated and input-isolated transmitters do not require isolation between the power supply and the output transmitter and receiver.
Tomi Engdahl says:
Ground Loop Problems with Measurement Systems and How to Avoid Them
http://www.pfinc.com/paper_briefs/Ground_Loop_Problems.pdf
Tomi Engdahl says:
Solving ground loop problems in pH process installations
http://www.all-about-ph.com/ground-loop-problems.html
If you are struggling with unstable pH values for your process measurement, there may be ground loop problems. A ground loop exists when an electric circuit is connected to earth ground at two or more points with different potentials. Different earth grounds are supposed to be at the same potential, but the potential of the earth varies from point to point.
In a typical process pH measurement, the electrode is connected through the process liquid and piping to earth ground. The pH transmitter is in most cases grounded with a grounding wire to the power outlet or safety ground and via the electrode to the process liquid. Those are two grounding points, and what will probably happen is that current will flow through the electrode wiring. The currents created by ground loops are often fluctuating a lot, and will therefore, produce an erratic and unpredictable pH measurement.
Tomi Engdahl says:
How To Measure Voltage
http://www.ni.com/tutorial/7113/en/
By providing electrical isolation, you can break ground loops, increase the common-mode range of the data acquisition system, and level shift the signal ground reference to a single system ground. Safety isolation references standards that have specific requirements for isolating humans from contact with hazardous voltages. It also characterizes the ability of an electrical system to prevent high-voltage and transient voltages to be transmitted across its boundary to other electrical systems with which the user may come in contact.
Incorporating isolation into a data acquisition system has three primary functions: preventing ground loops, rejecting common-mode voltage, and providing safety.
Isolation Types and Considerations when Taking a Measurement
http://www.ni.com/white-paper/3410/en/
Tomi Engdahl says:
Measuring current using a transmitter’s test connection – don’t make this mistake!
https://blog.beamex.com/measuring-current-using-a-transmitters-test-connection-dont-make-this-mistake
Using a mA meter with an internal impedance that is too high to measure current over the transmitter’s test connection will result in erroneous measurement results!
Tomi Engdahl says:
Pt100 temperature sensor – useful things to know
https://blog.beamex.com/pt100-temperature-sensor