Issue 142 May2002
You're Not Alone
Dealing with Isolation

Ground Loops

When equipment using different power supplies is tied together (with a common ground connection) there is a potential for ground loop currents to exist. This is an induced current in the common ground line as a result of a difference in ground potentials at each piece of equipment. (Note: improper house wiring can cause ground loops when the neutral side of the line, or the ground, is not properly grounded.)

We normally think of all grounds as being of the same potential. If this were so, there would never be a ground loop problem. Here at Circuit Cellar world headquarters, I’ve measured a considerable difference between the grounds of different outlets (and different phases) within the same room.

There are a number of reasons that could explain these findings. It doesn’t take a large difference in potential to cause ground currents to flow through a common ground connection. The potentials (and currents created) are also load related, so, most of the time, these currents will not be steady state.

If sensor circuitry is based on its own ground as a reference and the system ground is not the same, you can’t expect to be able to take accurate measurements. You’d think that making the common ground heavier might be the solution. But, in many cases, this only increases ground loop current. Breaking this common ground is a better solution. However, if the common ground connection is broken, the differential in ground potentials remains and will affect any signal between the two pieces of equipment. You need to isolate the grounds as well as the other signals, otherwise you run the risk of exceeding the maximum or minimum allowable input specs.

To eliminate ground loop problems when connecting devices using grounded supplies located on different circuits, do not make a common ground connection between the devices you want connected. Although this eliminates ground currents from flowing between devices, it creates a problem for the signals, which are ground referenced.

Take communications interfaces for instance. RS-232 circuitry must have a ground connection because it is the reference for the remaining signal lines. On the other hand, RS-422/485 uses differential signals not referenced to ground. You can use this twisted pair connection without a common ground unless there is a difference of more than 7 V between them. The RS-485 receivers can withstand up to a 7-V ground-referenced difference before exceeding the maximum or minimum ratings. Would you gamble with circuit failure over a 7-V spread? This is where signal isolation payoff comes into play.

When dealing with sensors, ground loop currents cause changes in an analog signal. These changes often look like signal noise. A ground loop can even be caused by a mechanical and electrical (if uninsulated) connection to a grounded object being sensed. To eliminate all of the common ground loop problems between sensor and measurement circuitry, always power and measure sensors with the same local supply. By measuring right at the sensor, lengthy leads will carry digital data (easily isolated) rather than analog data (difficult to isolate).

The available IsoLoop products will handle most isolation problems. Be-sides the speed advantage over most optoisolators, the IsoLoop products have a latching output. Because the output state is latched on magnetic field change (controlled by the input to the device), even if the power is removed from the input side, the output side’s logic state would remain latched (memorized). This would require an extra set of latches when using an optoisolator.

NVE introduced its first GMR product in 1994. These days, GMR sensors compete with Hall effect devices for many magnetic sensing applications and additional research continues on the use of GMR materials for magnetoresistive random access memory (MRAM) technology. Can you say core memory? What goes around….