Dealing with the state of isolation

When we think isolation in the electronics context, we think of the transformer, because for years, transformers have been used to isolate signals. Magnetic fields transfer information without an electrical connection between primary and secondary windings creating the isolation barrier.

More recently, opto devices have become a popular method of obtaining isolation. Opto devices rely on light (or the absence of light) to transfer information between the physically separated primary (input) and secondary (output).

Although the transformer is inherently a bidirectional isolation device, most isolation application requirements are unidirectional.

Lightsolation

Opto devices have the advantage of operating down to DC whereas transformers are specified at some AC bandwidth. From an isolated control point of view, transformers can't be used directly for the steady-state isolation. Opto devices work well in this scenario.

An opto device does require a chunk of current to operate and the transfer ratio of input current to output current will usually require a trade-off of speed for drive current. This trade-off is not a problem until you want to transfer information at high speeds.

Most opto devices have minimum TON/TOFF times in the microsecond range.

Magnesolation

You may already be familiar with Hall effect devices, which can be used to measure the strength of a magnetic field. The Hall effect is the presence of a voltage produced across (x-axis) a current-carrying conductor (y-axis) as a result of exposure to a magnetic field passing through the conductor (z-axis).

The giant magnetoresistive (GMR) effect is a change in a thin film non-magnetic conductive layer's resistance caused by an external magnetic field overcoming parallel but opposing magnetic coupling of adjacent magnetised layers.

You'll remember that the spins of electrons in a magnet are aligned to produce a magnetic moment. Magnetic layers with opposing spins (magnetic moments) impede the progress of the electrons (higher scattering) through a sandwiched conductive layer.

This arrangement causes the conductor to have a higher resistance to current flow. An external magnetic field can realign all the layers into a single magnetic moment. When all this happens, electron flow will be less affected (lower scattering) by the uniform spins of the adjacent ferromagnetic layers.

This causes the conduction layer to have a lower resistance to current flow. Note that this phenomenon takes place only when the conduction layer is thin enough (less than 5 mm) for the ferromagnetic layer's electron spins to affect the conductive layer's electron's path.

To put this phenomenon to work, NVE uses a Wheatstone bridge configuration of four GMR sensors. Its manufacturing process allows thick film magnetic material to be deposited over the sensor elements to provide areas of magnetic shielding or flux concentration.

Various op-amp or in-amp configurations can be used to supply signal conditioning from the bridge's outputs. This forms the basis of an isolation receiver.

The isolation transmitter is simply coil circuitry deposited on a layer between the GMR sensors layers and the thick film magnetic shielding layer. Current through this coil layer produces the magnetic field, which overcomes the antiferromagnetic layers reducing the sensor's resistance.

NVE obtains isolate specifications of 2500 VRMS using its manufacturing process. Unlike typical microsecond TON/TOFF times of optoisolators, isoloop-isolators are typically 1 ns, which is more than 100 times faster than its light-based rival.

The isoloop-isolators also have identical TON/TOFF times, which produce no pulse-width distortion as is the case with many optoisolators having differing TON/TOFF times.

Propagation delays are less than 10 ns with inter-channel skewing of less than 2 ns.

Isoloop-isolators have up to four channels per package in a variety of device direction configurations. These standard devices are great for bus isolation, serial ADCs and DACs and communication isolation.

Power isolation

Isolation can't be achieved using a common power supply. This need creates a requirement for additional circuitry beyond the signal isolation. There must be a separate power supply for the circuitry on each side of the isolation barrier. Often this will be more expensive than the signal isolation.

The simplest solution might be to buy a DC-DC converter. In this case the converter is used as an isolator instead of converting from one voltage to another.

Take your time choosing a converter - if you aren't careful you can find yourself being locked into a single-source manufacturer, especially if you choose a converter with a weird pinout or physical size.

An alternative might be to build one directly onboard. Both Linear Technologies and Maxim specialise in power products. Linear's LT1424-5 is good to about 2 W. For lower currents up to a 1 W converter, take a look at Maxim's MAX253.

Few external components are required to use the 253 as a transformer driver for an isolated power supply circuit.

C&D Technologies makes an isolation transformer specially designed for the 253. The 78250 series transformer is available in 1500 and 4000 V isolation.

I made prototypes of the circuits to create an isolated RS485 transceiver for use with a microcontroller. Notice the surface mount IL-485 mounted on a dip header. Pins 4, 5, 12 and 13 of the IL-485 are soldered to the header and the other pins are wired to the appropriate dip header pins.

Using a dip header for surface mount parts makes them easier to handle and re-use elsewhere (after you get past the surgical wiring). Although the IL-485 is only available in the surface mount variety, other devices are through-hole components.

I measured typical required currents in the isoloop IL-485 circuit and found the isolated RS485 side required considerably less than 50 mA without any load on the twisted pair. Using the isolated voltage to supply the termination load adds 50 mA to the load requirements of the 2563.

The 253 circuit easily supplied this current at just over 5 V after the Schottky rectifier drop, thanks to the 1:1.31 winding of the transformer. No regulator is needed because this is within the recommended range for the 485 (4.5-5.5 VDC) and well below the 7 VDC absolute maximum rating.

The 7253 transformer from C&D Technologies is rated at 200 mA, so there is plenty of overhead even with a heavy RS485 termination. Large spikes were seen on the isolated side of the MAX253's circuitry, but a series choke tamed them nicely.

Are isolation techniques necessary? In many situations, they aren't necessary. Overlooking the more obvious safety issues (such as distances between devices) in some applications there is an increased potential for ground loop problems.

Ground loops

When pieces of equipment using different power supplies are tied together (with a common ground connection) there is a potential for ground loop currents. This is an induced current in the common ground line as a result of a difference in ground potentials on each piece of equipment. (Note: improper house wiring can cause ground loops when at 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. We've measured considerable different outlets (and different phases) within the same room. There are several 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 specs.

To eliminate ground loop problems when connecting devices using grounded supplies 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. RS232 circuitry must have a ground connection because it is the reference for the remaining signal lines. On the other hand, RS422/485 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 RS485 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 the common ground loop problems between sensor and measurement circuitry, always power and measure sensors with the same local supply.

By measuring 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. Besides 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 (memorised).

This would require an extra set of latches when using optoisolators.

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 magneto-resistive random access memory (MRAM) technology. Can you say core memory? What goes around...