Issue 142 May2002
You're Not Alone
Dealing with Isolation

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). This differs from the GMR effect.

The GMR effect is a change in a thin film nonmagnetic conductive layer’s resistance caused by an external magnetic field overcoming the parallel but opposing magnetic coupling of adjacent magnetized layers (see Figure 2). You’ll remember that the spin 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 of the layers into a single magnetic moment. When this happens, electron flow will be less effected (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 nm) 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 (see Figure 3). Their 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 (see Figure 4). Current through this coil layer produce the magnetic field, which overcomes the antiferromagnetic layers thereby reducing the sensor’s resistance.

NVE obtains isolation 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 1000 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. Figure 5 shows typical RS-232 isolation. Specialty devices include isolated RS-422 and RS-485 communications transceivers (see Figure 6).