The discovery in 1988 of the giant magnetoresistive effect (GMR) has provided us with a new technology which offers many advantages over the traditional methods used for isolation such as the optocoupler or capacitive barrier. GMR offers two significant advantages over competitive methods of isolation. First, the large change in resistance of the GMR material allows for a stronger signal. Second, and probably more important, the technology is compatible with integrated circuit (IC) technology, allowing the GMR devices to be included as part of the chip package giving smaller, faster and less expensive devices such as digital isolators, sensors and MRAM.

GMR is a large change in electrical resistance observed in artificial thin-film materials composed of alternating ferromagnetic and non-magnetic layers as a function of applied magnetic field. The electrical resistivity is dependent on the direction of the electron spin in relation to the magnetic moment of the films. If the magnetic moments of the ferromagnetic layers are aligned (parallel), electron scattering is greatest and resistance is lowest. If the magnetic moments of the ferromagnetic layers are in opposing directions (antiparallel), electron scattering is at minimum and resistance is highest. The resistance of the structure is proportional to the cosine of the angle between the magnetic moments in adjacent magnetic layers.

A spin valve is generally 3-layer GMR structure which consists of a magnetically “soft” free layer. As shown in figure 4, this layer is sensitive to lower magnetic fields (10 –30 Oersteds) than the “pinned” layer which switches far from zero field (~1000 Oersteds) due to exchange bias between itself and the antiferromagnet. This basic spin valve structure is constructed as a serpentine resistor element and is used in a Wheatstone bridge formation as the core of the IL7xx series of digital isolators. This is shown in Figure 5.

The directional current in the field coil creates the magnetic field required to switch the GMR Wheatstone bridge. This gives the corresponding directional Vo.

The IsoLoop  products based on this patented GMR process provide the designer with the most advanced and versatile isolation devices currently available, especially when compared to the traditional optocoupler solution.

• Higher data rates – 100M b/s
• Propagation delay – 15ns max
• P.W.D. – 3ns
• Prop. Delay skew – 4ns (device to device), 2ns channel to channel.
• Temp Range : -40oC to +100oC, no derating.
• True logic devices. No extra components

 Devices include isolated single, dual and quad channels, isolated RS485 (Profibus approved), and isolated RS422 transceivers. All devices are in standard JEDEC packages and are UL and IEC approved.

[Product selections]

IL600-Series
Passive-Input Isolators 

Revolutionary monolithic isolators with passive inputs and fail-safe outputs. Drop-in replacements for passive-input optocouplers but with data rates to 100 Mbps and PWD as low as 3 ns. One, two, or three channel configurations; CMOS or open-drain outputs. PDIP, SOIC, or unique MSOP packages.