Sensors - March 2002 - Magnetic Couplers in Industrial Systems

A new generation of couplers conquers noise with high speed and multiple channels.

Sensing is only half the battle in industrial control systems. Getting the sensor data where they need to go is the other half. The data path can be strewn with ground loops, noise, temperature extremes, and speed bottlenecks. For years, optical couplers were the only option. Although revolutionary when they arrived on the scene a generation ago, optocouplers have failed to keep pace with advances in industrial networks and sensor technology, leaving many designers frustrated by their bulk, slow speed, high power consumption, and limited temperature range. But in the past year, the introduction of a new generation of solid-state couplers—magnetic couplers—has been overcoming many of these limitations.

Galvanic Couplers
Magnetic couplers are analogous to optocouplers in a number of ways. Optocouplers transmit signals by means of light through a bulk dielectric that provides galvanic isolation (see Figure 1).

Figure 1. Both optical (A) and magnetic isolators (B) provide galvanic isolation between electronic input and output. Magnetic isolators transmit the signal by a magnetic field rather than by photons.

Magnetic couplers transmit signals via a magnetic field, rather than a photon transmission, across a thin film dielectric that provides the galvanic isolation. As is true of optocouplers, magnetic couplers are unidirectional and operate down to DC. But in contrast to optocouplers, magnetic couplers offer the high-frequency performance of an isolation transformer, covering nearly the entire combined bandwidth of the two conventional isolation technologies.

Industrial Networks Need Isolation
Widespread electrical and communications networks often have nodes with different ground domains. The potential difference between these grounds can be AC or DC, and can contain various noise components. Grounds connected by cable shielding or logic line ground can create a ground loop—unwanted current flow in the cable. Ground-loop currents can degrade data signals, produce excessive EMI, damage components, and, if the current is large enough, present a shock hazard.

Galvanic isolation between circuits or nodes in different ground domains eliminates these problems, seamlessly passing signal information while isolating ground potential differences and common-mode transients. Adding isolation components to a circuit or network is considered good design practice and is often mandated by industry standards. Isolation is frequently used in modems, LAN and industrial network interfaces (e.g., network hubs, routers, and switches), telephones, printers, fax machines, and switched-mode power supplies.

Magnetic Technology and GMR Isolation
Magnetic couplers are based on the giant magnetoresistance (GMR) effect, discovered by French scientists in 1988. GMR materials are made from exotic metal alloys deposited in extremely thin layers and formed into tiny resistors that exhibit a large change in resistance (the “giant” in GMR) when exposed to a magnetic field. GMR sensors, with their high sensitivity and stable, repeatable switch points, enable today’s ultra-high-speed hard disk drives.

One version of these couplers, the IsoLoop, adds an integrated insulating layer and a microscopic coil on top of a small sensor element bridge. A magnetic field proportional to the input current signal is generated beneath the coil winding, and the resulting magnetic field is sensed across the dielectric film (see Figure 2).

Figure 2. A magnetic coupler consists of an onchip microscopic coil that generates a magnetic field and a GMR sensor that detects that field.

GMR resistors are sensitive to magnetic fields in the plane of the substrate, permitting a more compact integration scheme than would be possible with a Hall sensor, for example, that measures fields perpendicular to the substrate. And because of GMR’s high sensitivity, its propagation delay is shorter and more stable than a simple MEMS pickup coil.

The sensed magnetic field is amplified and conditioned with integrated electronic circuits to produce an isolated replica of the input signal. Ground potential variations, however, are common to both sides of the input coil, so they do not generate a current. Therefore, no magnetic field results, and these variations are not sensed by the GMR structures. The signal is transparently passed from the input to the output circuits and ground potential variations are rejected, resulting in a very large common-mode rejection ratio (CMRR) and true galvanic isolation.

Advantages of Magnetic Coupling
The advantages of magnetic coupling include high bandwidth, small footprint, excellent noise immunity, and temperature stability.

Bandwidth. IsoLoop couplers are 5–10 times faster than the fastest optocouplers, and have correspondingly faster rise, fall, and propagation times (see Figure 3).

Figure 3. Magnetic couplers (blue trace) have faster rise, fall, and propagation times than the fastest optocouplers (yellow trace) for the same input (purple trace).

Shorter rise and fall times also reduce power consumption in the device and system by minimizing time in active regions.

Figure 4. The four-channel magnetic coupler die in the photomicrograph has a footprint of only 2.1 mm².

Small Footprint. IsoLoop couplers can be fabricated in <1 mm² of die area per channel (see Figure 4), allowing multichannel devices in SSOP packages. Less board real estate means both more room for other functions and lower prices. Furthermore, because of their small die size, IsoLoop couplers cost no more than high-performance optocouplers.

Noise Immunity. Magnetic couplers provide transient immunity up to 25 kV/µs, compared to 10 kV/µs for optocouplers. Transient immunity is especially important in harsh industrial and process control environments.

Temperature Stability. Because the transmission and sensing elements are not subject to semiconductor temperature variations, magnetic couplers operate to 100°C and above; for most optocouplers the upper limit is 75°C. Magnetic couplers are also immune to optocouplers’ inheren. performance decay with age.

Magnetic Coupler Applications
Magnetic isolators are quickly finding their way into process control and industrial applications. Isolation of A/D interfaces is one popular use. In addition, magnetic isolators’ combination of speed and packaging density provides a good method of efficient data channel management when multiple A/Ds need to be interfaced on the same circuit card. A four-channel part with three channels going one way and one going the other is available for A/D interface applications. Magnetic couplers also enable higher speed factory networks such as Profibus and other protocols.

The Future of Magnetic Couplers
Magnetic couplers will in time be even faster and have more channels. More types of integrated bus transceivers will be available. Several manufacturers are planning to introduce magnetic couplers either by licensing NVE’s technology or developing their own. In addition, the U.S. military is providing significant funding for advanced magnetic coupler development because of the value of their high speed and noise immunity in aircraft and other systems.

Speeds, currently limited by the silicon electronics and not the coil/GMR structure, are expected to increase as ICs scale down and become faster. NVE has reported prototype devices with speeds of 300 Mbaud and switching times of <1 ns.

Also under development are higher-density parts (full byte-wide couplers) and more functionality (latching bus transceivers). In addition to the isolated RS-485 transceivers available now, CAN bus, USB, and RS-232 are on the horizon. Finally, the inherent linearity of a resistive coil and resistive sensing elements make magnetic couplers well suited for linear data protocols such as low-voltage differential signaling.

Magnetic Couplers Currently Available

NVE offers the IsoLoop line of single- and multi-channel couplers in both unidirectional and bi-directional configurations and in DIP and SOIC packages. Because they are fabricated by means of semiconductor technology, GMR isolation elements are easily combined with silicon processes to provide single-chip isolated transceivers. The first single-package, magnetically isolated bus transceiver is the IsoLoop IL485, which combines magnetic isolation with an RS-485 driver in a 16-pin SOIC package. The device has a data rate of 35 Mbaud—more than 100 times as fast as other single-package isolated transceivers.