A new generation of couplers conquers noise with high speed and multiple channels.
John Myers, NVE Corp.
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.
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
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
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).
The dielectric provides 4500 VDC of galvanic isolation.
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
Bandwidth. IsoLoop couplers are 5–10 times faster than the fastest optocouplers, and have correspondingly faster rise, fall, and propagation times (see Figure 3).
Shorter rise and fall times also reduce power consumption in the device and system by minimizing time in active regions.
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
The Future of Magnetic Couplers
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.
IsoLoop is a registered trademark of NVE Corp.
John Myers is Vice President, Isolation Products Business Unit, NVE Corp., 11409 Valley View Rd., Eden Prairie, MN 55344; 952-996-1610, fax 952-996-1600, firstname.lastname@example.org.