isoloop magnetic coupler

Isoloop Magnetic Couplers

1. INTRODUCTION

Couplers, also known as "isolators" because they electrically isolate as

well as transmit data, are widely used in industrial and factory networks,

instruments, and telecommunications. Every one knows the problems with

optocouplers. They take up a lot of space, are slow, optocouplers age and their

temperature range is quite limited. For years, optical couplers were the only

option. Over the years, most of the components used to build instrumentation

circuits have become ever smaller. Optocoupler technology, however, hasn’t kept

up. Existing coupler technologies look like dinosaurs on modern circuit boards.

Magnetic couplers are analogous to optocouplers in a number of ways.

Design engineers, especially in instrumentation technology, will welcome a

galvanically isolated data coupler with integrated signal conversion in a single IC.

My report will give a detailed study about ‘ISOLOOP MAGNETIC COUPLERS’.

Department of ECE, MRCE 1


2. INDUSTRIAL NETWORKS NEED ISOLATION

2.1 Ground Loops

When equipment using different power supplies is tied together

(with a common ground connection) there is a potential for ground loop currents

to exist. This is an induced current in the common ground line as a result of a

difference in ground potentials at each piece of equipment. Normally all grounds

are not in the same potential.

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.



3. GALVANIC COUPLERS

Optocouplers transmit signals by means of light through a bulk

dielectric that provides galvanic isolation (see Figure 1).Magnetic couplers are

analogous to optocouplers in a number of ways.

Figure 1: Optical Isolator

Figure 2: Isoloop Isolator

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.



4. PHYSICS OF THE GIANT MAGNETORESISTANCE

4.1 Giant Magnetoresistive

Large magnetic field dependent changes in resistance are possible in

thin film ferromagnet/nonmagnetic metallic multilayers. The phenomenon was first

observed in France in 1988, when changes in resistance with magnetic field of up

to 70% were seen. Compared to the small percent change in resistance observed in

anisotropic magnetoresistance, this phenomenon was truly ‘giant’

magnetoresistance.

The spin of electrons in a magnet is 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 these

phenomenon takes places 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.


http://www.wsrcc.com/alison/magnetism.html


Figure 3: Nonmagnetic Conductive Layers

The resistance of two thin ferromagnetic layers separated by a thin

nonmagnetic conducting layer can be altered by changing the moments of the

ferromagnetic layers from parallel to antiparallel, or parallel but in the opposite

direction.

Layers with parallel magnetic moments will have less scattering at the

interfaces, longer mean free paths, and lower resistance. Layers with antiparallel

magnetic moments will have more scattering at the interfaces, shorter mean free

paths, and higher resistance (see Figure 2 & 3).



Figure 4: Magneto Resistive Sensor

For spin-dependent scattering to be a significant part of the total resistance, the layers must be thinner than the mean free path of electrons in the bulk material. For many ferromagnets the mean free path is tens of nanometers, so the layers themselves must each be typically <10 nm (100 Å). It is therefore not surprising that GMR was only recently observed with the development of thin film deposition systems.

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.

4.2 Gmr Materials

There are presently several GMR multilayer materials used in sensors

and sensor arrays. The following chart shows a typical characteristic for a GMR

material:



Figure 5: : Characteristics Of Gmr Materials

Notice that the output characteristic is omnipolar, meaning that the

material provides the same change in resistance for a directionally positive

magnetic field as it does for a directionally negative field. This characteristic has

advantages in certain applications.

For example, when used on a magnetic encoder wheel, a GMR

sensor using this material will provide a complete sine wave output for each pole

on the encoder thus doubling the resolution of the output signal.

The material shown in the plot is used in most of GMR sensor

products. It provides a 98% linear output from 10% to 70% of full scale, a large

GMR effect (13% to 16%), a stable temperature coefficient (0.15%/°C) and

temperature tolerance (+150°C), and a large magnetic field range (0 to ±300

Gauss).



For spin-dependent scattering to be a significant part of the total resistance, the

layers must be thinner than the mean free path of electrons in the bulk material. For

many ferromagnets the mean free path is tens of nanometers, so the layers

themselves must each be typically <10 nm (100 Å). It is therefore not surprising

that GMR was only recently observed with the development of thin film deposition

systems.



5. CONSTRUCTION OF ISOLOOP MAGNETIC COUPLER

Figure 6: Isolator Data Travel

Figure 5. In a GMR, isolator data travels via a magnetic field through

a dielectric isolation to affect that resistance elements arranged in a bridge

configuration.

Figure 7: Magnetic Coupler



To put this phenomenon to work, a Wheatstone bridge configuration

of four GMR sensors (see Figure 5 & 6). The 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 5). Current through this coil layer produce the magnetic

field, which overcomes the antiferromagnetic layers there by reducing the sensor’s

resistance.

5.1 Sensor Arrays

GMR elements can be patterned to form simple resistors, half

bridges, Wheatstone bridges, and even X-Y sensors. Single resistor elements are

the smallest devices and require the fewest components, but they have poor

temperature compensation and usually require the formation of some type of

bridge by using external components. Alternatively they can be connected in series

with one differential amplifier per sensor resistor. Half bridges take up more area

on a chip but offer temperature compensation, as both resistors are at the same

temperature. Half bridges can be used as field gradient sensors if one of the

resistors is some distance from the other. They can function as field sensors if one

of the resistors is shielded from the applied field. Figure 4 shows a portion of an

array of 16 GMR half bridge elements with 5 µm spacing. The elements are 1.5

µm wide by 6 µm high with a similar size element above the center tap. The

bottoms of the stripes are connected to a common ground connection and the tops

of the half bridges are connected to a current supply. The center taps are connected

to 16 separate pads on the die. A bias strap passes over the lower elements to

provide a magnetic field to bias the elements.



5.2 Signal Processing

Adding signal processing electronics to the basic sensor element

increases the functionality of sensors. The large output signal of the GMR sensor

element Introduction means less circuitry, smaller signal errors, less drift, and

better temperature stability compared to sensors where more amplification is

required to create a usable output.

For the GMR products, we add a simple comparator and output

transistor circuit to create the world’s most precise digital magnetic sensor. For

these products, no amplification of the sensor’s output signal is necessary. A block

diagram of this circuitry is shown in the figure 8.



Figure 8: Signal Processing Circuit

The GMR Switch holds its precise magnetic operate point over

extreme variations in temperature and power supply voltage. This is a low cost

method.



6. WORKING OF A ISOLOOP MAGNETIC COUPLER

In the Isoloop magnetic couplers, a signal at the input induces a

current in a planar coil (see figure no: 5).The current produces a magnetic field,

which is proportional to the current in the planar coil. The resulting magnetic field

produces a resistance change in the GMR material, which is separated from the

planar coil by a high voltage insulating material. Since the GMR is sensitive

parallel to the plane of the substrate, this allows a considerably more compact

construction than would be possible with Hall sensors. The resistance change in

GMR material, which was caused by the magnetic field, is amplified by an

electronic circuit and impressed upon the output as a reproduction of the input

signal. Since changes in the ground potential at the input, output or both doesn’t

produce a current in the planar coil, no magnetic field is created. The GMR

material doesn’t change. In this way safe galvanic signal isolation is achieved and

at the same time a corresponding common mode voltage tolerance.



7. ADVANTAGES OF MAGNETIC COUPLING

The advantages of magnetic coupling include high bandwidth,

small footprint, excellent noise immunity, and temperature stability.

7.1 Bandwidth

IsoLoop couplers are 5–10 times faster than the fastest

optocouplers, and have correspondingly faster rise, fall, and propagation times

(see Figure 9). Shorter rise and fall times also reduce power consumption in the

device and system by minimizing time in active regions.

Figure 9: Magnetic Couplers Traces

7.2 Small Footprint

IsoLoop couplers can be fabricated in <1 mm2 of die area per channel

(see Figure 10). 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.

Figure 10: Four-Channel Magnetic Coupler Die

7.3 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.

7.4 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’ inherent performance decay with age.



8. DIGITAL ISOLATORS

These devices offer true isolated logic integration in a level not

previously available. All transmit and receive channels operate at 110 Mbd over

the full temperature and supply voltage range. The symmetric magnetic coupling

barrier provides a typical propagation delay of only 10 ns and a pulse width

distortion of 2 ns achieving the best specifications of any isolator device. Typical

transient immunity of 30 kV/µs is unsurpassed.

8.1 Dynamic Power Consumption

Isoloop devices achieve their low power consumption from the

manner by which they transmit data across the isolation barrier. By detecting the

edge transitions of the input logic signal and converting these to narrow current

pulses, a magnetic field is created around the GMR Wheatstone bridge.

Depending on the direction of the magnetic field, the bridge causes the output

comparator to switch following the input logic signal. Power consumption is

independent of mark-to-space ratio and solely dependent on frequency. This has

obvious advantages over optocouplers whose power consumption is heavily

dependent on its on-state and frequency. The maximum power supply current per

channel for IsoLoop is:



8.2 Data Transmission Rates

The reliability of a transmission system is directly related to the

accuracy and quality of the transmitted digital information. For a digital system,

those parameters, which determine the limits of the data transmission, are pulse

width distortion and propagation, delay skew.

Propagation delay is the time taken for the signal to travel through

the device. This is usually different when sending a low-to-high than when

sending a high-to-low signal. This difference, or error, is called pulse width

distortion (PWD) and is usually in ns. It may also be expressed as a percentage:

PWD% = (Maximum Pulse Width Distortion (ns) /Signal Pulse

Width (ns)) x 100%

Propagation delay skew is the difference in time taken for two or

more channels to propagate their signals. This becomes significant when clocking

is involved since it is undesirable for the clock pulse to arrive before the data has

settled. A short propagation delay skew is therefore critical, especially in high data

rate parallel systems, to establish and maintain accuracy and repeatability. The

IsoLoop range of isolators all has a maximum propagation delay skew of 6 ns,

which is five times better than any optocoupler. The maximum channel to channel

skew in the IsoLoop coupler is only 3 ns, which is ten times better than any

optocoupler.



9. COMPARISON BETWEEN OPTOCOUPLER AND ISOLOOP MAGNETIC COUPLER

Unlike typical microsecond TON/TOFF times of optoisolators, 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. The

working range of optocouplers is only between zero and ten megahertz. The

IsoLoop couplers have data transmission speeds up to100 mega baud. IsoLoop

devices will operate over a wide temperature range of -40 to +100C, compared

with the restricted range of 0 to +70C for optoisolators. The power consumption

of IsoLoop devices is solely dependent on frequency. This makes for lower power

consumption than optoisolators, whose power consumption is heavily dependent

on state and frequency. With data rates up to 100Mbaud, the IsoLoop technology

offers rates of up to ten times that of optoisolators.



10. CURRENT 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. These devices are great for bus

isolation, serial ADCs and DACs, and communication isolation. The combination

of the fast and high-density IsoLoop couplers with high packing density allows

efficient data channel management where several A/D channels must be isolated

on a board.

10.1 Digital Isolation Applications

ADCs

DACs

Multiplexed Data Transmission

Data Interfaces

Digital Noise Reduction

Ground Loop Elimination



11. THE FUTURE

Magnetic field detection has vastly expanded as industry has used a

variety of magnetic sensors to detect the presence, strength, or direction of

magnetic fields from the Earth, permanent magnets, magnetized soft magnets, and

the magnetic fields associated with current. These sensors are used as proximity

sensors, speed and distance measuring devices, navigation compasses, and current

sensors. They can measure these properties without actual contact to the medium

being measured and have become the eyes of many control systems.



12. CONCLUSION

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. 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. It 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). 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.



13. REFERENCES1. J.Daughton and Y. Chen. "GMR Materials for Low Field Applications,"

IEEE Trans Magn, Vol. 29:2705-2710, 2003.pp.18-21

2. Michael J. Caruso, Tamara Bratland, C. H. Smith, and Robert Schneider,

“A New Perspective on Magnetic Field Sensing,” Sensors Magazine, vol.

15, no. 12, (December 2002), pp. 34-46.

3. Carl H. Smith and Robert W. Schneider, “Low-Field Magnetic Sensing

with GMR Sensors, Part 1: The Theory of Solid-State Sensing,” Sensors

Magazine, vol. 16, no. 9, (September 2002), pp. 76-83.

4. Carl H. Smith and Robert W. Schneider, “Low-Field Magnetic Sensing

with GMR Sensors, Part 2: GMR Sensors and their Applications,”

Sensors Magazine, vol. 16, no. 10, (October 2002), pp. 84- 91.

5. http://www.circuitcellar.com/library/print/0502/JEFF/4.asp

6. http://www.sensorsmag.com/

7. http://www.nve.com/isoloop/news/hispdnr.php

8. http://www.electronicstalk.com/news/rho/rho000.html


http://www.nve.com/isoloop/news/hispdnr.php

isoloop magnetic coupler

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