ISOLOOP MAGNETIC COUPLERS REPLACED OPTOCOUPLERS

ISOLOOP MAGNETIC COUPLERS REPLACE OPTO-COUPLERS

ABSTRACT

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.

Everyone knows the problems with opto-couplers. They take up a lot of space, are slow, opto-

couplers 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. Opto-coupler technology, however, hasn’t kept up. Existing coupler

technologies look like dinosaurs on modern circuit boards. Magnetic couplers are analogous to

opto-couplers 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’.

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INDEX

SL NO CHAPTERS PAGE NO

CHAPTER 1

INTRODUCTION 04-05

CHAPTER 2INDUSTRIAL NETWORKS NEED ISOLATION 06

2.1 Ground Loops

CHAPTER 3GALVANIC COUPLERPHYSICS OF THE 07

GIANT MAGNETO RESISTANCE (GMR)

CHAPTER 4 GIANT MAGNETO RESISTIVE 08-12

4.1 GMR MATERIALS

CHAPTER 5 CONSTRUCTION OF ISOLOOP MAGNETICCOUPLER 13-16

5.1 Sensor Arrays

CHAPTER 6WORKING OF A ISOLOOP MAGNETIC COUPLER 17

CHAPTER 7 18-19

ADVANTAGES OF MAGNETIC COUPLING

7.1 Bandwidth

7.2 Small Footprint

7.3 Noise Immunity

7.4 Temperature Stability

CHAPTER 8 20-21DIGITAL ISOLATORS

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8.1 Dynamic Power Consumption

8.2 Data Transmission Rates

CHAPTER 9 22COMPARISON BETWEEN OPTOCOUPLER AND ISOLOOP MAGNETIC COUPLER

CHAPTER 10 23CURRENT APPLICATIONS

10.1 Digital Isolation Applications

CHAPTER 11 24THE FUTURE SCOPE

CHAPTER 12 25CONCLUSION

CHAPTER 13 26REFERENCES

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CHAPTER 1

INTRODUCTION

In many industrial control systems getting valuable data to its destination is often a

headache. Factors such as ground loops, noise, temperature extremes, and speed

bottlenecks can adversely affect data transmission. Opto-isolators have been until

recently the obvious solution, but their bulk, slow speed, high power consumption, and

limited temperature range now presents serious shortcomings for the designer.

A new generation of solid-state couplers IsoLoop magnetic couplers, has been

introduced which overcomes many Opto-coupler limitations. IsoLoop devices can be

combined with conventional integrated circuits to create single-package isolated

transceivers. Isolated transceivers are available in single 16-pin SOIC packages. These

devices support data rates as high as 40Mbps.IsoLoop couplers add 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. The resulting magnetic field is sensed across the dielectric film. The dielectric

provides 4,500 Volts DC of galvanic isolation Magnetic couplers are based on the Giant

Magneto resistance (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. The GMR resistors are sensitive to magnetic fields in the plane

of the substrate.

This enables a more compact integration scheme than would be possible with a

Hall sensor. The sensed magnetic field is amplified and conditioned with integrated

electronic circuits to produce an isolated replica of the input signal. Ground potential

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variations, however, are common to both sides of the input coil, so they do not generate a

current. Thus no magnetic field results and these variations are not sensed by the GMR

structures. In this way, the signal is transparently passed from the input to the output

circuits while ground potential variations are rejected to achieve a very large common-

mode rejection ratio (CMRR) and true galvanic isolation.

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CHAPTER 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 datasignals, 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.

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CHAPTER 3

GALVANIC COUPLERS

0Opto-couplers transmit signals by means of light through a bulk dielectric that provides

galvanic isolation (see Figure 3.1). Magnetic couplers transmit signals via a magnetic field,

rather than a photon transmission, across a thin film dielectric that provides the galvanic isolation

(see figure 3.2).

Figure 3.1: Optical Isolator

Figure 3.2: Isoloop Isolator

As is true of opto-couplers, magnetic couplers are unidirectional and operate down to

DC. But in contrast to opto-couplers, magnetic couplers offer the high-frequency performance of

an isolation transformer, covering nearly the entire combined bandwidth of the two conventional

isolation technologies.

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CHAPTER 4

PHYSICS OF THE GIANT MAGNETO RESISTANCE (GMR)

4.1 Giant Magneto resistive

Large magnetic field dependent changes in resistance are possible in thin film Ferro

magnet /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 an isotropic magneto resistance, this phenomenon

was truly ‘giant’ magneto resistance. 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 than5 nm) for the ferromagnetic layer’s electron spins to

affect the conductive layer’s electron’s path.

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Figure: 4.1.1

Figure: 4.1.2

Figure 4.1.A: 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 4.1.1 & 4.1.2).

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Figure 4.1.B: 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.

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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 4.2: Characteristics of GMR Materials

Notice that the output characteristic is Omni polar, 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.

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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 (0to 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. And 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.

CHAPTER 5

CONSTRUCTION OF ISOLOOP MAGNETICCOUPLER

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Figure 5.1: Isolator Data Travel

Figure 5.1 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 5.2: Magnetic Coupler

To put this phenomenon to work, a Wheatstone bridge configuration of four GMR

sensors (see Figure 5.1 & 5.2). 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 configuration scan 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 5.1). Current through this

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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 t he sma l l e s t

dev i ce s and r equ i r e t he f ewes t componen t s , bu t t hey have poo r 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 t empe ra tu r e . Ha l f b r i dges c an be

u sed a s f i e l d g r ad i en t s enso r s i f one o f t he 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 1.5µm wide by 6 µm h igh w i th a s im i l a r s i z e e l emen t above t he

cen t e r t ap . 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.

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5.1.1 Sensor Arrays

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 isrequired 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 5.2.1.

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Figure 5.2.1: 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.

CHAPTER 6

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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 comp

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

ma te r i a l , wh i ch was caused by t he magne t i c f i e l d , i s amp l i f i ed 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.

CHAPTER 7

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ADVANTAGES OF MAGNETIC COUPLING

The advantages of magnetic coupling include high bandwidth, small footprint, excellent

noise immunity, and temperature stability.

7.1 Bandwidth

I soLoop coup l e r s a r e 5–10 t imes f a s t e r t han t he f a s t e s t opto-couplers,

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 7.1: Magnetic Couplers Traces

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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 opto-

couplers.

Figure 7.2: Four-Channel Magnetic Coupler Die

7.3 Noise Immunity

Magnetic couplers provide transient immunity up to 25 kV/µs,

compa red t o 10 kV/µs fo r op tocoup l e r s . T rans i en t immun i ty i s e spec i a l l y i

mportant 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 opto-

couplers the upper limit is 75°C. Magnetic couplers are also immune to opto-couplers’ inherent

performance decay with age.

CHAPTER 8

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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 obv ious advan t ages ove r op to - coup l e r s whose power

consumpt ion i s heav i l y dependent on its on-state and frequency. The maximum power

supply current per channel for IsoLoop is:

I (input) =40(f /fmax) (4/5) mA

Where f =operating frequency

fmax= 50 MHz

8.2 Data Transmission Rates

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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 t he dev i ce .

Th i s i s u sua l l y d i f f e r en t when s end ing a l ow- to -h igh t han when s end ing a

h igh - to - l ow s igna l . Th i s d i f f e r ence , o r e r ro r , i s c a l l ed pu l s e w id th distortion

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

P W D % = ( 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 opto-

coupler. The maximum channel to channel skew in t he I soLoop coup l e r i s on ly 3 n s ,

t en t imes be t t e r t han any o the r opto-couplers.

CHAPTER 9

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COMPARISON BETWEEN OPTOCOUPLER ANDISOLOOP MAGNETIC COUPLER

Unlike typical microsecond Ton/Toff times of opto-isolators, The IsoLoop-isolators also

have identical TON/TOFF times, which produce no pulse-w i d t h d i s t o r t i o n a s

i s t h e c a s e w i t h m a n y o p t o - i s o l a t o r s h a v i n g d i f f e r i n g Ton /Tof f

t imes . P ropaga t i on de l ays a r e l e s s t han 10 n s w i th i n t e r - channe l 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 f o r bus i so l a t i on ,

s e r i a l ADCs and DACs , and commun ica t i on i so l a t i on . The work ing r ange

o f op to - coup l e r s i s on ly be tween ze ro and t en megahe r t z . The I soLoop

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 opto-isolators. The power consumption of IsoLoop devices is solely

dependent on frequency. This makes for lower power consumption than opto-isolators, 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 opto-isolators.

CHAPTER 10

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CURRENT APPLICATIONS

Magnetic isolators are quickly finding their way into process control and industrial

applications. Isolation of A/D interfaces is one popular use.

Inadd i t i on , magne t i c i so l a to r s ’ combina t i on o f speed and packag ing dens i t

y 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 t h r ee channe l s

go ing one way and one go ing t he o the r i s ava i l ab l e fo r A /D in t e r f ace

app l i c a t i ons . Magne t i c coup l e r s a l so enab l e h ighe r speed f ac to ry 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 allow s ufficient data channel

management where several A/D channels must be isolate don a board.

10.1 Digital Isolation Applications

ADCs DACs Multiplexed Data Transmission Data Interfaces Digital Noise Reduction Ground Loop Elimination

CHAPTER 11

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THE FUTURE SCOPE

Magnetic field detection has vastly expanded as industry has used

ava r i e t y o f magne t i c s enso r s t o de t ec t t he p r e sence , s t r eng th , o r d i r e c t i on

o f 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.

CHAPTER 12

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CONCLUSION

Magnetic couplers will in time be even faster and have more channels. More types of

integrated bus transceivers will be available. Several manufacturers a r e p l ann ing t o

i n t roduce magne t i c coup l e r s . The U .S . m i l i t a ry i s p rov id ing significant

funding for advanced magnetics 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 <1ns. Also under development are higher-

density parts (full byte-wide couplers) and more functionality (latching bus transceivers).

Finally, the inherent linearity of are sensitive coil and resistive sensing elements make magnetic

couplers well suited for linear data protocols such as low-voltage differential signaling.

CHAPTER 13

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REFERENCES

1. J .Daugh ton and Y . Chen . "GMR Mate r i a l s f o r Low F i e ld App l i c a t i ons , " IEEE Trans Magn, Vol. 29:2705-2710, 2003.pp.18-21

2. Michae l J . Ca ruso , Tamara Bra t l and , C . H . Smi th , and Robe r t Schne ide r , “A New Perspective on Magnetic Field Sensing,” Sensors Magazine, vol.15, no. 12, (December 2002), pp. 34-46

3. Ca r l H . Smi th and Robe r t W. Schne ide r , “Low-F ie ld Magne t i c Sens ing with GMR Sensors, Part 1: The Theory of Solid-State Sensing,” Sensors Magazine, vol. 16, no. 9, (September 2002), pp. 76-83.

4. Ca r l H . Smi th and Robe r t W. Schne ide r , “Low-F ie ld Magne t i c Sens ing w i t h G M R S e n s o r s , P a r t 2 : G M R S e n s o r s a n d t h e i r A pp l i c a t i o n s , ” Sensors Magazine, vol. 16, no. 10, (October 2002), pp. 84- 91.

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

6 . h t t p : / / w w w . s e n s o r s m a g . c o m /

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

8 . h t t p : / /www.e l ec t ron i c s t a lk . com/news / rho / rho000 .h tml

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

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ISOLOOP MAGNETIC COUPLERS REPLACED OPTOCOUPLERS

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