Giant magnetoresistive digital signal isolators
Summary:
With the help of BMDO SBIR funding, NVE Corporation (Eden Prairie, MN) has developed giant magnetoresistive (GMR) digital signal isolators that, compared to optical isolators, are faster and smaller, yet cost-competitive. These isolators can reduce or eliminate ground noise in communications carried by wire. NVE offers a product family of IsoLoop™ GMR isolators, which are ideally suited for industrial data bus, analog to digital conversion, and telecommunications applications.
Technology Description:
With the help of BMDO SBIR funding, NVE Corporation (Eden Prairie, MN) has developed giant magnetoresistive (GMR) digital signal isolators that, compared to optical isolators, are faster and smaller, yet cost-competitive. Optical isolators transmit a signal with light where the distance of light between the light-emitting diode and photon-sensing device provides the signal isolation. NVE’s GMR devices transmit the signal via a magnetic field. This provides many design advantages, including:
Higher bandwidth. With speeds up to 100 Mbaud, GMR isolators transmit data 4 to 20 times faster and have correspondingly faster rise, fall, and propagation times. NVE has demonstrated prototype GMR isolators with speeds of 300 Mbaud.
Lower power consumption. GMR isolators draw ½ to _ less power.
Smaller footprints. GMR isolators are 20-percent smaller.
Greater noise immunity. GMR isolators provide transient immunity up to 25 kV/µs, compared to 10 kV/µs for optical isolators.
Greater temperature stability. GMR isolators operate up to 100°C, compared to 75°C for optical isolators.
In the GMR isolators, a signal at the input induces a current in a planar coil. The current produces a magnetic field that 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. This resistance change is amplified by an electronic circuit and impressed upon the output as a reproduction of the input signal. Because changes in the ground potential at the input, output, or both don’t produce a current in the planar coil, no magnetic field is created. In this way, safe galvanic isolation is achieved.
Under an ongoing BMDO SBIR contract, NVE is developing a faster, more rugged isolator by combining a novel thin-film linear spin valve structure and low-voltage differential signaling (LVDS) circuitry. The spin valve structure is well-suited for high-speed operation because it is never driven into saturation. Hence, smaller fields/currents can be used to switch the electronics.
Using this design, NVE believes it can reduce the isolator’s current usage from 50 to 10 milliamps, yielding a corresponding increase in speed from 100 to 500 Mbaud. The company recently demonstrated a prototype with speeds up to 300 Mbaud. The LVDS circuitry is silicon-based, therefore it is radiation tolerant, which significantly improves the isolator’s ruggedness.
The spin valve structure consists of a linear spin valve bridge located on an on-chip coil separated by a thick layer of dielectric material. In operation, the coil driver circuit translates an input signal to current through the coil, which is electrically isolated from the bridge by the dielectric (designed to withstand load voltages of approximately 2,500 VAC for 1 minute). Current through the coil generates a local magnetic field to which the bridge responds with a small differential signal proportional to the field. The signal from the bridge is sensed across the isolation barrier and translated to the desired output. An on-chip shield covers the isolator structure to further reduce sensitivity to ambient fields. This highly integrated structure allows single- or multichannel devices to be built in smaller packages than previous isolators.
MDA Origins:
NVE has a long history with BMDO. In its early years, the company was supported by several BMDO SBIR contracts through which it explored the use of GMR materials for magnetic random access memory (MRAM) devices. MRAM interests BMDO because it combines the memory durability and radiation hardness of magnetic storage with the small packaging and fast access times of current silicon memory chip technology. Although this research did not yield any immediate MRAM products, it is still ongoing with funding from various government and private sources. However, NVE leveraged its expertise in GMR materials to develop GMR sensors and isolators.
In 2000, BMDO funded NVE through a SBIR Phase II contract to develop a high-speed, radiation-hardened GMR isolator. This device would be ideal for high-speed interfaces, such as flat-panel displays, used in BMDO satellite and other aerospace systems.
Spinoff Applications:
In communications carried by wire, isolators reduce or eliminate ground noise, which results from connecting electrical circuits with independent grounds. In some cases, these devices may also provide for limited protection against electrical damage.
They are commonly used in a wide range of communication networks. Differences in ground potentials between pieces of electronics are virtually impossible to eliminate, and the resulting noise is often much larger than the logic signals transmitted. NVE’s isolator can virtually eliminate this noise.
Signal isolation in high-speed communications will be possible for systems using random signal transmission in wires. Isolation in back planes of personal computers and other high-speed systems may enable these systems to operate at higher speeds.
Commercialization:
NVE offers a product family of IsoLoop™ GMR isolators, which are ideally suited for industrial data bus, analog to digital conversion, and telecommunication applications. This family includes single-channel, dual-channel, and four-channel devices (unidirectional and bidirectional). An isolated RS485 chip is also available. The single- and dual-channel isolators are available in 9-pin DIP and 8-pin SOIC packages. The four-channel version and the RS485 interface IC are available in wide-body 16-pin SOIC packages.
Distribution channels for NVE’s GMR isolators have been established in the Europe, Japan, Korea, China, and Taiwan. NVE has also teamed with Digi-Key Corporation, one of the fastest-growing electronic component distributors in the United States. “In the domestic market, Digi-Key is among the top 15 of the more than 800 electronic distributors and has done business with 99 of the top 100 electronic companies,” commented Dr. Daniel A. Baker, NVE’s president and chief executive officer. “This agreement is a great endorsement of NVE’s new components and a key element of our strategy to dramatically broaden our component distribution.”
NVE has received a basic patent on the use of a magnetic sensor and integrated coils to construct an isolator. Patents containing several improvements on this basic patent have also been filed. These patents, together with know-how in the design, processing, packaging, and testing of the IsoLoop products, are the primary barriers to those competing with NVE’s isolator products.
Company Profile:
Founded in 1989, NVE is an electronics component manufacturer specializing in the combination of magnetically-sensitive materials with integrated circuits. Its solid-state magentic technology has been applied to sensors, isolators, and magnetoresistive random access memory. NVE’s products and services include magnetic field sensors, gradiometers, arrays, and assemblies; magnetic-based isolators; MRAM research and development; magnetic modeling/simulation; custom design services; back-end IC processing; and custom thin-film sputtering. A 21,000 square-foot facility is capable of producing up to 40 million devices per year. The company has over 60 employees.
In November 2000, NVE shareholders approved the merger with Premis Corporation, a publicly traded corporation, which sold off all its assets and ceased operations in September 2000. The new company retained the NVE name and business and is now publicily traded (OTBB: NVEC).
Contact Information:
Bob Schneider
NVE Corporation
11409 Valley View Road
Eden Prairie, MN 55344-3617
Tel: (952) 829-9217
Fax: (952) 996-1600
email: bobsch@home.nve.com
web: http://www.nve.com
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