快板ICs Based on Giant Magnetoresistance (GMR)
快板ICs Based on Giant Magnetoresistance (GMR)
By Bryan Cadugan,
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摘要
快板MicroSystems is a world leader in developing, manufacturing, and marketing high-performanceintegrated circuits (ICs), which incorporate high performance magnetic transducers. This whitepaper provides a basic understanding of the giant magnetoresistance (GMR) effect and how Allegro uses this technology in market-leading ICs to meet today’s application requirements.
The Giant Magnetoresistance (GMR) Effect
The GMR effect was discovered in 1988 by both Albert Fert of Unité Mixte de Physique CNRS/Thales and Peter Grünberg of Institut für Festkörperforschung Forschungszentrum Jülich GmbH. Both of these individuals won the Nobel Prize for this discovery in 2007. The fundamental principle of the GMR effect is based on electron spins. In a magnetoresistor, electron scattering rates increase or decrease as a function of the interaction of the spin state of the electrons and the magnetic orientation of the medium in which the electrons are traveling. Electron scattering increases the mean free path of the electron flow, effectively altering the resistance of the medium. In summary, a magnetoresistor is a resistor that changes its resistance value in the presence of a magnetic field.
GMR transducers are manufactured by creating a sequence of very thin layers made of different magnetic and nonmagnetic materials. The sequence and thickness of these materials enable the stack of thin films (GMR stack) to change its resistance in the presence of magnetic fields.
Over time, advances in GMR led to the development of a “spin-valve” type structure, which is what Allegro uses in their newest ICs. In a spin-valve, one of the two magnetic layers is considered a “reference” and is pinned or fixed in its orientation, and the other is called a “free” layer and is free to align with the magnetic field in the surrounding environment (see Figure 1). In typical magnetic sensor applications, this magnetic field is generated by a magnet or an electrical current, and is referred to as Bapp整个本文件的其余部分。“旋转阀”如此命名，因为它类似于水龙头，水流与套管的旋转程度有关。GMR旋转阀的打开位置涉及当对准轨道层时（如图1中的方向A所示），其中电阻最低。当磁性层被抗对准时发生闭合位置（或低流量位置）（如图1中的方向B所示），其中电阻最高。对于“参考”和“自由”层之间的任何角度差，GMR换能器的电阻与该角度的余弦成比例。
r = R.min+ (Rmin- r.马克斯) × cos(θ)
抗性变化的百分比称为MR％，或磁阻百分比。Allegro的GMR传感器通常在全范围的现场响应范围内的5％至8％。这种响应级别创建了比Allegro的霍尔效应传感器高出50倍的信号，从而使用GMR传感器而不是Hall效应传感器在IC中实现更高的信噪比。
The GMR Response
The native response of the GMR to an applied field (Bapp) in the plane of the resistor (and therefore the die surface or IC surface) is proportional to the cosine of the angle of the applied magnetic
field. However, the resistance value of the GMR does not always indicate the strength of the field. A basic GMR transducer is more of a magnetic angle sensor (as shown in Figure 1). However, in
many cases, a linear response to the field in one axis is desired from a GMR transducer. In order to create this linear response, an anisotropy is created 90 degrees from the “reference” layer that
acts like another magnetic field to be vector summed with the applied field (this anisotropy-induced field, Ban, is indicated by yellow arrow in Figure 2). The response then has a linear region around the state of zero magnetic field. This method of linearizing the response is used in many of Allegro’s ICs. It is important to note the saturated response present at either extreme of the range of field response. When in a linear application, the maximum operating range is specified to account for stray magnetic field and the magnetic stimulus to be sensed. GMR product datasheets can be referenced to indicate the operating boundary conditions. One item of note is that Allegro Hall-effect solutions have no such native saturated response. Allegro’s Hall ICs have a saturated response based on application or electrical circuit conditions, not a result of the Hall technology itself.
Using GMR in an IC Application
Typically, GMR resistors are created and placed in a Wheatstone bridge configuration. Half of the Wheatstone bridge (elements A and C in Figure 3) is positioned under one magnetic condition
and the other half of the Wheatstone bridge (elements B and D) is positioned under another magnetic condition. Ideally, these conditions present an equal but opposite response, allowing for the maximum output signal from the bridge. As shown by blue arrows and text in Figure 3, elements A and C sense a field in an orientation pointing left (in an anti-parallel state in this example, denoted as R马克斯在图1）中，元件B和D感测在右侧的方向上的场（在该示例中的并行状态下，表示为rminin Figure 1). The result is that resistors A and C will be in a high resistance state and those in B and D will be in a low resistance state. The differential output will then be positive.
With a Wheatstone bridge, the output is always scaled with the applied VCC, and with no magnetic field applied, centers the differential output at 0 V. The differential bridge output will then swing positively or negatively according to the direction of the applied magnetic field on the Wheatstone bridge. This bridge configuration allows for both a cancellation of temperature effects and also for a level of immunity to stray magnetic field.
For current sensors, field is steered over elements A and C of the Wheatstone bridge in one direction, and the field over elementsB and D of the Wheatstone bridge in the opposite direction (see
Figure 4). The output of the Wheatstone bridge is fed into a differential amplifier and then through Allegro’s normal sensitivity and offset correction circuits, and possibly more advanced signal processing circuits in the analog or digital domains. In other applications where the conductor is not integrated, the physical spatial separation of the GMR elements is used to affect a differential
signal, allowing response to a variety of magnetic stimulus.
GMR的另一个申请用于环形磁体速度感测应用，例如ABS或变速器传感器。雷竞技最新网址使用交替的北极和南磁化产生磁性材料环，如图5所示。可以将GMR传感器放置在该材料下方，使得管芯的平面是水平的。A和C GMR元件和B和D GMR元件之间的间隔基于环磁体处于其旋转循环的位置，产生由这些元件的这些元件感测的不同磁场。当N（北）杆以芯片置于模具时，磁场指向左侧元件A和C以及右边的元素B和D.这将在GMR上创建响应，如图3所示，最大响应于GMR桥。当过度S（南）杆时，响应将最大负。当在极之间时，该字段大约相等，每个元件均相等，并且桥的响应接近0.这导致来自传感器的正弦输出，因为环磁体旋转。通过在输出中的阈值之间计数阈值之间的时间，可以测量环形磁铁的速度。与传统的霍尔传感器相比，GMR的敏感性较高，提供了更高的气隙传感能力，以及输出中的更高的重复性，以便在速度输出中更高的精度。
快板Has a Monolithic GMR Solution
Many vendors selling GMR solutions do so using a multi-chip approach: a “sensor” chip and an “interface” chip. Allegro is one of very few IC manufacturers who directly integrate GMR technology
on top of their semiconductor wafers.
这种综合方法提供了许多优点，包括通过避免额外的管芯键合来改善可靠性，并且在整合电流承载线或定位元件与外部参考时，允许更简单的整体设计。
晶圆进入包裹
Since Allegro’s GMR solution is monolithic in nature, GMR IC wafers are managed in the same manner as Hall-effect sensors IC wafers. The fabricated wafers are ground to the proper thickness for their packages, and the wafer is cut into the appropriate die size. Following this step, the part is packaged in Allegro’s standard range of semiconductor IC packages.
选择一个解决方案或GMR的解决方案
GMR transducers offer some advantages over Hall-effect transducers. However, it is very important to understand the desired application of these transducers, as in many cases a Hall solution is the better solution.
| Factor | Hall | GMR（基于 example stack) |
| Sensitive direction | Through plane (1 axis) | In plane (2 axis), usually 1 primary |
| 响应 | Perfectly one axis linear | Cosine type response in 2 axes that is more complex to interpret |
| Sensitivity (native) | ~10-20 μV / G | 0.5-2 mV / G (50+ X hall) |
| Linear range | Unrestricted | ±55 G |
| Responsive range | Unrestricted | ±100 G |
Conclusion
快板’s new integrated GMR technology provides an additional tool in the designer’s toolbox to address new applications and to extend the capabilities of their ICs in existing applications. GMR offers the ability to improve signal to noise, increase resolution, or reduce the required field level for a given solution (smaller magnets, larger air gaps, etc.). Additionally, sensing in-plane to the wafer or IC surface gives the ability to create new, more robust, differentially magnetic solutions than possible with through-plane sensitive Hall technology. Allegro will be releasing products across all relevant magnetic sensor IC portfolios to take advantage of the new capability that GMR technology provides.