New manufacturing methods and signal correction techniques have shrunk the size of the newest generations of Hall sensors from the initial bipolar designs.
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Hall Effect sensors have been around for a long time. While the early bipolar versions were great at detecting magnetism, they had several drawbacks. For one, they couldn't control their response because they lacked error correction circuitry. Another drawback was that they were effected by temperature and stress, which often changed output voltage readings. They were also expensive to make and there were no economies of scale to help lower cost.

That was about twenty years ago. New manufacturing methods and error correction techniques have turned Hall sensors into sleek, accurate switches that have a lot to offer motion designers. And, the cost has come down to the point where the technology is economical for practically any application requiring the sensing of speed, direction, position, and current.

A change in direction

A typical Hall effect sensor consists of a sensing cell (Hall plate) and an operational amplifier. In the presence of a magnetic field, the Hall cell produces a small voltage that the op amp increases. Ideally, when the field is removed, the output voltage goes to zero. However, both the Hall cell and the op amp can produce substantial offset voltages that will vary the actual response.

Early manufacturing and assembly procedures often failed to remove these offset errors, and in many cases played a role in creating them. Wafer fabrication processes - such as heating and cooling, thin film deposition, sawing, die mount, encapsulation, and lead trim - contributed piezoresistive effects and resistive changes, which in turn, resulted in errors that were costly to correct later in production.

Trimming was the most common method of offset correction. Unfortunately, trimming techniques and the extensive testing needed to verify correction, plus low chip yield, totaled about 50% of the cost of Hall effect sensors.

But that was before CMOS. The main benefit of CMOS technology is that it shrinks the size of the sensors. Depending on the design, the die can be as small as 1 mm².

Programmable Hall sensors are self-contained units, including the output drive. Set up can be easy with some using a type of level-shifting logic to program.
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CMOS also makes it easier to build switches. This was an important development because most error-correction circuitry is based on switching technology. The combination of smaller die and CMOS switch also meant that Hall sensors were more stable over a wider temperature range.

Today, thanks to CMOS, Hall sensors incorporate chopper stabilization and quadrature switching to decrease offset errors. Chopper stabilization is used to reduce input offset errors at the op amp, and is a benefit for both digital and linear (analog) Hall sensors. The quadrature scheme involves actively switching the direction of current through the Hall elements. The combined effect of both techniques is an order of magnitude improvement in switch point drift and gain and offset errors.

As with other electronic devices, newer digital design techniques are also helping Hall sensors. Circuit techniques reduce the number of external components needed to implement certain functions. Recently, engineers took advantage of this capability to develop programmable Hall sensors. Now it's possible to have sensors with user defined sensitivity and offset.

More recently, engineers developed 3-V logic systems, which make Hall sensors compatible with the newest Pentium- class processors.

Sensing range

Hall switches typically have a Hall integrated circuit, a magnet, and a means of moving the magnet or the magnetic field. Operation is simple. The switch is ON in the presence of the field and OFF when the field is removed. Both the operate point and release point, as well as the differential can be precisely set by the designers.

The operate point is where the magnetic flux density turns the sensor ON, allowing current to flow from the output to ground. Conversely, the release point is where the magnetic flux density turns the sensor OFF. The absolute difference between them is referred to as hysteresis. Its purpose is to eliminate false triggering, which can be caused by minor variations in input, electrical noise, and mechanical vibration.

Depending on how designers employ these characteristics, they can solve a wide range of motion sensing problems.

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