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As the term implies, linear Hall sensor ICs develop an output signal that is proportional to the applied magnetic field. Normally, in any current-sensing application, this flux field is focused by a 'slotted' toroid to develop an adequate field intensity, and this magnetic field is induced by current flowing in a conductor. A 'classic' transfer curve for a ratiometric linear is illustrated in figure 1.

Note that, at each extreme of its range, the output saturates. Most recent linear Hall ICs provide a ratiometric output voltage. The quiescent i. This quiescent output voltage signal equates to no applied magnetic field and, for current sensing, is equivalent to zero current flow. Output saturation voltages are typically 0. Any systems problems associated with low-level signals and noise are minimized by the monolithic integration of magnetic Hall element, amplifier, output, and allied signal processing circuitry.

Existing very stable, linear HEDs exploit dynamic quadrature offset cancellation circuitry and utilize electronic switching to change the current path in the Hall element.

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Sample-and-hold circuitry and a low-pass filter are exploited to properly 'recondition' the internal dynamic signals of these innovative linear HEDs. Linear Hall-effect ICs can detect small changes in flux intensity, and are generally more useful than digital Hall ICs for current sensing. Linear HEDs are often capacitively coupled to op amps, or DC connected to comparators, to attain system design objectives. As mentioned, Hall-effect current sensing usually necessitates the use of a slotted toroid made of ferrous materials.

The toroid both concentrates and focuses an induced magnetic field toward the location of the Hall-effect element within the IC package. The conductor current flows through the turns wound upon the toroid, and the induced flux field is concentrated on the sensor IC in the gap or slot in the toroid. Usually, this gap is made to closely match the Hall IC package thickness approx. The current flow with this 'tight' magnetic coupling induces a flux intensity per the formula.

Widening the slot gap reduces the flux coupling and can increase the upper current limit, which is predicated upon the Hall sensor IC sensitivity more to follow. However, decoupling the induced field to extend the maximum current limit may affect linearity, usable range, etc.


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This 'loose' coupling is under evaluation, but not yet complete; hence, no new formulas for magnetic flux and conductor current and larger gaps have been documented. The two newest [ Ed. Note: Article originally presented in Presently, though seldom sold, 'calibrated' linear Hall-effect ICs are superb circuits for setting up and measuring system magnetic parameters, and represent an excellent entry to the performance, characteristics, and limitations of ratiometric ICs.

The elemental distinction between the A and A is magnetic sensitivity. From these plots figures 3 and 4 it is apparent that neither linearity nor symmetry the deviation in the slope from the quiescent or null voltage is a vital design consequence as neither surpasses 0. The practical current limit maximum with 'tight' coupling is derived using the range and flux per turn in the prior formula per the approximation:. Obviously, this is a situation that a designer would prefer to avoid. However, low-cost options are scarce or non-existent. The latest linear HEDs incorporating the dynamic quadrature DC offset cancellation are illustrated in figure 5.

This precludes most of the earlier offset related factors DC imbalances due to resistive gradients, geometrical dissimilarities, piezoresistive effects, etc. A low-pass filter and a sample-and-hold circuit are employed to recondition the signal fed to the linear, ratiometric Hall sensor IC output. Although the power requirements for linear HEDs are small, external power is needed. Easy, on-board, 'down' regulation from a system supply is simple with low-cost IC regulators. A listing of absolute maximum limits for the new linear, ratiometric sensor ICs follows in table 4. The high-impedance inputs of today's analog or conversion circuitry usually necessitates microamperes not milliamperes of Hall sensor IC output current.

However, magnetic fields beyond the usable range force the output into saturation and non-linear operation without harm to the HED. However, the newest linears permit infrequent i. Internal power is usually not a HED limitation, but designers should comprehend the basic results of device power dissipation and its relationship to elevating the sensor IC junction temperature.

IC and system reliability is inversely correlated to the temperature of all system components. Higher ambient and junction temperatures reduce the life expectancy and dependability of any system. Various, numerous linear-HED characteristics are of concern in current-sensing applications, and brief descriptions of these follow.

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Subsequently, many of these characteristics and parameters will be embodied in a focus on accuracy, temperature effects, linearity, symmetry, etc. A stable, well-regulated supply is very necessary for proper operation, otherwise the output voltage will fluctuate and follow any variations in supply. Note: For latest performance characteristics, refer to the Allegro selection guide. A 'no-disconnect,' 'non-intrusive' technique is based upon forming a 'toroid' around the conductor being sensed.

Rather than pass the wire through the toroid figures 6A and 6B , a soft iron piece is formed around the conducting wire.


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This permits sensing currents without the need for disconnecting any conductors in the power system 'no-disconnect' formed toroid is shown in figure 6C. Excessive current does not impair or damage the sensor IC.

However, extreme, sustained overcurrent could be a fire or safety hazard if the conductor overheats and creates a dangerous situation. Signal voltage changes little up to this frequency. However, noticeable phase shift becomes distinct at somewhat lower frequencies. Representative oscilloscope plots show the effects of frequency on the Hall sensor IC signal. The top signal is the HED voltage, and the lower trace is the winding coil current. Note: Testing performed with 20 turns on a gapped toroid; and the voltage scales of the three plots are not identical.

Other intermediate-frequency plots exhibit similar phase shifts, but were not included due to space limits. Note: Limitations refer to the strictures of the original publication. Also, it should be mentioned that this bandwidth limitation is correlated with the linear sensor IC. The magnetics and induced coupling is definitely not a restricting factor to bandwidth within this range of operating frequencies. Battery-powered and battery 'backup' designs are particular concerns, and any method capable of curtailing power is scrutinized.

A recurring technique is to periodically activate the sensor IC circuitry by switching the power supply on for brief intervals, and then off for longer periods. Average power is related to duty cycle. Thus, for low duty-cycle applications, the power consumed can be decreased substantially. Clearly, the time required for a linear Hall IC to provide a stable, usable signal is very important, and two different linear HEDs were evaluated to ascertain their power-up response characteristics.

The devices exhibit dissimilar properties, and the oscilloscope plots portray their dynamic operation upon applying power to the linears. The latest linear HEDs with dynamic quadrature offset cancellation have a slower response than an earlier generation that exploits the orthogonal Hall element. The previous series A, etc.

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The very obvious tradeoff: speed vs. These plots reveal basic tradeoffs in performance vs. Inherently, linear Hall sensor ICs exhibit no hysteresis. However, different slotted toroids and varied magnetic materials may possess differing hysteretic properties.

Hysteresis is a minor concern when using ferrite cores, but other ferrous cores such as powdered iron may exhibit different characteristics.

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Thus, a complete, thorough evaluation of specific toroids and the associated linear sensor IC would be a very prudent and recommended suggestion. A current-sensor application design that employs sufficient turns to drive the output voltage of the HED to nearly full scale at the maximum design current first induces saturation of the sensor IC. For optimum accuracy, the number of turns used should induce output voltage transitions that just fall short of saturating the sensor IC more on this.

The testing specifications for the recent, stable linear Hall IC series are:.