HALL-EFFECT

GEAR-TOOTH SENSORS

—AC COUPLED

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

6

Low-Frequency Operation. Low-frequency opera-

tion of the sensor is set by the value of an external

capacitor. Figure 3 provides the low-frequency cut-off (-3

dB point) of the filter as a function of capacitance value.

This information should be used with care. The graph

assumes a perfect sinusoidal magnetic signal input.

In reality, when used with gear teeth, the teeth create

transitions in the magnetic field that have a much higher

frequency content than the basic rotational speed of the

target. This allows the device to sense speeds much

lower than those indicated by the graph for a given

capacitor value.

codes Z5S, Y5S, X5S, or X7S (depending on operating

temperature range) or better are recommended. The

commonly available Z5U temperature code should not be

used in this application.

Magnet Selection. The UGS3059KA or UGx3060KA

can be used with a wide variety of commercially available

permanent magnets. The selection of the magnet de-

pends on the operational and environmental requirements

of the sensing system. For systems that require high

accuracy and large working air gaps or

an extended temperature range, the usual magnet mate-

rial of choice is rare-earth samarium cobalt (SmCo). This

magnet material has a high energy product and can

operate over an extended temperature range. For sys-

tems that require low-cost solutions for an extended

temperature range, AlNiCo 8 can be used. Due to its

relatively low energy product, smaller operational air gaps

can be expected. Neodymium iron boron (NeFeB) can be

used over moderate temperature ranges when large

working air gaps are required. Of these three magnet

materials, AlNiCo 8 is the least expensive by volume and

SmCo is the most expensive.

System Issues. Optimal performance of a gear-tooth

sensing system strongly depends on four factors: the IC

magnetic parameters, the magnet, the pole piece configu-

ration, and the target.

Sensor Specifications. Shown in Figure 4 are

graphs of the differential field as a function of air gap.

A 48-tooth, 2.5” (63.5 mm) diameter, uniform target similar

to that used in ABS applications is used. The samarium

cobalt magnet is 0.32” diameter by 0.20” long

(8.13 x 5.08 mm). The maximum functioning air gap with

this typical gear/magnet combination can be determined

using the graphs and specifications for the sensor IC.

able air gap would be approximately 0.120”. If the worst

case switch points of

±35 G for the UGx3060KA are used,

the maximum air gap is approximately 0.105”.

All system issues should be translated back to such a

profile to aid the prediction of system performance.

Figure 3

Capacitor Characteristics. The major requirement

for the external capacitor is its ability to operate in a

bipolar (non-polarized) mode. Another important require-

ment is the low leakage current of the capacitor (equiva-

lent parallel resistance should be greater than 500k

Ω). To

maintain proper operation with frequency, capacitor

values should be held to within

±30% over the operating

temperature range. Available non polarized capacitors

include ceramic, polyester, and some tantalum types. For

low-cost operation, ceramic capacitors with temperature

0.1

1.0

10

1.0

10

CAPACITANCE IN

µµµµF

100

0.1

0.01

Dwg. GH-025

1 k

APPLICATIONS INFORMATION (cont’d)