HALL-EFFECT

GEAR-TOOTH SENSORS

—AC COUPLED

www.allegromicro.com

5

24 V

MAX

0+B

0

DIFFERENTIAL FLUX DENSITY, B

E1

Dwg. GH-034

-B

OP

RP

B

B

Figure 2

OP

B

B – B

E1

E2

GEAR

4300 G

4130 G

150 G

0 G

-150 G

RP

B

V

OUT(SAT)

V

OUT

B & B

E1

E2

OUTPUT DUTY CYCLE

≈ 50%

Dwg. WH-003-1

DIRECTION

OF ROTATION

LEADING

EDGE

TRAILING

EDGE

NORTH

SOUTH

E2

E1

(a)

(b)

(c)

Sensor Operation. These sensor ICs each contain

two integrated Hall transducers (E1 and E2) that are used

to sense a magnetic field differential across the face of the

IC (see Sensor Location drawing). Referring to Figure 1,

the trigger switches the output ON (output LOW) when

Figure 2 relates the output state of a back-biased

sensor IC, with switching characteristics shown in Figure

1, to the target gear profile and position. Assume a north

pole back-bias configuration (equivalent to a south pole at

the face of the device). The motion of the gear produces

a phase-shifted field at E1 and E2 (Figure 2(a)); internal

conditioning circuitry subtracts the fields at the two

elements (Figure 2(b)); this differential field is band-pass

filtered to remove dc offset components and then fed into

a Schmitt trigger; the Schmitt trigger switches the output

(Figure 2(c)), the IC output is LOW whenever sensor E1

sees a (ferrous) gear tooth and sensor E2 faces air. The

output is HIGH when sensor E1 sees air and sensor E2

sees the ferrous target.

AC-Coupled Operation. Steady-state magnet and

system offsets are eliminated using an on-chip differential

band-pass filter. The lower frequency cut-off of this

patented filter is set using an external capacitor the value

of which can range from 0.01

µF to 10 µF. The high-

frequency cut-off of this filter is set at 30 kHz by an

internal integrated capacitor.

The differential structure of this filter enables the IC to

reject single-ended noise on the ground or supply line

and, hence, makes it resistant to radio-frequency and

electromagnetic interference typically seen in hostile

remote sensing environments. This filter configuration

also increases system tolerance to capacitor degradation

at high temperatures, allowing the use of an inexpensive

external ceramic capacitor.

Figure 1

TYPICAL TRANSFER

CHARACTERISTIC

APPLICATIONS INFORMATION (cont’d)