3059 Datasheet, PDF (5/8 Pages) Allegro MicroSystems – HALL-EFFECT GEAR-TOOTH SENSORS -AC COUPLED

3059 AND 3060
HALL-EFFECT
GEAR-TOOTH SENSORS
—AC COUPLED
Figure 1
TYPICAL TRANSFER
CHARACTERISTIC
24 V
MAX
B _OP
0
-B
B _RP
V _OUT(SAT)
0
DIFFERENTIAL FLUX DENSITY, BE1 – BE2
+B
Dwg. GH-034
4300 G
B &B
E1 E2
B –B
E1 E2
4130 G
150 G
B _OP
0G
B RP
-150 G
V
OUT
V
OUT(SAT)
Figure 2
LEADING
EDGE
TRAILING
EDGE
GEAR
DIRECTION
OF ROTATION
E2
E1
NORTH
SOUTH
(a)
(b)
OUTPUT DUTY CYCLE ≈ 50%
(c)
Dwg. WH-003-1
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
B_E1 - B_E2 < B_OP and switches the output OFF (output
HIGH) when B_E1 - B_E2 < B_RP. The difference between B_OP
and B_RP is the hysteresis of the device.
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
transistor at the thresholds B_OP and B_RP. As shown
(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.