High Precision Hall Effect AC-Coupled Differential
Sensor IC with Integrated Filter Capacitor
A1421, A1422,
and A1423
12
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Applications Information
The A1421, A1422, and A1423 are versatile high-precision dif-
ferential sensor ICs that can be used in a wide range of applica-
tions. Proper choice of the target material and shape, magnet
material and shape, and assembly techniques enables large
working air gaps and high switchpoint accuracy over the device
operating temperature range.
Device Operation
The device IC contains two integrated Hall transducers that are
used to differentially respond to a magnetic field across the sur-
face of the IC. Referring to figure 1, which shows curves for the
A1421 as an example, the trigger switches the output when the
differential magnetic field crosses the B
OP
level while increasing
in strength (referred to as the positive direction). In the example,
the A1421 output voltage switches high (off), and switches the
output low (on) when the differential magnetic field crosses B
RP
while decreasing (the negative direction).
The operation is achieved through the use of two separate com-
parators. One comparator has a positive hysteresis, B
HYS1
, and
the other a negative hysteresis, B
HYS2
. Therefore, one comparator
switches at the B
OP
crossing on an increasing differential signal
and the other switches at the B
RP
crossing on a decreasing differ-
ential signal. The hysteresis on each comparator precludes false
switching on noise or target jitter.
The behavior is similar for the A1422 and the A1423. The
switchpoints are as shown in the magnetic charactersitics table,
and the output polarity is inverted. This is illustrated in figure 2,
on the next page.
Start-up
During power-on time, t
PO
, the output signal, V
OUT
, is high.
Beyond this time, if the applied magnetic field, B
diff
, is smaller
than B
HYS
, the switching state and V
OUT
polarity are indeter-
minate. V
OUT
will be valid for B
diff
> B
HYS
, after the additional
settling time, t
Settling
, has also elapsed.
Delay
The bandpass filter induces delay in the output signal, V
OUT
, rel-
ative to the applied magnetic field, B
diff
. Simulation data shown
Figure 1. Typical output characteristics with dual comparator operation. The example shown is for the A1421. Characteristics
shown without delay, see characteristic data charts for delay and phase shift contributions.
On
Off
Off
B
RP(typ)1421
B
OP(typ)1421
1421 Output Signal, V
OUT
1421 Switching State
Applied Magnetic
Field, B
diff
Comparator 1, A1421
Comparator 2, A1421
B
HYS2
A
B
HYS1
A
0.0
15.0