A1425 Allegro MicroSystems, A1425 Datasheet - Page 10

• Manufacturer Part Number: A1425
• Description: High Accuracy Analog Speed Sensor with Integrated Filter Capacitor and Dual Zero-Crossing Output Signal
• Manufacturer: Allegro MicroSystems

The A1425 is a versatile high-precision differential sensing
device that can be used in a wide range of applications. Proper
choice of the target material and shape, and assembly techniques
enables large working air gaps and high switchpoint accuracy
over the device operating temperature range.
Sensor Operation
The A1425 sensor IC contains two integrated Hall transducers
that are used to differentially sense a magnetic fi eld across the
surface of the IC. Referring to fi gure 1, the trigger switches the
output off (output high) when the differential magnetic fi eld
crosses zero while increasing in strength (referred to a the posi-
tive direction), and switches the output on (output low) when
the differential magnetic fi eld crosses zero while decreasing (the
negative direction).
The operation is achieved through the use of two separate com-
parators. Both comparators use the same reference point, 0 G, to
provide high accuracy, but one comparator has a positive hystere-
sis, B
A1425-DS Rev. 0b
Preliminary - Subject to Change
Without Notice November 10, 2004
High Accuracy Analog Speed Sensor with Integrated Filter Capacitor and Dual Zero-Crossing Output Signal
HYS1
, and the other a negative hysteresis, B
Output Signal, V
Applied Magnetic
Figure 1. Typical output characteristics with dual comparator operation. Characteristics shown without delay, see characteristic
data charts for delay and phase shift contributions.
Switching State
Comparator 1
Comparator 2
Field, B
OUT
diff
–11.0
11.0
0.0
Off
HYS2
Applications Information
. Therefore,
A1425
B
B
HYS2
RP(typ)
one comparator switches at the zero crossing on an increasing
differential signal and the other switches at the zero crossing on a
decreasing differential signal. The hysteresis on each comparator
precludes false switching on noise or target jitter.
Start-up
During power-on time, t
Beyond this time, if the applied magnetic fi eld, B
less than 50 G peak-to-peak, the switching state and V
ity are indeterminate. V
after the additional settling time, t
Also during t
charges the on-chip capacitor. This feature reduces t
to the long RC time constant of a high-pass fi lter.
Delay
The on-chip band-pass fi lter induces delay in the output signal,
V
data shown in the Characteristic Data section quantify the effect
of the input signal amplitude on the phase shift of the output.
On
OUT
B
OP(typ)
, relative to the applied magnetic fi eld, B
B
HYS1
PO
, a circuit in the A1425 is briefl y enabled that
OUT
PO
, the output signal, V
Off
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
will be valid for B
S
, has also elapsed.
B
B
OP(max)
OP(min)
/ B
/ B
diff
RP(min)
diff
RP(max)
OUT
. Simulation
diff
> 50 Gp-p,
, is high.
, is absent or
PO
OUT
, relative
polar-
10