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A1425
High Accuracy Analog Speed Sensor IC with Integrated Filter
Capacitor and Dual Zero-Crossing Output Signal
Features and Benefits
Used in sensing motion of ring magnet or ferrous targets
Integrated filter capacitor
Wide operating temperature range
Operation with magnetic input signal frequency from
20 Hz to 20 kHz
Resistant to EMI
Large effective air gaps
4.0 to 26.5 V supply operating range
Output compatible with both TTL and CMOS logic
families
Reverse battery protection
Resistant to mechanical and thermal stress
Accurate true zero-crossing switchpoint
Package: 4 pin SIP (suffix K)
Description
The A1425 AC-coupled Hall-effect sensor IC is a monolithic
integrated circuit that switches in response to changing
differential magnetic fields created by rotating ring magnets
and, when coupled with a magnet, by ferrous targets.The device
is a true zero-crossing detector: the output switches precisely
when the difference in magnetic field strength between the
two Hall elements is zero. A unique dual-comparator scheme
provides for accurate switching at the zero crossing on both the
positive and negative-going regions of the differential signal,
while utilizing hysteresis to prevent false switching. The zero-
crossing nature of this device provides excellent repeatability
and accuracy for crankshaft applications.
Changes in field strength at the device face, which are
induced by a moving target, are sensed by the two integrated
Hall transducers. The transducers generate signals that are
differentially amplified by on-chip electronics.This differential
design provides immunity to radial vibration within the
operating air gap range of the A1425, by rejection of the
common mode signal. Steady-state magnet and system offsets
are eliminated using an on-chip differential band-pass filter.
This filter also provides relative immunity to interference from
electromagnetic sources.
Continued on the next page…
VS+
VCC
(Pin 1)
Functional Block Diagram
Dual Hall
Transducers
Regulator
0.1 uF
Hall
Amp
Gain
Stage
Bandpass Filter Integrated
Tracking Capacitor
VREF
Diagnostic
Circuitry
Comparator
TEST
(Pin 3)
VOUT
(Pin 2)
1425-DSa, Rev.3
GND
(Pin 4)
(Required)

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A1425
High Accuracy Analog Speed Sensor IC with Integrated
Filter Capacitor and Dual Zero-Crossing Output Signal
Description (continued)
The device utilizes advanced temperature compensation for the
high-pass lter, sensitivity, and Schmitt trigger switchpoints,
to guarantee optimal operation to low frequencies over a wide
range of air gaps and temperatures.
Each Hall effect digital integrated circuit includes a voltage
regulator, two Hall effect elements, temperature compensating
circuitry, a low-level amplier, band-pass lter, Schmitt trigger,
and an output driver, which requires a pull-up resistor. The on-
board regulator permits operation with supply voltages from 4.0
to 26.5 V. The output stage can easily switch 20 mA over the full
frequency response range of the device, and is compatible with
both TTL and CMOS logic circuits.
The device is packaged in a 4-pin plastic SIP. It is lead (Pb) free,
with 100% matte tin plated leadframe.
Selection Guide
Part Number
Packing*
A1425LK-T
Bulk, 500 pieces/bag
*Contact Allegro for additional packing options.
Switchpoints
BRP(MIN)
(G)
BOP(MAX)
(G)
–11 11
Absolute Maximum Ratings
Characteristic
Supply Voltage
Reverse Supply Voltage
Continuous Output Current
Continuous Reverse-Output Current
Operating Ambient Temperature
Maximum Junction
Storage Temperature
Symbol
VCC
VRCC
IOUT
IROUT
TA
TJ(max)
Tstg
Notes
Refer to Power Derating section
Range L
Pin-out Diagram
Terminal List Table
Name
VCC
Number
1
VOUT
2
TEST
3
GND
4
1 2 34
Rating
28
–18
25
–50
–40 to 150
165
–65 to 170
Units
V
V
mA
mA
ºC
ºC
ºC
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
2

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A1425
High Accuracy Analog Speed Sensor IC with Integrated
Filter Capacitor and Dual Zero-Crossing Output Signal
OPERATING CHARACTERISTICS Valid at TA = – 40ºC to 150ºC, TJ 165°C; over operational air gap range and VCC within
operating range, unless otherwise noted. Typical operating parameters: VCC = 12 V and TA= 25°C.
Characteristic
Symbol
Test Conditions
Min. Typ. Max. Units
ELECTRICAL CHARACTERISTICS
Supply Voltage
Supply Current
Output Saturation Voltage
Output Leakage Current
VCC Operating; TJ < TJ(max)
ICC
VOUT(SAT) ISINK = 20 mA
IOFF VOUT = 24 V, Bdiff = 0
PROTECTION COMPONENT CHARACTERISTICS
Reverse Supply Current
Supply Zener Current
Supply Zener Clamp Voltage1
Output Zener Current
Output Zener Clamp Voltage
Output Short Circuit Current Limit2
IRCC VCC = –18 V
IZSupply VS = 28 V
VZSupply ICC = 10 mA, TA = 25°C
IZOutput VOUT = 28 V
VZOutput IOUT = 3 mA, TA = 25°C
IOUTS(lim)
RESPONSE CHARACTERISTICS
4.0 – 26.5 V
– 4.2 7.0 mA
– 140 400 mV
– – 5 μA
– – –1 mA
– – 10 mA
28 33 37
V
– – 3 mA
28 – – V
– – 50 mA
Power-On State
POS t < tResponse
– High –
V
Power-On Time3,7
Settling Time4,7
Response Time7
Upper Corner Frequency
Lower Corner Frequency
tPO
tSettle
tResponse
fcu
fcl
VCC > VCC(min)
fBdiff 100 Hz
Equal to tPO + tS; fBdiff 100 Hz
–3 dB, single pole
–3 dB, single pole
– 4.5 9 ms
0–
4.5 –
20 –
––
50 ms
59 ms
– kHz
20 Hz
OUTPUT CHARACTERISTICS
Output Rise Time5
Output Fall Time
MAGNETIC CHARACTERISTICS
Output Off Switchpoint6,7
tr RPU = 1 kΩ, COUT2 = 10 pF
tf RPU = 1 kΩ, ISINK = 20 mA, COUT2 = 10 pF
– – 200 ns
– – 200 ns
BOP
Bdiff increasing, fBdiff = 200 Hz, Bdiff = 50 Gp-p;
digital output signal switches low to high
–11
0
11
G
Output On Switchpoint6,7
BRP
Bdiff decreasing, fBdiff = 200 Hz, Bdiff = 50 Gp-p;
digital output signal switches high to low
–11
0
11
G
Applied Magnetic Field7,8
Bdiff Differential p-p magnetic eld
50 – 1250 G
1ICC equivalent to ICC(max) + 3 mA.
2IOUT does not change state when IOUT > IOUTS(lim), regardless of changes in the impinging magnetic eld.
3Time required to initialize device.
4Time required for the output switchpoints to be within specication.
5Output Rise Time will be dominated by the RC time constant.
6For other sinusoidal signal frequencies and magnetic elds, –BOP = BRP = sinα(Bdiff 2) ± 25%, where α is the phase shift shown in the Characteristic
Data section.
7See Denitions of Terms section.
8Exceeding the maximum magnetic eld may result in compromised absolute accuracy.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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