PRELIMINARY TECHNICAL DATA

Wheel Speed Sensor For

ABS Systems
Preliminary Technical Data AD22157
FEATURES Functional Block Diagram
Speed and direction from 0Hz to 2500Hz
Air gap diagnostics Vhigh
2-wire current-loop operation
Wide Operating Temperature Range
Functional during temperature excursions to 190C
Tracker 1
Reverse Supply Protected (-30V)

APPLICATIONS

Hall Array

In Amps
7mA 7mA
Automotive PWM
Wheel speed and direction sensing

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Transmission speed sensing
Industrial
Tracker 2
Incremental position sensing
Proximity switching

LE AD22157 Vlow

GENERAL DESCRIPTION associated with Hall cell characteristics. The sensor is compen-
The AD22157 is a mixed signal magnetic field transducer sated to work optimally with SmCo magnets. The architecture
designed for applications where both speed sensing and direc- maximizes the advantages of fine line CMOS and high voltage
tion sensing of a ferrous target wheel are required over a wide DMOS allowing the device to operate accurately in demanding
speed range. environments.
SO

The device operates from a 2 wire high compliance current loop Principle features of the AD22157 include an adaptive differen-
and is suitable for continuous operation from -40C to 150C with tial zero crossing detector which accurately determines the posi-
supplies up to +20 Vdc. The sensor is designed to remain func- tion of target wheel edges. This architecture eliminates the
tional during voltage transients up to +27V. effects of package and thermal stress on the Hall sensor array
The sensor output format is a current pulse from 7mA to 14mA resulting in 2% repeatability of the time interval from rising
(the quiescent bias is 7mA) whose rising edge is accurately edge to rising edge of the sensor output.
placed relative to the edges of the target wheel. The pulse width The sensor takes 4 edges from either power on or a stopped con-
B

is determined by both target wheel direction and field strength. dition to achieve full accuracy. The architecture employs digital
The output pulse is coded in multiples of a well defined time signal processing to provide robust functionality and eliminate
interval depending on direction and field strength in conform- spurious or missing pulses under extreme conditions of EMC.
ance with industry standards currently being promoted by lead- The AD22157 is housed in a 5 lead single-in-line (SIP) package
O

ing systems manufacturers. suitable for mounting with a back biasing magnet in a typical
Pulse widths corresponding to differential magnetic fields mea- wheel speed sensor assembly.
sured at the sensor of ∆B >4mT (normal operation),
2mT<∆B<4mT (low field range), and ∆B<2mT (very low field
range) are provided. Direction is indicated in the normal and
low ranges.
A fail safe stop signal repeating at approximately 1.5Hz is pro-
vided initially at power on, if the target wheel is stopped, or if
no dynamic signal is detected for some other reason.
The sensor combines integrated bulk Hall cell technology and
instrumentation circuitry to minimize temperature related drifts
REV. PrA
Information furnished by Analog Devices is believed to be accurate and reliable. One Technology Way, P.O. Box 9106, Norwood, MA02062-9106,USA
However, no responsibility is assumed by Analog Devices for its use, nor for any www.analog.com
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No license is granted by implication or otherwise under any patent or patent rights of
Analog Devices.

-1-
 PRELIMINARY TECHNICAL DATA
Preliminary AD22157 - Specifications (TA=+25C and V+=12V unless otherwise noted)
Parameters Min Typ Max Units
OPERATING TEMPERATURE
-40 150 C
POWER SUPPLY
Vcc Operating 4.5 20.0 V
Vcc max transient 27.0
Reverse Supply -30.0
SUPPLY CURRENT
Iout low 5.5 7.0 8.5 mA
Iout high (pulsed) 11.0 14.0 17.0
OUTPUT CURENT RATIO 1.8 2 2.2

OUTPUT CURRENT PULSE WIDTH1

Nominal field operation (left/right) 72/144 90/180 108/216

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Low field operation (left/right) 288/576 360/720 432/664 uS
Air gap limit 36 45 54
No field or stopped for >737mS 1152 1440 1728

PULSE PERIOD IN STOPPED MODE 589 737 884 mS

THRESHOLDS FOR OPERATION MODES
Nominal field threshold 4<Bth
Low field threshold
Field too low threshold
POWER ON TIME
CALIBRATION CYCLE
LE 2<Bth<4
Bth<2
4
4
mT

mS
edges
CALIBRATION UPDATE CYCLE each edge
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TIMING ACCURACY2 2 %

NOTES
1 Left: wheel moving from pin 1 to pin 5 with the front of the AD22157 facing the wheel (see figure 7).
2 Timing wheel with 2.5mm tooth/2.5mm valley and 5mmx4mm SmCo magnet.
B

ABSOLUTE MAXIMUM RATINGS* Pin Configuration

O

Maximum Supply Voltage . . . . . . . . . . . . . . . . . . . . . +27 V

Maximum Output Current (Pin 2) . . . . . . . . . . . . . . 18 mA
5 NC
Operating Temperature Range . . . . . . . . . –40°C to +150°C
Die Junction Temperature . . . . . . . . . . . . . . . . . . . .+190°C 4 NC
Storage Temperature Range . . . . . . . . . . –65°C to +160°C 3 Vlow
Lead Temperature (Soldering, 10 sec) . . . . . . . . . . +300°C
2 Vhigh
*Stresses above those listed under “Absolute Maximum Ratings” may cause permanent
damage to the device. This is a stress rating only and functional operation of the device 1 NC
at these or any other conditions above those listed in the operational sections of this spec-
ification is not implied. Exposure to absolute maximum rating conditions for extended
periods may affect device reliability. PIN 1 IDENTIFIER

CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection. Although the
AD22157 features proprietary ESD protection circuitry, permanent damage may occur on devices
subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended
to avoid performance degradation or loss of functionality.

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 PRELIMINARY TECHNICAL DATA
AD22157
CIRCUIT OPERATION current. The three Hall effect sensors are connected to instrumen-
The AD22157 is a two wire current modulating transducer which tation amplifiers as two pairs with the center plate shared between
generates current pulses in response to spacial differential changes the two amplifiers. In this configuration two spacial differential
in a magnetic field. A typical application is wheel speed sensing magnetic signals are transformed into electrical signals whose
where the field to be sensed is generated by the interaction of a peak to peak amplitude is directly proportional to the differential
permanent magnet behind the sensor and a notched or hole magnetic field component and the Hall plate bias current.
stamped ferromagnetic target wheel in front of the sensor. Under Pitch matching the Hall array to the wheel results in an approxi-
these conditions the sensor must reject that portion of the ‘bias’ mately sinusoidal field variation being sensed by the spatial differ-
field which is constant, and amplify the remaining differentially ential array.
modulated portion of the field and determine accurately the posi-
tion of edge transitions on the wheel.
SIGNAL DETECTION SOURCES OF ERROR PRIOR TO SIGNAL CONDITIONING
The bias field rejection is accomplished by a spacial differential The Hall sensors generate a number of error components in addi-
measurement of the field using integrated Hall plate structures tion to the desired spatial differential signal:
within the silicon substrate. A linear array of three Hall cells is Uncompensated magnetic bias field due to mismatch of Hall
used. The AD22157 is designed to give optimum quadrature sig- plate sensitivities, Hall bias current mismatch and variations in

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nals at a tooth/ notch pitch of 5mm. magnetic flux density across the surface of the bias magnet.
Each of the three Hall devices is constructed of four individual Intrinsic Hall plate offset due to lithographic misalignment of
plates of 200um diameter connected in parallel and spatially orien- Hall plate contacts, local planar variations in Hall plate diffusion
tated in each of four cross quadrature positions in order to relieve due to manufacturing tolerances and mechanical stress imposed by
encapsulation.
Temperature dependent sensitivity of the Hall cells is approxi-

Hall Array A Hall Array B Hall Array C

LE mately +450 ppm/ C........(+/-150 ppm / C).
Temperature dependent components of offsets are beyond the
scope of this functional description, however it may be assumed
that their total contribution at the output of the pre amplifiers is in
the order of several hundred millivolts, which may drift with tem-
200um dia.
perature by tens of millivolts in either direction.
From a circuit perspective, the amplifiers will contribute further
SO
1.25mm 1.25mm
input referred offset to the signals. This component is less than 1
mV and typically is of the order of several hundred micro volts.
Cross Quad Hall Cell SIGNAL CONDITIONING
Array
The primary function of the signal conditioning is to compensate
Figure 1. AD22157 Hall Array Spacing for offset errors and accurately determine the zero crossings of the
differential Hall cell signal component. The differential signals
process gradient induced offsets in the Hall signal voltage. approximate quadrature sine waves whose frequency is deter-
mined by the rotational speed of the target wheel. The phase rela-
B

The Hall plate arrays are biased by three matched current sources. tionship of the quadrature signal is used to determine the direction
The sensitivity of the plates to magnetic field is 5uV / Gauss at this of wheel rotation.
Two separate measurement channels are used for signal condition-
Hall signal A Hall signal B
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Hall signal C ing. The first channel circuitry (Tracker1) is used to determine the
zero crossing information and is the primary source of edge infor-
mation. The second channel (Tracker2) is used only for obtaining
1.414 *Hall signal direction information by comparison of signal phase. Each chan-
nel comprises two infinite sample hold circuits built around ten bit
tracking analog to digital convertors.
Peak detection of each of the channel signals is performed by
Tracker1 and Tracker2 using two A/D converter based sample
Channel 1 signal A-B
Channel 2 signal l B-C hold circuits per Tracker. One sample hold circuit follows positive
peaks, the other negative peaks. The potentials of the DAC’s rep-
Figure 2. Quadrature Fields Sensed By Hall Array resent the positive and negative peak values of the signal at any
given time.

REV. PrA -3-

 PRELIMINARY TECHNICAL DATA

AD22157
The mid point of these potentials is used as a reference for a tor and watchdog timer.
zero crossing detector in the PWM. This system assures that a The timing sequence is as follows:
phase jitter specification of +/- 2% for 1kHz signal (rising edge
to rising edge) can be met over all operational conditions. i. The counter is reset upon receipt of a zero crossing event.

Tracker1 also makes an absolute measurement of peak to peak ii. The leading edge of the pulse is output after a delay of 45uS.
signal. The digital result is a measure of field strength which iii. Amplitude thresholds are decoded with direction and the
can be related directly to air gap or used for diagnostic purposes appropriate output pulse width is generated.
in the application. The digital result is combined with direction iv. The counter is reset.
information from Tracker 2 and used to program the output
pulse width modulator (PWM). v. If a zero crossing is not received before the counter overflows
(745 uS), a STOP pulse is output.
The absolute peak to peak value of the Hall signal may vary due
to air gap settings and alter dynamically due to wheel run out. A The purpose of resetting the trackers is to enable the offset cor-
fixed resolution converter may fail to maintain acquisition of rection circuitry to remain active when no zero crossing events
the signal peaks using only single 1lsb steps. To compensate for occur but when thermally induced drift may invalidate the offset

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this, the resolution of the converters adapts to changes in the correction over extended periods of inactivity.
signal that cannot be followed using 1lsb steps. The sensor supply loop current is modulated in response to the
HALL PLATE BIAS pulse input between two discrete current levels of approxi-
The Hall cells are biased so that the temperature coefficient of mately 7 mA and 14 mA. The lower current value being the qui-
sensitivity of the AD22157 is of similar magnitude but opposite escent or logic low state.
polarity to rare earth magnetic materials i.e. SmCo = - 450ppm/
C. or Alnico 5 -7 = -300 ppm/C. This results in good stability of
the PWM thresholds.

OPERATIONAL MODES
LE
On receipt of a power on reset or a stopped or no field condition
Channel1 Signal Tracker1(S/H_max) Tracker1(S/H_min)

the sample hold circuits in each tracker channel reset to their

maximum and minimum voltages. They then track inwards until
SO
the Hall signal is acquired.
Tracker1(S/H_max) increments to the most positive Hall signal
peak, Tracker1(S/H_min) decrements to negative peaks.
Four zero crossing events are required to ensure Hall signal
peak acquisition. No output edges are enabled during this time.
On the third zero crossing after the reset condition the acquire
mode of operation is disabled. The DAC signals are then coinci-
dent with the peak values of the Hall signal. After the four zero
B

crossing delay, the converters enter dither mode. This mode of 0

operation keeps the DAC voltages at the peaks of the Hall signal Figure 3. Signal Acquisition From a Power On or Stopped (no
and maintains a valid zero crossing in the presence of run out field) Condition
and offset drift.
O

PWM AND OUTPUT STAGE

The pulse width modulator is the final stage of the signal condi-
tioning. Its primary function is to convert the Hall signal infor-
mation of zero crossings, signal amplitude, and direction, into a
single bit pulse width modulated signal.
The leading edge of the pulse is determined by a zero crossing
event from Tracker 1. The duration of the pulse is determined
by direction and signal amplitude. (See Fig. 4 and 5)
All events within the signal conditioning are synchronized to
the internal clock. Asynchronous zero cross events are aligned
to the following clock edge which results in a maximum delay
of 1. 4 us. Output pulse widths are modulated by means of a 19
bit counter. The counter functions both as a pulse width modula-

-4- REV. PrA

 PRELIMINARY TECHNICAL DATA

AD22157

Chan. A

Pulse Width Modulated

Output

80 mT

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Diff B Field

NORMAL
Noise Floor
1.5 mT

0
LE LOW

Air Gap
Air Gap Limit

Figure 4. AD22157 Output Signal / Field Relationship

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Normal Field

Left 90uS

Right 180uS

Low Field
B

Left 360uS

Right 720uS
O

Air Gap Limit 45uS (independent of direction)

Tooth
Notch Notch
Note: Wheel stopped or no field pulse width: 1440uS / pulse period: 737mS

Figure 5. AD22157 PWM vs. Field and Direction of Wheel Rotation

REV. PrA -5-

 PRELIMINARY TECHNICAL DATA

AD22157

Sensor Circuit

NC
ECU
AD22157 NC

Vlow

Vhigh
4.7nF
NC

75Ω Cecu

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Vbat_high
(Battery Voltage) 27V

Vbat_low

Supply to additional sensors

LE
Figure 6. Typical Connection in a Wheel Speed Sensing Application
A typical automotive application is shown above. The digital output signal is developed across a 75
SO
ohm resistor. The power supply to the device is conditioned in an ECU to limit “load dump” pulses to
27v.

Front of AD22157
B

faces the wheel

Magnet placed
against the back side
O

of the AD22157
5
4
3
2
1

Right: Pin 5 to Pin 1 Left: Pin 1 to Pin 5

Magnet Pin 1

Wheel Front View

Package Rear View

Figure 7. Wheel Direction Diagram

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 PRELIMINARY TECHNICAL DATA

AD22157

Outline Dimensions
Dimensions shown in mm and (inches)

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LE
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B
O

REV. PrA -7-