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GENERAL DESCRIPTION
The AD22157 is a mixed signal magnetic field transducer
designed for applications where both speed sensing and direc-
tion sensing of a ferrous target wheel are required over a wide
speed range.
The device operates from a 2 wire high compliance current loop
and is suitable for continuous operation from -40C to 150C with
supplies up to +20 Vdc. The sensor is designed to remain func-
tional during voltage transients up to +27V.
The sensor output format is a current pulse from 7mA to 14mA
(the quiescent bias is 7mA) whose rising edge is accurately
placed relative to the edges of the target wheel. The pulse width
is determined by both target wheel direction and field strength.
The output pulse is coded in multiples of a well defined time
interval depending on direction and field strength in conform-
ance with industry standards currently being promoted by lead-
ing systems manufacturers.
Pulse widths corresponding to differential magnetic fields mea-
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
associated with Hall cell characteristics. The sensor is compen-
sated to work optimally with SmCo magnets. The architecture
maximizes the advantages of fine line CMOS and high voltage
DMOS allowing the device to operate accurately in demanding
environments.
Principle features of the AD22157 include an adaptive differen-
tial zero crossing detector which accurately determines the posi-
tion of target wheel edges. This architecture eliminates the
effects of package and thermal stress on the Hall sensor array
resulting in 2% repeatability of the time interval from rising
edge to rising edge of the sensor output.
The sensor takes 4 edges from either power on or a stopped con-
dition to achieve full accuracy. The architecture employs digital
signal processing to provide robust functionality and eliminate
spurious or missing pulses under extreme conditions of EMC.
The AD22157 is housed in a 5 lead single-in-line (SIP) package
suitable for mounting with a back biasing magnet in a typical
wheel speed sensor assembly.
Wheel Speed Sensor For
ABS Systems
Preliminary Technical Data AD22157
FEATURES
Speed and direction from 0Hz to 2500Hz
Air gap diagnostics
2-wire current-loop operation
Wide Operating Temperature Range
Functional during temperature excursions to 190C
Reverse Supply Protected (-30V)
APPLICATIONS
Automotive
Wheel speed and direction sensing
Transmission speed sensing
Industrial
Incremental position sensing
Proximity switching
Information furnished by Analog Devices is believed to be accurate and reliable.
However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties which may result from its use.
No license is granted by implication or otherwise under any patent or patent rights of
Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA02062-9106,USA
www.analog.com
Analog Devices, Inc.,2001
Functional Block Diagram
Tracker 1
Tracker 2
PWM 7mA 7mA
Vhigh
Vlow
In Amps
Hall Array
AD22157
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PRELIMINARY TECHNICAL DATA
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Preliminary AD22157 - Specifications (TA=+25C and V+=12V unless otherwise noted)
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.
Parameters Min Typ Max Units
OPERATING TEMPERATURE
-40 150 C
POWER SUPPLY
Vcc Operating
Vcc max transient
Reverse Supply
4.5 20.0
27.0
-30.0
V
SUPPLY CURRENT
Iout low
Iout high (pulsed)
5.5
11.0
7.0
14.0
8.5
17.0
mA
OUTPUT CURENT RATIO 1.8 2 2.2
OUTPUT CURRENT PULSE WIDTH1
Nominal field operation (left/right)
Low field operation (left/right)
Air gap limit
No field or stopped for >737mS
72/144
288/576
36
1152
90/180
360/720
45
1440
108/216
432/664
54
1728
uS
PULSE PERIOD IN STOPPED MODE 589 737 884 mS
THRESHOLDS FOR OPERATION MODES
Nominal field threshold
Low field threshold
Field too low threshold
4<Bth
2<Bth<4
Bth<2
mT
POWER ON TIME 4 mS
CALIBRATION CYCLE 4 edges
CALIBRATION UPDATE CYCLE each edge
TIMING ACCURACY22%
ABSOLUTE MAXIMUM RATINGS*
Maximum Supply Voltage . . . . . . . . . . . . . . . . . . . . . +27 V
Maximum Output Current (Pin 2) . . . . . . . . . . . . . . 18 mA
Operating Temperature Range . . . . . . . . . –40°C to +150°C
Die Junction Temperature . . . . . . . . . . . . . . . . . . . .+190°C
Storage Temperature Range . . . . . . . . . . –65°C to +160°C
Lead Temperature (Soldering, 10 sec) . . . . . . . . . . +300°C
*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
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
1
2
3
4
5
Pin Configuration
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.
NC
NC
NC
Vlow
Vhigh
PRELIMINARY TECHNICAL DATA
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REV. PrA
CIRCUIT OPERATION
The AD22157 is a two wire current modulating transducer which
generates current pulses in response to spacial differential changes
in a magnetic field. A typical application is wheel speed sensing
where the field to be sensed is generated by the interaction of a
permanent magnet behind the sensor and a notched or hole
stamped ferromagnetic target wheel in front of the sensor. Under
these conditions the sensor must reject that portion of the ‘bias’
field which is constant, and amplify the remaining differentially
modulated portion of the field and determine accurately the posi-
tion of edge transitions on the wheel.
SIGNAL DETECTION
The bias field rejection is accomplished by a spacial differential
measurement of the field using integrated Hall plate structures
within the silicon substrate. A linear array of three Hall cells is
used. The AD22157 is designed to give optimum quadrature sig-
nals at a tooth/ notch pitch of 5mm.
Each of the three Hall devices is constructed of four individual
plates of 200um diameter connected in parallel and spatially orien-
tated in each of four cross quadrature positions in order to relieve
process gradient induced offsets in the Hall signal voltage.
The Hall plate arrays are biased by three matched current sources.
The sensitivity of the plates to magnetic field is 5uV / Gauss at this
current. The three Hall effect sensors are connected to instrumen-
tation amplifiers as two pairs with the center plate shared between
the two amplifiers. In this configuration two spacial differential
magnetic signals are transformed into electrical signals whose
peak to peak amplitude is directly proportional to the differential
magnetic field component and the Hall plate bias current.
Pitch matching the Hall array to the wheel results in an approxi-
mately sinusoidal field variation being sensed by the spatial differ-
ential array.
SOURCES OF ERROR PRIOR TO SIGNAL CONDITIONING
The Hall sensors generate a number of error components in addi-
tion to the desired spatial differential signal:
Uncompensated magnetic bias field due to mismatch of Hall
plate sensitivities, Hall bias current mismatch and variations in
magnetic flux density across the surface of the bias magnet.
Intrinsic Hall plate offset due to lithographic misalignment of
Hall plate contacts, local planar variations in Hall plate diffusion
due to manufacturing tolerances and mechanical stress imposed by
encapsulation.
Temperature dependent sensitivity of the Hall cells is approxi-
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-
perature by tens of millivolts in either direction.
From a circuit perspective, the amplifiers will contribute further
input referred offset to the signals. This component is less than 1
mV and typically is of the order of several hundred micro volts.
SIGNAL CONDITIONING
The primary function of the signal conditioning is to compensate
for offset errors and accurately determine the zero crossings of the
differential Hall cell signal component. The differential signals
approximate quadrature sine waves whose frequency is deter-
mined by the rotational speed of the target wheel. The phase rela-
tionship of the quadrature signal is used to determine the direction
of wheel rotation.
Two separate measurement channels are used for signal condition-
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
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
hold circuits per Tracker. One sample hold circuit follows positive
peaks, the other negative peaks. The potentials of the DAC’s rep-
resent the positive and negative peak values of the signal at any
given time.
Figure 1. AD22157 Hall Array Spacing
1.25mm 1.25mm
Hall Array A Hall Array B Hall Array C
Cross Quad Hall Cell
Array
200um dia
.
Hall signal A Hall signal B
Hall signal C
Channel 1 signal A-B Channel 2 signal l B-C
1.414 *Hall signal
Figure 2. Quadrature Fields Sensed By Hall Array
AD22157
PRELIMINARY TECHNICAL DATA
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The mid point of these potentials is used as a reference for a
zero crossing detector in the PWM. This system assures that a
phase jitter specification of +/- 2% for 1kHz signal (rising edge
to rising edge) can be met over all operational conditions.
Tracker1 also makes an absolute measurement of peak to peak
signal. The digital result is a measure of field strength which
can be related directly to air gap or used for diagnostic purposes
in the application. The digital result is combined with direction
information from Tracker 2 and used to program the output
pulse width modulator (PWM).
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
fixed resolution converter may fail to maintain acquisition of
the signal peaks using only single 1lsb steps. To compensate for
this, the resolution of the converters adapts to changes in the
signal that cannot be followed using 1lsb steps.
HALL PLATE BIAS
The Hall cells are biased so that the temperature coefficient of
sensitivity of the AD22157 is of similar magnitude but opposite
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
On receipt of a power on reset or a stopped or no field condition
the sample hold circuits in each tracker channel reset to their
maximum and minimum voltages. They then track inwards until
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
crossing delay, the converters enter dither mode. This mode of
operation keeps the DAC voltages at the peaks of the Hall signal
and maintains a valid zero crossing in the presence of run out
and offset drift.
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-
tor and watchdog timer.
The timing sequence is as follows:
i. The counter is reset upon receipt of a zero crossing event.
ii. The leading edge of the pulse is output after a delay of 45uS.
iii. Amplitude thresholds are decoded with direction and the
appropriate output pulse width is generated.
iv. The counter is reset.
v. If a zero crossing is not received before the counter overflows
(745 uS), a STOP pulse is output.
The purpose of resetting the trackers is to enable the offset cor-
rection circuitry to remain active when no zero crossing events
occur but when thermally induced drift may invalidate the offset
correction over extended periods of inactivity.
The sensor supply loop current is modulated in response to the
pulse input between two discrete current levels of approxi-
mately 7 mA and 14 mA. The lower current value being the qui-
escent or logic low state.
Channel1 Signal Tracker1(S/H_max) Tracker1(S/H_min)
0
Figure 3. Signal Acquisition From a Power On or Stopped (no
field) Condition
AD22157
PRELIMINARY TECHNICAL DATA
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REV. PrA
Air Gap
Diff B Field
NORMAL
LOW Air Gap Limit
0
80 mT
Noise Floor
1.5 mT
Chan. A
Pulse Width Modulated
Output
Figure 4. AD22157 Output Signal / Field Relationship
Normal Field
Low Field
Tooth
Notch Notch
Air Gap Limit 45uS (independent of direction)
Left 90uS
Right 180uS
Left 360uS
Right 720uS
Note: Wheel stopped or no field pulse width: 1440uS / pulse period: 737mS
Figure 5. AD22157 PWM vs. Field and Direction of Wheel Rotation
AD22157
PRELIMINARY TECHNICAL DATA
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A typical automotive application is shown above. The digital output signal is developed across a 75
ohm resistor. The power supply to the device is conditioned in an ECU to limit “load dump” pulses to
27v.
1
2
3
4
5
Package Rear View
Wheel Front View
Pin 1
Magnet
Left: Pin 1 to Pin 5
Right: Pin 5 to Pin 1
Front of AD22157
faces the wheel
Magnet placed
against the back side
of the AD22157
Figure 7. Wheel Direction Diagram
Figure 6. Typical Connection in a Wheel Speed Sensing Application
Cecu
75
Ω
27V
Vbat_high
Vbat_low
(Battery Voltage)
4.7nF
Vlow
Vhigh
NC
AD22157
ECU
Sensor Circuit
Supply to additional sensors
NC
NC
AD22157
PRELIMINARY TECHNICAL DATA
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REV. PrA
Outline Dimensions
Dimensions shown in mm and (inches)
AD22157
PRELIMINARY TECHNICAL DATA
