AD AD22157

PRELIMINARY TECHNICAL DATA
Wheel Speed Sensor For
ABS Systems
Preliminary Technical Data
AD22157
Functional Block Diagram
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)
Tracker 1
In Amps
Hall Array
APPLICATIONS
Automotive
Wheel speed and direction sensing
Transmission speed sensing
Industrial
Incremental position sensing
Proximity switching
Vhigh
PWM
The AD22157 is a mixed signal magnetic field transducer
designed for applications where both speed sensing and direction 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 functional 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 conformance with industry standards currently being promoted by leading systems manufacturers.
Pulse widths corresponding to differential magnetic fields measured 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.
7mA
Tracker 2
AD22157
GENERAL DESCRIPTION
7mA
Vlow
associated with Hall cell characteristics. The sensor is compensated 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 differential zero crossing detector which accurately determines the position 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 condition 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.
A fail safe stop signal repeating at approximately 1.5Hz is provided 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
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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
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PRELIMINARY TECHNICAL DATA
Preliminary AD22157 - Specifications (TA=+25C and V+=12V unless otherwise noted)
Parameters
Min
Typ
Max
Units
-40
150
C
4.5
20.0
27.0
-30.0
V
mA
OPERATING TEMPERATURE
POWER SUPPLY
Vcc Operating
Vcc max transient
Reverse Supply
SUPPLY CURRENT
Iout low
Iout high (pulsed)
5.5
11.0
7.0
14.0
8.5
17.0
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
PULSE PERIOD IN STOPPED MODE
589
737
884
THRESHOLDS FOR OPERATION MODES
Nominal field threshold
Low field threshold
Field too low threshold
uS
mS
4<Bth
2<Bth<4
Bth<2
mT
POWER ON TIME
4
mS
CALIBRATION CYCLE
4
edges
CALIBRATION UPDATE CYCLE
each edge
2
TIMING ACCURACY2
%
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.
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
Pin Configuration
*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 specification is not implied. Exposure to absolute maximum rating conditions for extended
periods may affect device reliability.
5
NC
4
3
NC
Vlow
2
Vhigh
1
NC
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
current. The three Hall effect sensors are connected to instrumentation 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.
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 position of edge transitions on the wheel.
Pitch matching the Hall array to the wheel results in an approximately sinusoidal field variation being sensed by the spatial differential array.
SIGNAL DETECTION
SOURCES OF ERROR PRIOR TO SIGNAL CONDITIONING
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 signals at a tooth/ notch pitch of 5mm.
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.
The Hall sensors generate a number of error components in addition to the desired spatial differential signal:
Each of the three Hall devices is constructed of four individual
plates of 200um diameter connected in parallel and spatially orientated in each of four cross quadrature positions in order to relieve
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 approximately +450 ppm/ C........(+/-150 ppm / C).
Hall Array A
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 temperature by tens of millivolts in either direction.
Hall Array C
Hall Array B
200um dia.
1.25mm
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.
1.25mm
SIGNAL CONDITIONING
Cross Quad Hall Cell
Array
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 determined by the rotational speed of the target wheel. The phase relationship of the quadrature signal is used to determine the direction
of wheel rotation.
Figure 1. AD22157 Hall Array Spacing
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
Hall signal A
Hall signal B
Two separate measurement channels are used for signal conditioning. The first channel circuitry (Tracker1) is used to determine the
zero crossing information and is the primary source of edge information. The second channel (Tracker2) is used only for obtaining
direction information by comparison of signal phase. Each channel comprises two infinite sample hold circuits built around ten bit
tracking analog to digital convertors.
Hall signal C
1.414 *Hall signal
Channel 1 signal A-B
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 represent the positive and negative peak values of the signal at any
given time.
Channel 2 signal l B-C
Figure 2. Quadrature Fields Sensed By Hall Array
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PRELIMINARY TECHNICAL DATA
AD22157
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.
tor and watchdog timer.
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).
ii. The leading edge of the pulse is output after a delay of 45uS.
The timing sequence is as follows:
i. The counter is reset upon receipt of a zero crossing event.
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 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.
The purpose of resetting the trackers is to enable the offset correction 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 approximately 7 mA and 14 mA. The lower current value being the quiescent or logic low state.
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.
Channel1 Signal
Tracker1(S/H_max)
Tracker1(S/H_min)
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 coincident 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.
0
Figure 3. Signal Acquisition From a Power On or Stopped (no
field) Condition
PWM AND OUTPUT STAGE
The pulse width modulator is the final stage of the signal conditioning. Its primary function is to convert the Hall signal information 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-
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PRELIMINARY TECHNICAL DATA
AD22157
Chan. A
Pulse Width Modulated
Output
80 mT
Diff B Field
NORMAL
Noise Floor
Air Gap Limit
LOW
1.5 mT
0
Air Gap
Figure 4. AD22157 Output Signal / Field Relationship
Normal Field
Left 90uS
Right 180uS
Low Field
Left 360uS
Right 720uS
Air Gap Limit 45uS (independent of direction)
Notch
Tooth
Notch
Note: Wheel stopped or no field pulse width: 1440uS / pulse period: 737mS
Figure 5. AD22157 PWM vs. Field and Direction of Wheel Rotation
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PRELIMINARY TECHNICAL DATA
AD22157
Sensor Circuit
ECU
AD22157
NC
NC
Vlow
Vhigh
4.7nF
NC
75Ω
Cecu
Vbat_high
27V
(Battery Voltage)
Vbat_low
Supply to additional sensors
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
ohm resistor. The power supply to the device is conditioned in an ECU to limit “load dump” pulses to
27v.
Front of AD22157
faces the wheel
Magnet placed
against the back side
of the AD22157
1
2
3
4
5
Left: Pin 1 to Pin 5
Right: Pin 5 to Pin 1
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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