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AD22157

Wheel Speed Sensor For ABS Systems

Analog Devices 

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

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-

Pitch matching the Hall array to the wheel results in an approxi-

mately sinusoidal field variation being sensed by the spatial differ-

ential array.

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

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

E plates of 200um diameter connected in parallel and spatially orien-

tated in each of four cross quadrature positions in order to relieve

T Hall Array A

Hall Array B

Hall Array C

LE 1.25mm

200um dia.

1.25mm

O Figure 1. AD22157 Hall Array Spacing

Cross Quad Hall Cell

Array

S 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

B Hall signal A

Hall signal B

Hall signal C

O 1.414 *Hall signal

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.

Channel 1 signal A-B

Channel 2 signal l B-C

Figure 2. Quadrature Fields Sensed By Hall Array

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.

REV. PrA

-3-

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