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AD22157 Datasheet(PDF) 3 Page - Analog Devices |
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AD22157 Datasheet(HTML) 3 Page - Analog Devices |
3 / 7 page -3- 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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