DETECTION OF PASSING MAGNETIC ARTICLES WHILE ADAPTING THE DETECTION THRESHOLD

Abstract: A method for detection of passing magnetic articles which includes an initial step of sensing an ambient magnetic field and generating a voltage, Vsig, proportional to the magnetic field. A threshold voltage is generated as a percentage of the peak-to-peak voltage of Vsig. The method further includes the step of generating a detector output voltage that becomes one binary level when Vsig rises to exceed the threshold voltage and another binary level when Vsig falls to below the threshold voltage. The threshold voltage is updated to track the positive and negative peaks of the Vsig voltage upon each transition of the Vout voltage.

Description: DETECTION OF PASSING MAGNETIC ARTICLES WHILE ADAPTING THE DETECTION THRESHOLD BACKGROUND OF THE INVENTION This invention relates to a proximity detector including a Hall-voltage peak-to- peak percentage threshold detector, and especially to a ferrous-gear-tooth Hall-transducer, or other magnetic-field-to-voltage transducer, capable of detecting the leading and trailing gear tooth edges of an adjacent rotating ferrous gear, or other magnetic articles, and more particularly relates to such a Hall sensor with detection thresholds that adapt to the peak to peak amplitude of the Hall voltage. The term "magnetic" as used herein applies to magnetized bodies, ferrous bodies and other bodies having a low magnetic reluctance that tend to alter the ambient magnetic field. In the patent U.S. 5,442,283, issued August 15, 1995 there is described a proximity detector including a Hall-voltage slope-activated, or peak-referenced detector capable of detecting the rising and falling edges of a gear tooth. The detector includes a circuit for tracking a slope of a Hall voltage and briefly holding the ensuing peak voltage before producing a pulse signal indicating the onset of the following Hall-voltage slope of opposite direction. The Hall voltage holding circuit includes a capacitor and circuit means for controllably leaking charge out of or into the capacitor for preventing false tripping of a comparator that provides a pulse output signal. The holding voltage of the capacitor thus has a droop which leads to increasing loss of holding accuracy as the speed of gear tooth passage becomes slower, and therefore the detector has a minimum gear tooth speed at which accurate detection is possible. The changes in the ambient magnetic field and corresponding changes in the transducer voltage caused by the passing of magnetic articles tend to vary. Most such proximity detectors of the prior art produce a high binary output voltage indicating proximity of a passing article, and produce a low binary voltage when the article recedes from the detector. The transition in detector output voltage from low to high typically is triggered by a comparator that determines when the transducer voltage rises to equal a fixed internal threshold voltage reference, or in the case of the above described slope-activated, or peak- referenced detector, determines when a transducer voltage peak has just occurred and the signal voltage drops a predetermined incremental voltage from the peak value. These prior art proximity detectors, having fixed threshold voltages, produce a low to high (or high to low) output voltage that corresponds to different locations in the transducer voltage waveform when there are changes in the amplitude of the transducer voltage. The sources of such changes in transducer voltage amplitude are many. For example, gear teeth (articles) may have different ferro- magnetic properties from tooth to tooth and undulating changes in the spacings (air gap) gear teeth to transducer caused by eccentricity of the gear. Also, changes in temperature can cause changes in air gap dimensions and in the sensitivity of the transducer and transducer- voltage amplifier. Furthermore, the magnetic-fϊeld-to voltage transducer in a proximity detector typically includes an internal DC offset voltage that varies with mechanical stresses and temperature. Such changes in the transducer voltage therefore cause shifts in the timing of proximity detection relative to the actual distances of article approach and receding at which these transducer voltages exceed or fall below the fixed thresholds. This results in loss of accuracy in proximity detection that has become less and less tolerable especially when employed for detection of the rotational position of a gear by sensing the proximity of the gear teeth. It is an object of this invention to provide a proximity detector that generates a binary output voltage wherein the transitions accurately correspond to a definite point of approach and a definite point of receding of a passing magnetic article. It is a further object of this invention to provide a magnetic article proximity detector that periodically determines when the amplitude or offset of the magnetic-field- to- voltage transducer voltage has changed significantly, and adjusts the detection threshold as needed to be essentially a predetermined constant percentage of the peak to peak value of a changing detector-transducer- voltage amplitude. BRIEF SUMMARY OF THE INVENTION The invention relates to a method for detecting passing magnetic articles which includes an initial step of sensing an ambient magnetic field and generating a voltage, Vsig, proportional to the magnetic field. A threshold voltage is generated as a percentage of the peak-to-peak voltage of Vsig. The method further includes the step of generating a detector output voltage that becomes a first binary level when Vsig rises to exceed the threshold voltage and a second binary level when Vsig falls to below the threshold voltage. More particularly, a PDAC voltage is generated as a function of the positive peak values of Vsig and an NDAC voltage is generated as a function of the negative peak values of Vsig. The threshold voltage is updated by a predetermined amount upon each transition of the detector output voltage and is further updated to track the positive and negative peaks of the Vsig voltage. With this arrangement, a relatively simple and robust circuit and technique are provided for updating the PDAC and NDAC voltages to ensure that the threshold voltage remains, within a predetermined tolerance, a percentage of the peak-to-peak Vsig voltage. This method further provides additional hysteresis which serves to reduce the susceptibiltiy of the circuit to noise on the Vsig signal and which is introduced at a time when the switch points defining transitions of the detector output voltage are not affected. Further, the method has a relatively fast response time, since complex threshold voltage updating decisions are eliminated by updating the threshold voltage by a predetermined amount after transitions of the detector output voltage. Additionally, the simplicity of the threshold voltage updating technique results in simplified circuit testing and thus, a reduction in manufacturing time and cost. The threshold voltage is at a first level corresponding to a first percentage of the peak-to-peak Vsig voltage when Vsig exceeds the threshold voltage and is at a second level corresponding to a second percentage of the peak-to-peak Vsig voltage when Vsig is less than the threshold voltage. With this arrangement, the threshold voltage is provided with hysteresis. More particularly, the first level of the threshold voltage is a first percentage of the voltage PDAC - NDAC and the second level of the threshold voltage is a second percentage of the voltage PDAC - NDAC. The threshold voltage updating step includes decreasing the PDAC voltage by the predetermined amount upon transitions of the detector output signal from one of the binary levels to the other and increasing the NDAC voltage by the predetermined amount upon opposite transitions of the detector output signal. The threshold voltage updating step further includes permitting the PDAC voltage to track the positive peaks of the Vsig signal and permitting the NDAC voltage to track the negative peaks of the Vsig voltage. Also described is a magnetic article detector comprising a magnetic field sensor providing a voltage output signal, Vsig, proportional to the magnetic field, a threshold voltage generator operative to generate a threshold voltage that is a percentage of the peak-to-peak voltage of Vsig and a comparator comparing Vsig to the threshold voltage to generate a detector output voltage. The detector output voltage becomes one binary level when Vsig rises to exceed the threshold voltage and another binary level when Vsig falls to below the threshold voltage. The threshold voltage is updated by a predetermined amount upon each transition of the detector output voltage and is further updated to track the positive and negative peaks of the Vsig voltage. A hysteresis circuit provides the threshold voltage at a first level that is a first percentage of the peak-to-peak voltage of Vsig when Vsig exceeds the threshold voltage and at a second level that is a second percentage of the peak-to-peak voltage of Vsig when Vsig is below the threshold voltage. The threshold voltage generator includes a PDAC voltage generator, an NDAC voltage generator, and a circuit coupled between the PDAC voltage and the NDAC voltage for providing the threshold voltage as a percentage of the difference between the PDAC voltage and the NDAC voltage. The PDAC voltage generator includes a first counter having an output at which a first count signal is provided and a first digital to analog converter having an input coupled to the output of the first counter and an output at which the PDAC voltage is provided. The first counter counts up causing the PDAC voltage to increase when the Vsig voltage is greater than the PDAC voltage, thereby causing the PDAC voltage to track the positive peaks of the Vsig voltage. The first counter counts down for a predetermined duration upon transitions of the detector output voltage of a first polarity, thereby decreasing the PDAC voltage by a predetermined amount. Similarly, the NDAC voltage generator includes a second counter having an output at which a second count signal is provided and a second digital to analog converter having an input coupled to the output of the second counter and an output at which the NDAC voltage is provided. The second counter counts down causing the NDAC voltage to track the negative peaks of the Vsig voltage and counts up for a predetermined duration upon transitions of the detector output voltage of a second polarity, thereby increasing the NDAC voltage by a predetermined amount. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a block diagram of a first magnetic-article proximity detector. Figure 2 shows a waveform of the Hall voltage, Vsig, in the circuit of Figure 1 which waveform corresponds to the ambient magnetic field at the passage of one ferrous gear tooth (or other magnetic article). Figures 2 and 3 are drawn to the same scale. Figure 3 shows the waveform of the output signal, Vout, from the proximity detector of Figure 1, corresponding in time to the Hall voltage waveform of Figure 2. Figure 4 shows a waveform of signal, Vsig, attributable to the passing of a plurality of magnetic articles that generate uneven amplitudes from one to the other in the ambient magnetic field. Shown superimposed on Vsig are the simultaneously generated DAC output voltages Vp new and V Nne w Figure 5 shows a waveform of Vsig during one update interval of 64 pulses in Vsig and during a portion of a succeeding update interval, wherein the peak positive and negative values in Vsig are changing. Superimposed on Vsig are the DAC voltages Vp new and Vnn e w, in the one interval and Vp ne and V ne in the succeeding update interval. Figure 5a shows a waveform of the update signal, Vupdt, drawn to the scale as is Figure 5. Figure 6 shows a few periods of the transducer signal Vsig in which an update interval terminates at tupdate and a successive tupdate interval begins. Figures 7, 8, 9, 10, 11 and 12 show, for the proximity detector of Figure 1, the waveforms respectively of the input signal to latches 42 and 52, the reset signal to counters 17 and 27, Vpcomp, Vncomp, Q33b, and the proximity-detector output signal Vout, all drawn to the time scale corresponding to that of Figure 6. Figure 13 shows a schematic of a second magnetic-article proximity detector. Figure 14 shows a schematic of the update controller of the proximity detector of Figure 13. Figure 15 shows a waveform of Vsig with the VTH threshold voltage, the PDAC voltage and NDAC voltage generated by the detector of Figure 13 superimposed thereon. Figure 16 shows a waveform of Vout generated by the detector of Figure 13 during the same time interval shown in Figure 15. Figure 17 shows a waveform of Vsig during several update time intervals in which the positive peak values in Vsig are changing. Superimposed on Vsig are the PDAC, PDAC - ΔV, NDAC and NDAC + ΔV voltages generated by the detector of Figure 13. Figure 17a shows a waveform of Vsig during several update time intervals in which the negative peak values in Vsig are changing. Superimposed on Vsig are the PDAC, PDAC - ΔV, NDAC and NDAC + ΔV voltages generated by the detector of Figure 13. Figure 18 is a flow diagram illustrating operation of the detector of Figure 13 in updating the PDAC voltage in accordance with certain changes in the positive peak values of Vsig. Figure 19 is a flow diagram illustrating operation of the detector of Figure 13 in updating the NDAC voltage in accordance with certain changes in the negative peak values of Vsig. Figure 20 shows a waveform of Vsig during two update time intervals, with the PDAC, PDAC - ΔV, NDAC and NDAC + ΔV voltages superimposed thereon. Figures 20a, 20b, 20c, 20d, 20e, 20f and 20g show, for the detector of Figure 13, respective waveforms of the cntl28 signal, the Vout signal, the p_ok signal, the upd_pdac signal, the dcrp signal, the p_cnt_up signal and the elk signal, all drawn to the same time scale as Figure 20. Figure 21 shows a schematic of a third magnetic-article proximity detector. Figure 22 shows a schematic of the update controller of the proximity detector of Figure 21. Figure 23 shows a waveform of Vsig during several update time intervals in which the positive peak values in Vsig are changing. Superimposed on Vsig are the PDAC, PDAC - ΔV, NDAC and NDAC + ΔV voltages generated by the detector of Figure 21. Figure 23 a shows a waveform of Vsig during several update time intervals in which the negative peak values in Vsig are changing. Superimposed on Vsig are the PDAC, PDAC - ΔV, NDAC and NDAC + ΔV voltages generated by the detector of Figure 21. Figure 24 is a flow diagram illustrating operation of the detector of Figure 21 in updating the PDAC voltage in accordance with certain changes in the positive peak values of Vsig. Figure 25 is a flow diagram illustrating operation of the detector of Figure 21 in updating the NDAC voltage in accordance with certain changes in the negative peak values of Vsig. Figure 26 shows a waveform of Vsig during three update time intervals, with the PDAC and PDAC - ΔV voltages superimposed thereon. Figures 26a, 26b, 26c, 26d, 26e, 26f, 26g, 26h and 26i show, for the detector of Figure 21, respective waveforms of the pcomp signal, the p latbig signal, the p_cnt_up signal, the p ok signal, the p_latsm signal, the p_update signal, the phold signal, the CLK signal and the cntl28 signal, all drawn to the same time scale as Figure 26. Figure 27 a schematic of a further magnetic-article detector. Figure 28 shows a schematic of the update controller of the detector of Figure 27. Figure 29 shows a waveform of Vsig having changing positive peak values, with the PDAC voltage and the VTH threshold voltage superimposed on Vsig. Figure 29a shows the Vout waveform for the Vsig waveform of Figure 29. Figure 29b is an expanded view of a portion of the Vsig and VTH threshold voltages of Figure 29. Figure 30 shows a waveform of Vsig having changing negative peak values with the NDAC voltage and the VTH threshold voltage superimposed on Vsig. Figure 30a shows the Vout waveform for the Vsig waveform of Figure 30. Figure 31 shows a waveform of Vsig, with the PDAC and NDAC voltages and the VTH threshold voltage superimposed thereon. Figures 31a, 31b, 31c, 31d, 31e, 31f, 31g, 31h, 31i, 3 lj, and 3 Ik show, for the detector of Figure 27, respective waveforms of the Vout signal, the dcrp signal, the CLK signal, the p_cnt_up signal, the count of the positive ring counter, the pdone signal, the dcrn signal, the CLK signal, the n cnt up signal, the cout of the negative ring counter, and the ndone signal, all drawn to the same time scale as Figure 31. Figure 32 shows an illustrative comparator of Figure 27 having an integrator. Figure 33 illustrates the operation of the comparator of Figure 32 in processing an illustrative noisy Vsig signal. Figure 34 is a schematic of a still another magnetic-article detector. Figure 35 is a schematic of the positive portion of the update controller of Figure 34. Figure 36 is a schematic of the negative portion of the update controller of Figure 34. Figure 37 shows state diagrams for the magnetic-article detector of Figure 34. Figure 38 shows an illustrative Vsig signal with the NDAC, PDAC, and VTH voltages generated by the detector of Figure 34. Figures 38a, 38b, 38c, 38d, and 38e show, for the signals of Figure 38, respective waveforms for the Vout signal, the N_HOLD signal, the N JPDN signal, the P HOLD signal, and the P UPDN signal as generated by the detector of Figure 34. Figure 39 shows a further illustrative Vsig signal with the NDAC and PDAC voltages generated by the detector of Figure 34. Figure 39a shows, for the signals of Figure 39, the Vout signal provided by the detector of Figure 34. DETAILED DESCRIPTION OF THE INVENTION The Hall element 10 of Figure 1 is energized by a current I H and has an output connected to the input of a Hall voltage amplifier 12. Hall element 10 may be mounted at a pole of a magnet (not shown), so that when a ferrous article approaches, the Hall voltage V H and the amplified Hall voltage Vsig increase (or decrease); and when the article recedes, V H and Vsig decrease (or increase depending on the polarity of the magnet pole). Alternatively, the sensor circuit of Figure 1 may be used to detect magnetic articles that themselves are magnetized, in which case the Hall element need not be mounted adjacent a magnet. A magneto resistors bridge (not shown) may be substituted for the Hall element. And two Hall elements with their outputs connected differentially to the input of the Hall voltage amplifier (not shown) represents a second alternative magnetic-field-to-voltage transducer. The amplified Hall voltage Vsig is manipulated by the remaining circuits in the proximity detector of Figure 1 to produce a binary square wave output signal, Vout, that like a shadow graph reflects the profile of the passing articles. Referring to Figure 1, the amplified Hall voltage Vsig is applied to the positive input of a first comparator 14 and is also applied to the positive input of one second comparator 16 and to the negative input of another second comparator 26 which generate Vpcomp and Vncomp respectively (Figures 9 and 10). A composite latch, composed of the invertors 32a and 32b and the clocked flip-flops 33a and 33b for producing an intermediate signal Q33b (Figure 11) and proximity-detector output voltage Vout (Figure 12). The circuitry including comparators 16 and 26, invertors 32a and 32b, and flip flops 33a and 33b comprise a peak-to-peak percentage threshold detector in which the comparator hysteresis circuitry biases the center of the comparator hysteresis loop to a level corresponding to a predetermined percentage of the peak-to-peak transducer signal. Referring to Figures 2 and 3, and also to Figures 6 and 9, during each positive going portion of Vsig, the voltage Vsig falls from the positive peak and at time ti goes below the threshold reference voltage Vp th , produced at a tap in the resistive voltage divider 34. At times ti, the output V pcomp of comparator 16 goes from a binary high to a low level as in Figure 9, and Vout goes from a high to a low level as seen in Figure 12. During each negative going portion of Vsig, the voltage Vsig rises (or falls away) from the negative peak and at time t 2 goes above the threshold reference voltage V H I , produced at low tap in the resistive voltage divider 34. At times t 2 , the output V nCo mp of comparator 16 goes from a binary low to a high as in Figure 10, and Vout goes from a low to a high level as seen in Figure 12. Assuming, as a starting point, that the counter 17 is at zero count, when the output of the first comparator 14 goes high the counter 17 begins counting the elk pulses from elk 18. The resulting count is presented to the digital-to-analog converter (PDAC1) 20 which produces an output analog voltage Vp new always lying somewhere within the range from zero to the DC supply voltage, +Vreg. At any instant the amplitude of Vp new is a direct linear function of the count signal from counter 17. When power is first applied to the detector circuit, a logic block (not shown) senses the time of turning on of the DC supply voltage, +Vreg, and resets the counters to zero count. The comparator 14 has hysteresis and so is a Schmitt type comparator. The output of the DAC 20 (PDAC1) is connected to the negative input of the comparator 14 so that whenever Vsig becomes greater than voltage Vp ne plus the small hysteresis threshold voltage V h y s i of the comparator 14, then the comparator 14 output goes high. If at that time Vout is low, then the output of AND gate 15 goes high and the counter 17 is enabled and counting. When Vsig is growing more positive, Vp ne is caused to track Vsig in stair step fashion, as is illustrated in Figure 4. The incremental vertical excursions of the stair stepped Vp new are equal to Vreg/2 n , where n is the number of DAC bits. The incremental horizontal times, Δtl, increase as the slope of Vsig decreases. As is illustrated in Figure 4, when a peak positive voltage of Vsig is reached, the counter 17 stops counting (e.g. at time t pp ι) and Vp new holds this peak voltage until at a subsequent positive pulse in the signal Vsig that is greater than the held voltage Vp new , Vpnew again begins to track the subsequent positive pulse to its peak and to hold (e.g. at time t pp2 ) that new peak voltage. An update counter 36 is a six bit counter which counts the low-to-high transitions, i.e. positive transitions, in the proximity-detector output signal Vout. When at time t update , 64 positive binary transitions have been counted, counter 36 wraps and starts again from zero count to count the ensuing positive transitions. Referring to Figures 5, 5a, 6, 7 and 8, at t up date the output of the update counter 36 goes high and latches 42 and 52 are enabled and then disabled before the counter 17 (and counter 27) is (are) reset via the delay circuit 39. Thus at the end of each update time interval of 64 counts, the PDAC1 output voltage V new holds the positive peak voltage of the highest positive excursion that has occurred in the signal Vsig during that update time interval. The most recent maximum peak positive voltage in Vsig is accordingly updated at the end of each update time interval of a predetermined number of excursions of one polarity in Vsig. The predetermined (count) number in this example was set at 64 positive excursion peaks and the counter 36 is accordingly a six bit counter, but this number is not critical. At the end of each count 64, the output of the update counter 36 goes high to enable the update AND gate 38. If the update signal, Vupdt, is simultaneously high, the output of the update AND gate 38 goes high and enables the latch 42. Latch 42 holds the count in counter 17 during the following update time interval (of 64 pulses in Vsig). That count is applied to the input of PDAC2 44 during that update time interval. At the end of the first update time interval following detector startup, PDAC2 44 generates at its output an analog signal Vp 0 i d that is equal to the initial amplitude of Vsig at the beginning of the first update time interval (left-most update time interval as seen in Figure 5). A window comparator 46 has a reference-voltage input connected to the output of PDAC1 20. The output signal v Po i d from PDAC2 is applied via a unity gain buffer stage 48 to one end of the resistor voltage divider 34. And from there the signal Vp 0 i d is applied to the plus and minus inputs of the window comparator 46 via respectively the additive and subtractive DC bias voltages Δv. Referring to Figure 5, at the end of an update time interval, time t up date, the output of window comparator 46 goes high only if Vp ne l es outside of the range from V Po i d +Δ and Vp o i d -Δ. If at the end of an update interval, Vp ne is higher than or lower than this range then the output of the window comparator goes high causing the output of OR gate 50 to go high. This in turn causes the output of AND gate 38 to go high, and causes latch 42 to latch in the current count in counter 17. As is illustrated in Figure 5, this updates Vp 0 i , namely PDAC2 44 generates at its output an analog signal Vp 0 i d that is equal to the initial amplitude of the held Vp new generated during the preceding update time interval. All update time intervals following the first update time interval begin with a decision as to whether to update V xo id to the last held value of V xnew of the preceding time interval as illustrated in the second time interval of Figure 5. If the current value of V new , namely the maximum peak positive peak value of Vsig during an entire update time interval, is not outside of and lies within the range from Vp o i d +Δ and Vp 0 i d -Δ at the end of that update time interval, then the output of comparator 46 remains low and latch 42 will not be enabled. A lower circuit portion in the proximity detector of Figure 1 essentially mirrors the construction of the upper portion just described. The lower circuit portion manipulates the negative pulses in Vsig in the same way as does the upper portion with respect to positive pulses in Vsig. Except for the holding of negative peaks in Vsig by NDAC1 30, latch 52 and NDAC2, there is no action going on in the lower circuit portion during positive going portions of Vsig. For example, the output of comparators 24 and 26 go high only when Vsig goes negative. Thus only when Vsig is going negative are there changes of state in the signals of AND gate 25, counter 27, NDAC1 30, latch 52, NDAC2 54, buffer 58 and window comparator 56. The upper (P) and lower (N) portions of the circuit share the elk 18, the reset delay circuit 39 and the OR gate 50. It should be noted that the DC reference voltages +Vreg and ground are connected to NDAC1 30 and NDAC2 54 inversely with respect to those connections to PDAC1 20 and PDAC2 44. Thus instead of the output voltage going up as the count increases (in counter 17) to the PDACs, the output voltage of the NDACs goes down as the count (in counter 27) goes up. Alternatively, the NDACS could have been connected to the DC reference voltages as are the PDACs if the counter 27 had been of the kind that counts down from maximum count. Also, the counters 17 and 27 are of the kind that include an anti-overflow feature that prevents wrapping of the count when maximum count is exceeded, whereas counter 36 is of the simple kind that does wrap. The resistive voltage divider 34 is a component that bridges the upper and lower circuit portions. The ends of voltage divider 34 are connected respectively to the outputs of buffer stages 48 and 58. The held signal Vp 0 i d is applied to the upper end and the held signal V NO M is applied to the lower end of voltage divider 34. At the resistance center (from which there are equal resistances to the two ends) a voltage Vos is generated which is equal to the center voltage between the held positive peak voltage (of Vsig) Vp 0 i and the held negative peak voltage V NO M- The threshold voltages V Λ and VPNΛ are shown respectively at about a third the way up and two thirds the way up the voltage divider 34. It will now be appreciated that these threshold voltages V N H I and V Ptn have been adjusted after every update time interval to remain at a fixed percentage of the peak to peak voltage of Vsig even when the peak voltages of Vsig vary and/or when the offset voltages included in Vsig vary. Voltage divider 34 is made up of six equal resistance resistors. The voltage Vos is 50% of the voltage Vreg. Vp^ and V NΛ are preferably set at about 67% and 33% of Vreg, and generally in the steepest portions of Vsig near Vos. In general, Vp th may be taken at the center point in the voltage divider, or may be at a higher point between 50% and 100%) of the difference voltage Vp new -V ne - Likewise, V N Λ may be at the center point in the voltage divider, namely at 50% of Vp new - V ne w, or may be at a lower point between 50%) and 0% of the difference voltage Vp ne - VNnew- The thresholds Vp t h and VN U I may be other than equal magnitudes from center voltage Vos, i.e. non symmetrical. In any case, these threshold values vary with time and are always a fixed percentage of the current (updated) peak to peak difference voltage in the signal (Vsig). This has the great advantage when operating in peak-to-peak percentage threshold mode as described above, that the proximity sensor provides a binary output voltage wherein the transitions correspond more accurately to a definite point of approach and point of receding of a passing magnetic article. Referring to Figure 13, an alternate embodiment of a magnetic-article proximity detector is shown, in which like reference numbers refer to like elements. The detector includes the Hall element 10 providing a Hall voltage to the amplifier 12 which provides the amplified Hall voltage, Vsig, as described above in conjunction with Figure 1. Like the detector of Figure 1, the detector of Figure 13 is operative to produce a binary square- wave output signal, Vout, at a first binary level when Vsig rises to exceed a threshold voltage and another binary level when Vsig falls to below the threshold voltage. In the embodiment of Figure 1, the threshold voltage is comprised of the individual Vp tn and V N Λ voltages whereas, in the embodiment of Figure 13, the single threshold voltage, VTH, is provided with hysteresis. The embodiments of Figures 1 and 13 are further similar in that, in both cases, the threshold voltage is a percentage of the peak-to-peak voltage of Vsig and is updated to remain the percentage, within a predetermined tolerance, of the Vsig voltage. Accordingly, both such detectors can be characterized as peak-to-peak percentage threshold detectors. In the embodiment of Figure 1, the V th threshold voltage is a first percentage of Vsig and the V N Λ threshold voltage is a second percentage of Vsig. In the embodiment of Figure 13 on the other hand, the VTH threshold voltage is at a first level corresponding to a first percentage of Vsig when Vsig exceeds the threshold voltage and is at a second level corresponding to a second percentage of Vsig when Vsig is less than the threshold voltage (i.e., the threshold voltage VTH is provided with hysteresis). It will be appreciated by those of ordinary skill in the art, and is within the scope of this invention, that the detectors described herein may be implemented in the form of, or to include, mid-signal detectors in which the threshold voltage is a fixed percentage of the Vsig voltage or peak-referenced (i.e., slope-activated detectors). As one example, the peak-to-peak percentage threshold detectors may include a peak-referenced detector such that the detector output signal Vout becomes one binary level when Vsig rises to exceed a held negative peak excursion by a predetermined amount and another binary level when Vsig falls to below a held positive peak excursion by a predetermined amount. The Vsig voltage is applied to the positive input of a first comparator 100 and to the negative input of a second comparator 104. The output signals of comparators 100 and 104 are coupled to an input of respective NOR gates 108 and 112. The second input of NOR gates 108 and 112 receives a respective control signal from an update controller 120, described further below in conjunction with Figure 14. Specifically, NOR gate 108 has a p_cnt_up_n signal applied to its second input and NOR gate 112 has an n cnt up n signal applied to its second input, as shown. The output of NOR gate 108 is applied to a HOLD input of an up/down counter 114. The counter output is held constant (i.e., the counter is disabled) when the HOLD input signal is at a first logic level and is released (i.e., the counter is enabled) when the HOLD input signal is at the second logic level. In the illustrative embodiment, the counter 114 is a six bit counter which is enabled when the HOLD input is low. A control signal, p cnt up from the update controller 120 (Figure 14) is applied to an UPDN input of the counter 114 in order to control the count direction. As will become apparent, the p_cnt_up signal normally causes the counter 114 to count up. Under certain conditions however, the p_cnt_up signal causes the counter 114 to count down for a single clock cycle. The counter 114 is clocked by a system clock signal, elk, and is further responsive to a pndac_resn signal which resets the counter 114 upon start up of the detector. The outputs of the counter 114 are coupled to inputs of a Positive Digital -to- Analog Converter (PDAC) 118. The output of the PDAC 118 provides a voltage, PDAC, which is used to generate the detector threshold voltage, VTH, as will be described. In operation, the PDAC voltage varies in accordance with certain variations in the positive peak values of the Vsig voltage. The resolution of the PDAC 118 is selected to ensure that changes in the VTH threshold voltage caused by changes in the PDAC voltage are substantially imperceptible. In the illustrative embodiment, the resolution of the PDAC 118 is 50mV. The PDAC voltage is coupled to a buffer 124 and is fed back to the negative input of comparator 100, as shown. The comparator 100, NOR gate 108, counter 114, PDAC 118 and buffer 124 comprise a "positive portion" of the detector circuitry. A "negative portion" of the detector is similarly arranged. Specifically, the output of the NOR gate 112 is coupled to a HOLD input of an up/down counter 130. The counter 130 is further responsive to the elk clock signal, the pndac resn reset signal, and an n cnt up control signal provided by the update controller 120 (Figure 14) which controls the count direction. The outputs of the up/down counter 130 are coupled to inputs of a Negative Digital-to- Analog Converter (NDAC) 134 which produces an NDAC voltage used, along with the PDAC voltage, to generate the VTH threshold voltage. The NDAC voltage varies in accordance with certain variations in the negative peak values of the Vsig voltage. Like the PDAC 118, the resolution of the NDAC 134 is selected to ensure that changes in the VTH threshold voltage caused by changes in the NDAC voltage are substantially imperceptible. In the illustrative embodiment, the resolution of the NDAC 134 is 50mV. The NDAC voltage is coupled to a buffer 136 and is further fed back to the negative input of comparator 104, as shown. The buffered PDAC and NDAC voltages are coupled to a resistor divider 140 comprising series resistors 142, 144, 146 and 148 in order to generate the VTH threshold voltage for comparison to the Vsig voltage by a comparator 160. The output of comparator 160 provides the detector output signal, Vout, which is at a first binary, or logic level when the Vsig voltage exceeds the VTH threshold voltage and is at a second binary level when the Vsig voltage is less than the VTH threshold voltage. The VTH threshold voltage is set at a percentage of the peak-to-peak Vsig voltage and is adaptive in the sense that it is updated in accordance with certain variations in the Vsig voltage so as to remain the percentage, within a predetermined tolerance, of the peak- to-peak Vsig voltage. As will become apparent, this arrangement is achieved by using the PDAC and NDAC voltages to generate the VTH threshold voltage and varying the PDAC and NDAC voltages in accordance with certain variations in the positive and negative peak values of Vsig, respectively. The VTH threshold voltage is provided with hysteresis in the sense that VTH is at a first level corresponding to a first percentage of the peak-to-peak Vsig voltage when Vsig exceeds the VTH threshold voltage and is at a second level corresponding to a second percentage of the peak-to-peak Vsig voltage when Vsig is less than the VTH threshold voltage. That is, once the Vsig voltage falls below the first level of the VTH threshold voltage, the VTH threshold voltage is increased so that the Vsig voltage has to exceed a second, higher level of the VTH threshold voltage before the Vout signal transitions. In the illustrative embodiment, the first and second percentages are fixed percentages of the peak-to-peak Vsig voltage, but providing the percentages as variable percentages of the peak-to-peak Vsig voltage is within the scope of the invention. More particularly, a pair of switches 166, 168 is provided for selectively "shorting" respective resistors 142, 148. To this end, switches 166 and 168 are coupled in parallel with resistors 142 and 148 of resistor divider 140, respectively. The Vout signal from comparator 160 is inverted by an inverter 164 to provide a pos_compn signal which is applied to a control input of switches 166 and 168, as shown. A second control input of switches 166 and 168 is responsive to the Vout signal. Referring also to Figures 15 and 16, when the Vout and pos compn control signals are in first respective logic states, one of the switches 166, 168 is open and the other is closed. In the illustrative embodiment, when the Vout signal is at a logic high level and the pos_compn signal is at a logic low level, switch 166 is open and switch 168 is closed. Under this condition, resistor 148 is "shorted" by closed switch 168 and the VTH threshold voltage is at a first level relative to the PDAC and NDAC voltages. In the illustrative embodiment, the first level is approximately 40% of the difference between the PDAC and NDAC voltages. When the Vout signal is at a logic low level and the pos_compn signal is high, switch 166 is closed and switch 168 is open, thereby causing resistor 142 to be "shorted". Under this condition, the VTH threshold voltage is at a second level which, in the illustrative embodiment, is approximately 60% of the PDAC - NDAC voltage. Since the PDAC and NDAC voltages are updated in accordance with certain variations in the Vsig voltage as will be described, the first VTH threshold voltage level corresponds to a first percentage, within a predetermined tolerance, of the peak-to- peak Vsig voltage and the second VTH threshold voltage level corresponds to a second percentage, within a predetermined tolerance, of the peak-to-peak Vsig voltage. Generally, both the first and second percentages are between 0% and 100%) of the peak-to- peak Vsig voltage. More preferably and in the illustrative embodiment, the first percentage is between 0% and 50% and the second percentage is between 50% and 100%). In order to update the PDAC and NDAC voltages in accordance with certain variations in the positive and negative peak values of the Vsig voltage, respectively, "voltage ok windows" are established for use by the update controller 120 (Figure 14). The voltage ok windows establish the "predetermined tolerance" between the VTH threshold voltage and percentage(s) of the Vsig voltage it represents. A first voltage source 170 is coupled to the buffered PDAC voltage to generate a voltage, PDAC - ΔV, which is a predetermined voltage less than the PDAC voltage. In the illustrative embodiment, ΔV is the equivalent of two bits, or lOOmV. Similarly, a voltage source 172 is coupled to the buffered NDAC voltage to generate a voltage, NDAC + ΔV, which, in the illustrative embodiment, is the equivalent of two bits higher than the NDAC voltage. A "positive voltage ok window" used to update the PDAC voltage is defined between the PDAC and PDAC-ΔV voltages and a "negative voltage ok window" used to update the NDAC voltage is defined between the NDAC and NDAC + ΔV voltages. Referring to Figures 17, 17a, 18 and 19, the manner by which the PDAC and NDAC voltages are updated in accordance with variations in the positive and negative peaks of the Vsig voltage, respectively, will be described. Specifically, Figure 17 illustrates how the PDAC voltage varies with certain variations in the positive peaks of Vsig over several updated time intervals and Figure 18 is a corresponding flow diagram illustrating operation of the detector in updating the PDAC voltage. Figure 17a illustrates the manner in which the NDAC voltage varies with certain variations in the negative peaks of Vsig over several update time intervals and Figure 19 is a corresponding flow diagram. It will be appreciated by those of ordinary skill in the art that the methods illustrated by Figures 18 and 19 are illustrative only and may be readily varied, such as by varying the sequence of the steps. The first time interval ending at time X follows power up of the detector and is referred to as a start up time interval. Subsequent time intervals, during and/or after which the PDAC and NDAC voltage may be updated, are referred to as update time intervals. During the start up time interval, the PDAC 118 and the NDAC 134 track the highest and lowest peaks of the Vsig voltage, respectively, as shown. Since the initial Vsig voltage is unknown, the PDAC voltage is set to a value lower than the least positive expected peak of Vsig, such as the negative voltage rail, and the NDAC voltage is set to a value greater than the least negative expected peak of Vsig, such as the positive voltage rail. The update controller 120 (Figure 14), which is operative to update the PDAC and NDAC voltages as necessary, is disabled during the startup time interval. After the startup time interval, at time X, if any positive peak of Vsig is within the positive voltage ok window during a given time interval, then the PDAC voltage is not updated either during or at the end of the interval, as is illustrated during the interval between times X and X+l in Figure 17. If, however, a positive peak value of the Vsig voltage exceeds the positive voltage ok window (i.e., exceeds PDAC), then the PDAC voltage is increased to the value of this positive peak voltage, as illustrated during the interval between times X+2 and X+3 in Figure 17. Finally, if all of the positive peaks of the Vsig voltage during a given time interval are less than the positive voltage ok window (i.e., are less than PDAC - ΔV), then the PDAC voltage is decremented by a predetermined amount at the end of that interval, as is illustrated in the interval between times X+l and X+2 in Figure 17. In one embodiment, the PDAC voltage is decremented by one bit at the end of any time interval during which all positive peaks of the Vsig voltage are less than the positive voltage ok window. Referring to Figure 18, the method for updating the PDAC voltage as a function of the positive peaks of the Vsig voltage commences in step 200. In step 204, it is determined whether Vsig is greater than the upper boundary of the positive voltage ok window, or PDAC. If the Vsig voltage is greater than PDAC, then the PDAC voltage is incremented in subsequent step 208 to the value of the Vsig voltage. In the event that the Vsig voltage is not greater than the PDAC voltage, then it is next determined in step 212 whether the current time interval is over. Steps 204 - 212 are repeated until the time interval has ended. An update time interval is comprised of a predetermined number of transitions of the Vout voltage. In the illustrative embodiment, preferably, each time interval is comprised of 128 positive (or alternatively 128 negative) transitions of the Vout signal. More generally however, it is advantageous for the time interval to be greater than one revolution of a passing magnetic article. Considering, for example, the case in which the magnetic article is a rotating gear having teeth spaced by valleys, this arrangement ensures that the highest tooth and deepest valley will be detected during each time interval. At the end of each time interval, in step 216, it is determined whether all of the positive peaks of the Vsig voltage during that particular time interval were less than the positive voltage ok window (i.e., PDAC - ΔV). If all of the positive Vsig peaks during the particular interval were less than PDAC- ΔV, then the PDAC voltage is decremented by a predetermined amount in step 220, following which the process terminates in step 224. In the illustrative embodiment, the predetermined amount by which the PDAC voltage is decremented is one bit, or fifty millivolts. If however, not all of the positive peaks of the Vsig voltage during the interval were less than the positive voltage ok window, then the process terminates in step 224, as shown. Referring to Figures 17a and 19, the operation of the detector in updating the NDAC voltage in accordance with certain negative peak values of the Vsig voltage will be described. In general, the NDAC voltage is updated in response to the negative peaks of Vsig conversely to the manner in which the PDAC voltage is updated in response to the positive peaks of Vsig. Specifically, and as illustrated between times X and X+l in Figure 17a, as long as one of the negative peaks of the Vsig voltage during a given interval is within the negative voltage ok window, then the NDAC voltage is not modified. If however, all of the negative peaks of the Vsig voltage during the interval were greater than the negative voltage ok window (i.e., greater than NDAC + ΔV), then the NDAC voltage is incremented by a predetermined amount, such as one bit, at the end of that interval, as illustrated at time X+2. Finally, in the event that any of the negative Vsig peaks is less than the negative voltage ok window (i.e., is less than the NDAC voltage), then the NDAC voltage is decremented to equal that negative peak voltage, as occurs between times X+2 and X+3 in Figure 17a. The operation of the detector in updating the NDAC voltage is illustrated in the flow diagram of Figure 19 which commences at step 230. In step 234, it is determined whether the Vsig voltage is less than the NDAC voltage. In the event that the Vsig voltage is less than the NDAC voltage, then the NDAC voltage is decremented to equal the value of the Vsig voltage. Alternatively, if the Vsig voltage is not less than the NDAC voltage, then step 242 is next performed in which it is determined whether the particular time interval has ended. Steps 234- 242 are repeated until an interval has ended, as shown. At the end of each time interval, it is determined, in step 246, whether all of the negative peaks of the Vsig voltage during the just ending interval were greater than the negative voltage ok window (i.e., greater than NDAC + ΔV). In the event that all of the negative Vsig peaks during the particular interval were greater than the negative voltage ok window, then the NDAC voltage is incremented by a predetermined amount, such as one bit, in step 250 as illustrated at time X+2 in Figure 17a, following which the process terminates in step 254. Otherwise, the process terminates directly after step 246, as shown. Referring to Figure 14, the update controller 120, which is operative to cause the detector to update the PDAC and NDAC voltages as necessary, includes a positive controller portion 122 and a negative controller portion 126. Since the arrangement and operation of the positive and negative controller portions 122 and 126 mirror each other, the circuitry and operation will be described with particular reference to the positive controller portion 122 for simplicity of illustration. A counter 128 counts transitions of the Vout voltage of a particular polarity, such as each positive going transition, to provide an update time interval clock signal, cntl28. As noted above, in the illustrative embodiment, each update time interval is comprised of 128 transitions of the Vout output voltage. It will be appreciated by those of ordinary skill in the art that the update time interval may be readily varied. The positive controller portion 122 includes a comparator 132 having a positive input to which the Vsig voltage is applied and a negative input to which the PDAC - ΔV voltage is applied. Thus, the comparator 132 compares the Vsig voltage to the lower boundary of the positive voltage ok window. The output signal of the comparator 132, p ok, is applied to the reset input of a cross-coupled NOR latch 150. The output signal of the NOR latch 150, upd_pdac, is applied to the input of a flip-flop 152 which is clocked by the cntl28 signal and which provides an output signal, dcrp. The dcrp signal is applied to the input of a further flip-flop 156 which is clocked by clkn, an inverted version of the system clock signal. The Q output of flip-flop 156 provides the p_cnt_up_n signal to the NOR gate 108 (Figure 13) and to an input of a NOR gate 154. The Qbar output of flip- flop 156 provides the p_cnt_up signal which is applied to the UPDN input of the counter 114 (Figure 13) to control the count direction. Flip-flop 152 is reset by the NOR gate 154 at detector start up and each time the counter 114 counts down. The negative controller portion 126 includes a comparator 138 which compares the Vsig voltage to the upper boundary of the negative voltage ok window, or NDAC + ΔV. The output signal of the comparator 138, n_ok, is coupled to the reset input of a NOR latch 176, which is set at the end of each update time interval by the cntl28 signal. The Q output of latch 176 is a signal, upd ndac, which is applied to a flip-flop 178. Flip-flop 178 is clocked by the cntl28 signal, as shown, and provides a dcrn output signal to a further flip-flop 182 which is clocked by the clkn signal. The Q output of flip-flop 182 provides the n_cnt_up_n signal to NOR gate 112 (Figure 13) and further to an input of a NOR gate 180. The flip-flop 182 is reset by the NOR gate 180 at detector start up and when the counter 130 counts down. The Qbar output of flip-flop 182 provides the n cnt up signal to the UPDN input of counter 130 (Figure 13) to control the count direction. The latch 150 is set by the cntl28 signal at the end of each update time interval. Further, at the end of an update time interval in which the latch 150 has not been reset, the dcrp signal goes high (i.e., the decrement bit is set). Upon the next clock cycle following the decrement bit being set, the p_cnt_up_n signal goes high for a single clock cycle and the p_cnt_up signal goes low for a single clock cycle. A low pulse on the p_cnt_up signal of a one clock cycle duration commands the counter 114 to count down one bit. Recall however, that the counter 114 is only able to count, either up or down, if its HOLD input signal is low. Referring also to Figures 20-20g, the operation of the update controller 120 will be illustrated by an example. Specifically, Figure 20 shows two illustrative update time intervals of the Vsig voltage, with the PDAC voltage, the PDAC - ΔV voltage, the NDAC voltage and the NDAC + ΔV voltage superimposed thereon. Figure 20a illustrates the cntl28 output signal of the counter 128 (Figure 14), Figure 20b illustrates the detector output signal Vout and Figure 20c illustrates the p_ok output signal of comparator 132 (Figure 14), during the same time intervals shown in Figure 20. Figure 20d shows the upd_pdac output signal of the NOR latch 150, Figure 20e illustrates the dcrp output signal of flip-flop 152, Figure 20f illustrates the p_cnt_up output signal of the flip-flop 156 and the elk signal is shown in Figure 20g, during the same time intervals shown in Figure 20. Consider, for example the interval between times X+l and X+2, in which all of the positive Vsig peaks are less than the lower boundary of the positive voltage ok window, PDAC - ΔV. In this case, the p_ok voltage remains low, so the latch 150 is not reset during this interval. Thus, at the end of the interval, the p_cnt_up signal goes low for a single clock cycle to command the counter 114 to count down by one bit. Further, the counter 114 is enabled under this condition, since the p_cnt_up_n input signal to NOR gate 108 is high and thus, the HOLD input to counter 114 is low. In the case where at least one positive Vsig peak during an interval is within the positive voltage ok window, as illustrated between times X and X+l in Figures 20-20g, the PDAC voltage is not modified. Under this condition, the p_ok signal goes high to reset the upd_pdac voltage, thereby preventing the decrement bit, dcrp, from being set. Thus, at the end of this interval, the p_cnt_up signal remains high to command the counter 114 to count up. However, the PDAC voltage is not modified since the counter 114 is disabled. More particularly, the HOLD input to the counter 114 is high since both the output of the comparator 100 and the p_cnt_up_n signal are low. Finally, when a positive Vsig peak exceeds the PDAC voltage, the PDAC voltage is updated to follow, or track the Vsig positive peak any time during the interval. In this case, the p_ok output signal of comparator 100 goes high to reset the latch 150. More particularly, the latch 150 is reset when Vsig exceeds the PDAC voltage. This condition causes the upd_pdac signal to go low and prevents the decrement bit, dcrp, from being set by flip-flop 152 at the end of the interval. Thus, at the end of the interval, the p_cnt_up signal remains high to keep the counter 114 in count up mode. This condition also presents a low to NOR gate 108, allowing the counter 114 to increment as needed any time during the interval by releasing the HOLD signal. Referring also to Figure 21, an alternate detector embodiment is shown. The detector of Figure 21 operates in a similar manner to the detector of Figure 13 to provide an output voltage Vout which is at one binary level when the Vsig voltage exceeds a threshold voltage VTH and is at another binary level when the Vsig voltage is less than the threshold voltage. Further, the detector of Figure 21 provides the VTH threshold voltage with hysteresis in a like manner to the detector of Figure 13. The detector of Figure 21 differs from that of Figure 13, however, in the manner in which the PDAC and NDAC voltages are updated in accordance with certain variations in the positive and negative peaks of the Vsig voltage, respectively. In general, the detector of Figure 21 is characterized in that any modifications, or updates to the PDAC and NDAC voltages occur at the end of a particular update time interval and further, in that the PDAC and NDAC voltages are incremented and decremented, respectively, by a predetermined amount. This arrangement is contrasted to varying the PDAC voltage to track a positive peak of Vsig as occurs between times X+2 and X+3 in Figure 17 or varying the NDAC voltage to track a negative peak of Vsig as occurs between times X+2 and X+3 in Figure 17a. By incrementing or decrementing the PDAC and NDAC voltages by only a predetermined amount, the occurrence of "hunting" (i.e., the PDAC and NDAC voltages fluctuating to track noise peaks) is reduced. The detector of Figure 21 includes the Hall element 10 and the Hall voltage amplifier 12 which provides the amplified Hall voltage, Vsig, as described above. The Vsig voltage is coupled to the positive input of a first comparator 260 and to the negative input of a second comparator 264, as shown. The output signal of the comparator 260, pcomp, is inverted by an inverter 266 for coupling to a multiplexer 268, as shown. The multiplexer 268 is further responsive to a phold control signal provided by an update controller 280 (Figure 22) for selecting between the output of the inverter 266 and the phold signal for coupling to the multiplexer output. The output signal 270 of the multiplexer 268 provides the HOLD input signal to an up/down counter 274. The select input signal to the multiplexer 268, en_update, is provided by a cross-coupled NOR latch 278, which is set by the startup signal and reset by the cntl28 signal, as shown. Similarly, the output of the comparator 264 is inverted by an inverter 316, the output of which provides an input to a multiplexer 320. A further input to the multiplexer 320 is provided by an nhold control signal generated by the update controller 280. The multiplexer 320 is responsive to the en_update signal for selecting between the output of the inverter 316 and the nhold signal for coupling to the multiplexer output. The multiplexer output provides the HOLD input signal to an up/down counter 322. The remainder of the detector of Figure 21 is substantially identical to the detector of Figure 13. Specifically, like counter 114 (Figure 13), the counter 274 is clocked by the CLK signal and reset by the pndac_resn signal. A p_cnt_up signal provided by the update controller 280 controls the direction of the count by the counter 274. The outputs of the counter 274 are coupled to inputs of a PDAC 284, the output of which provides the PDAC voltage. A buffer 286 buffers the PDAC voltage and a voltage source 290 generates the PDAC - ΔV voltage, which is a predetermined voltage, ΔV, less than the PDAC voltage, such as two bits, or lOOmV. In the "negative portion" of the detector, the counter 322, like counter 130 (Figure 13), is clocked by the CLK signal and is reset by the pndac_resn signal, as shown. An n_cnt_up signal, provided by the update controller 280, controls the direction in which the counter counts. The outputs of the counter 322 are coupled to inputs of an NDAC 324, the output of which provides the NDAC voltage, as shown. The NDAC voltage is buffered by a buffer 328 which provides a buffered NDAC voltage and a voltage source 330 generates the NDAC +ΔV voltage, which is a predetermined voltage, ΔV, greater than the NDAC voltage, such as two bits, or lOOmV. A resistor divider 292, including series-coupled resistors 294, 296, 298 and 300, is coupled across the buffered PDAC and NDAC voltages to provide the threshold voltage, VTH, as shown. Hysteresis switches 304 and 306 are arranged and operative to provide the VTH threshold voltage with hysteresis in the same manner described above in conjunction with Figure 13. The VTH threshold voltage is compared to the amplified Hall voltage, Vsig, by a comparator 310, the output of which is the Vout signal. An inverter 312 inverts the Vout voltage to provide a further control input signal to hysteresis switches 304 and 306, as shown. The logic circuitry which provides the HOLD input signals to the counters 274 and 322 is operative to hold the output of the respective counter constant (i.e., to disable the counter) during each update time interval and at the end of any update time interval during which at least one Vsig peak was within the respective voltage ok window. However, at the end of any update time interval during which a condition requiring updating of the PDAC or NDAC voltage occurred, the logic circuitry is operative to enable the respective counter for a single system clock cycle. In this way, the counter is able to count up or down (depending on the state of the respective p_cnt_up and n_cnt_up signals) by one bit. Considering the "positive portion" of the HOLD logic circuitry, during the first update time interval following startup of the detector, the en_update signal is high to select the A input to multiplexer 268. With the A input selected, the output signal of the multiplexer 268 follows the inverted version of the output of comparator 260. Stated differently, during the first update time interval following detector powerup, the HOLD input signal causes the counter 274 to increment for as long as Vsig is greater than PDAC. Thus, during the first update time interval, the PDAC voltage increases, as necessary, to track the most positive peak of the Vsig voltage. At the end of the first update time interval, the cntl28 signal resets the latch 278, causing the en update signal to go low to select the B input to the multiplexer 268. The B multiplexer input signal is the phold signal provided by the update controller 280. As will become apparent, the phold signal is normally high, causing the counter 274 to be disabled. However, at the end of any update time interval during which a condition requiring updating of the PDAC voltage occurred, the phold signal goes low for a single system clock cycle to enable the counter 274 to count up or down (depending on the logic level of the p_cnt_up signal) by a predetermined amount, such as one bit. The "negative portion" of the HOLD logic circuitry, including inverter 316 and multiplexer 320, is operative to provide the HOLD input signal to the counter 322 in a manner similar to that of inverter 266 and multiplexer 268. In particular, during the first update time interval, when the en update select input to the multiplexer 320 is high, the multiplexer 320 selects the A input for coupling to the HOLD input of counter 322. Accordingly, during the first update time interval, the NDAC voltage tracks the most negative peak of the Vsig voltage. At the end of the first update time interval, when the en_update signal goes low, the B input to the multiplexer 320 (i.e., the nhold signal) is selected to provide the HOLD input signal to counter 322. Like the phold signal, the nhold signal is normally high, causing the counter 322 to be disabled. However, at the end of any update time interval during which a condition requiring the NDAC voltage to be updated occurs, the nhold signal goes low for a single system clock cycle to permit the counter 322 to count up or down (depending on the logic level of the n cnt up signal) by a predetermined amount, such as one bit. Before discussing the update controller 280 shown in Figure 22 and the illustrative way in which the phold, nhold, p_cnt_up and n_cnt_up signals are generated, the manner by which the detector of Figure 21 updates the PDAC and NDAC voltages in accordance with certain variations in the positive and negative peaks of the Vsig voltage, respectively, will be described in conjunction with Figures 23, 23a, 24 and 25. Referring to Figure 23 and the corresponding flow diagram of Figure 24, the PDAC updating operation of the detector of Figure 21 commences at step 400, following which it is determined in step 404 whether the current update time interval is over. Step 404 is repeated until a time interval ends, at which time it is determined in step 408 whether any positive peak of the Vsig voltage during the just ending interval exceeded the PDAC voltage. If any positive peak of Vsig during the interval was greater than the PDAC voltage, then the PDAC voltage is incremented by a predetermined amount, such as one bit, in subsequent step 412, as illustrated at time X+l in Figure 23. In step 416, it is determined whether all the positive peaks of the Vsig voltage during the just ending interval were less than the PDAC - ΔV voltage (i.e., whether all positive peaks of Vsig were less than the positive voltage ok window). In the event that all positive Vsig peaks during the interval were less than the positive voltage ok window, then the PDAC voltage is decremented by a predetermined amount, such as one bit, in subsequent step 420, following which the process terminates in step 424. This condition is illustrated at time X+2 in Figure 23. Alternatively, the process terminates directly after step 416, as shown. Referring to Figures 23a and 25, the operation of the detector of Figure 21 in updating the NDAC voltage commences at step 430, following which it is determined in step 434 whether the particular update time interval has ended. Once a time interval ends, it is determined in step 438 whether any negative peak of the Vsig voltage during the just ending interval was less than the NDAC voltage (i.e., was less than the negative voltage ok window). In the event that a negative peak of the Vsig voltage was less than the NDAC voltage, then the NDAC voltage is decremented by a predetermined amount, such as one bit, in step 442. This condition is illustrated at time X+l in Figure 23a. Alternatively, step 442 is bypassed. Subsequently, in step 446, it is determined whether all of the negative peaks of the Vsig voltage during the just ending interval were greater than the NDAC + ΔV voltage (i.e., whether all negative Vsig peaks were greater than the negative voltage ok window). In the event that all negative Vsig peaks during the interval were greater than the negative voltage ok window, then the NDAC voltage is incremented in step 450 by a predetermined amount, such as one bit, following which the process terminates in step 454. This condition is illustrated at time X+2 in Figure 23a. Alternatively, the process terminates directly after step 446, as shown. Referring to the update controller 280 of Figure 22, like the controller 120 of Figure 14, the controller 280 includes a positive controller portion 350 and a negative controller portion 352. The update controller 280 will be described with particular reference to the positive controller portion 350 for simplicity. Also shown in the schematic of Figure 22 is the counter 128 which is responsive to the Vout signal for providing the cntl28 signal, in the manner discussed above in conjunction with Figure 14. The positive controller portion 350 includes a comparator 354 for comparing the Vsig voltage to the PDAC - ΔV voltage. The output signal of comparator 354, p_ok, is coupled to the reset input of a cross-coupled NOR latch 356, the set input to which receives the cnt 128 signal. The output of latch 356 is inverted by an inverter 358 to provide a p_latsm signal, as shown. The p latsm signal indicates whether or not the Vsig voltage exceeded the lower boundary of the positive voltage ok window, PDAC - ΔV, during the just ending update time interval. The comparator 260 (Figure 21), which compares the Vsig voltage to the PDAC voltage, provides a pcomp signal which is coupled to the update controller 280 and, specifically, to the set input of a cross-coupled NOR latch 360. The NOR latch 360 is reset by the cnt 128 signal and provides, at its output, a p latbig signal, as shown. The p latbig signal indicates whether or not the Vsig voltage exceeded the upper boundary of the positive voltage ok window, PDAC, during the just ending update time interval. The p latsm and p_latbig signals are coupled to respective inputs of an OR gate 364, the output of which is coupled to a first input of an AND gate 366. The second input to AND gate 366 is provided by the en update signal, as shown. The output signal of AND gate 366 is coupled to the D input of a flip-flop 368, the Q output of which provides a p_update signal. The flip-flop 368 is clocked by the cntl28 signal and is reset by the output of a NOR gate 370, as shown. The p_update signal provides the D input to a further flip-flop 374, the Q output of which is fed back to a first input of the NOR gate 370. The second input to NOR gate 370 receives the startup signal, as shown. The Qbar output of flip-flop 374 provides the phold signal to the B input of multiplexer 268 (Figure 21). The p_latsm signal is further coupled to a flip-flop 376, the Qbar output of which provides the p_cnt_up signal to the counter 274 (Figure 21) to control the direction of operation of the counter. The flip-flop 376 is clocked by the cntl28 signal and reset by the startup_n signal, as shown. As noted above, the negative controller portion 352 is substantially identical to the positive controller portion 350. More particularly, the negative controller portion 352 includes a comparator 378 which compares the Vsig voltage to the upper boundary of the negative voltage ok window, NDAC + ΔV. The n_ok output signal of comparator 378 is coupled to a latch 380, the output of which is inverted by an inverter 382. The comparator 264 (Figure 21), which compares the Vsig voltage to the NDAC voltage, provides an output signal, ncomp, which is coupled to a latch 384, as shown. The output of latch 384 is an n_latbig signal and the output of inverter 382 is an n latsm signal both of which are coupled to inputs of an OR gate 386. The n_latsm signal is further coupled to the input of a flip-flop 396. The output of flip-flop 396 provides the n_cnt_up signal which controls the direction of operation of counter 322 (Figure 21). The output of OR gate 386 is coupled to a first input of an AND gate 388, the second input to which is provided by the en update signal. The output of AND gate 388 is coupled to a flip-flop 390 which provides an n_update signal to a further flip-flop 394. The Qbar output of flip-flop 394 provides the nhold signal to the multiplexer 320 (Figure 21). The Q output of flip-flop 394 is fed back to a NOR gate 392 which is further responsive to the startup signal for resetting the flip-flop 390. Considering operation of the positive controller portion 350, the latch 356 is set at the end of each update time interval and is reset when the Vsig voltage exceeds the PDAC - ΔV voltage. Thus, the p latsm output signal of inverter 358 transitions to a logic low level during any update time interval in which at least one positive Vsig peak exceeded PDAC - ΔV. Latch 360 is set when the Vsig voltage exceeds the PDAC voltage and is reset at the end of each update time interval. Thus, the p latbig signal transitions to a logic high level when the Vsig voltage exceeds the PDAC voltage and remains high until the end of the particular update time interval. Accordingly, the output of OR gate 364 is high if either (1) the Vsig voltage has exceeded the PDAC voltage during the current time interval; or (2) the Vsig voltage has been less than the PDAC - ΔV voltage during the entire current time interval (i.e., if a condition has occurred requiring the PDAC voltage to be updated). If the output of OR gate 364 is high and the first update time interval following detector startup has passed, then the output of AND gate 366 goes high. Under this condition, at the end of the current update time interval, the flip-flop 368 latches in the high input signal to provide a logic high p update signal to the flip-flop 374. Further, at the end of the current system clock cycle, the logic high p_update signal is latched into flip-flop 374 to provide a logic low phold signal to the multiplexer 268 (Figure 21). With the phold signal low, the HOLD input to the counter 274 is likewise low, thereby enabling the counter 274 to count. Further, the phold signal remains low for only a single system clock cycle so that the counter 274 increments or decrements by a predetermined amount, such as one bit. The direction of operation of counter 274 is determined by the logic level of the p cnt up signal. As noted above, the p_cnt_up signal is normally high, causing the counter 274 to count up. However, when the p_latsm signal is low at the end of an update time interval (i.e., if all positive peaks of the Vsig voltage during the just ending interval were less than PDAC - ΔV), then flip-flop 376 causes the p_cnt_up signal to go low, causing the counter 274 to count down. Further, the counter 274 counts down for only a single clock cycle since the phold signal is low for only a single clock cycle. The operation of the positive controller portion 350 will become more apparent by reference to the three illustrative update time intervals of the Vsig voltage in Figure 26, which shcrws only the positive peaks of the Vsig voltage for simplicity of illustration. During the interval between times X and X+l, one of the positive peaks of the Vsig voltage exceeds the PDAC voltage. Thus, at the end of the interval, the PDAC voltage is incremented by one bit. To this end, when the Vsig voltage exceeds the PDAC voltage, the pcomp signal goes high and transitions back to a low level when the Vsig voltage decreases to below the PDAC voltage, as shown in Figure 26a. The positive going transition of the pcomp signal causes the latch 360 to be set and the p latbig signal to go high, as shown in Figure 26b. The p_ok output signal of comparator 354 transitions to a high level every time the Vsig voltage exceeds the PDAC - ΔV voltage and returns to a logic low level when the Vsig voltage falls to below the PDAC - ΔV voltage, as shown in Figure 26d. Thus, the p_latsm signal goes low following the first positive going transition of the p_ok signal during the interval, as shown in Figure 26e. At the end of the update time interval, at time X+l, the cnt 128 signal goes high, which causes flip-flop 368 to latch in the high input signal and the p update signal to go high. Further, at time X+l, flip-flop 376 latches in the low input signal, causing the p_cnt_up signal to go high and the counter 274 to count up. During the interval ending at time X+l, the D input to latch 368 went high due to the p_latbig signal going high as Vsig exceeded PDAC. Also during this interval, the D input to flip-flop 376 went low due to the p_ok signal resetting latch 356. On the falling edge of the system clock signal CLK following the end of the update time interval, the phold output signal of flip-flop 374 goes low until the next falling edge of the CLK signal. The p_update signal returns to a logic low level when the flip-flop 368 is reset by the phold signal going low. With this arrangement, the counter 274 is enabled by the phold signal to count for a single clock cycle. During the time interval between times X+l and X+2, all of the positive peaks of the Vsig voltage are less than the positive voltage ok window, or PDAC - ΔV. Under this condition, the PDAC voltage is decremented by one bit at the end of the interval. Since the Vsig voltage remains less than PDAC - ΔV, the latch 356 is not reset during this time interval and the p_latsm signal remains high. With p_latsm high, the output of OR gate 364 goes high and, since the time interval is not the first following detector start up, the output of AND gate 366 goes high. At time X+2, the p update signal goes high and upon the next falling edge of the CLK signal, phold goes low for a single clock cycle. Thus, under this condition, the counter 274 is enabled to count up or down by one bit. In particular, the counter 274 counts down, since the p cnt up signal is low due to the p latsm signal being high, as shown. Considering finally the interval between times X+2 and X+3, one of the positive Vsig peaks is within the positive voltage ok window during this interval. Thus, the PDAC voltage is not updated at the end of the interval. To this end, once the Vsig voltage exceeds the PDAC - ΔV voltage, the p_ok output signal of comparator 354 goes high and resets the latch 356 causing the p_latsm signal to go low. Since the p latsm signal is low at the end of the interval, the outputs of OR gate 364 and AND gate 366 are low. Thus, the p update signal and the phold signal remain low and the counter 274 remains disabled, thereby preventing the PDAC voltage from being updated. Referring to Figure 27, a further alternate magnetic field detector is shown. The detector of Figure 27, like the previously described embodiments, provides an output voltage Vout which is at one binary level when the Vsig voltage exceeds a VTH threshold voltage and is at another binary level when the Vsig voltage is less than the VTH threshold voltage. Further, the detector of Figure 27 provides the VTH threshold voltage with hysteresis in a like manner to the detectors of Figures 13 and 21 in that the VTH threshold voltage is a percentage of the peak-to-peak voltage of Vsig, and is updated to remain the percentage of the peak-to-peak voltage of Vsig, within a predetermined tolerance, even as the peak-to-peak voltage of Vsig varies. In particular, the VTH threshold voltage is at a first level corresponding to a first percentage of the peak-to-peak voltage of Vsig when Vsig exceeds the VTH threshold voltage and is at a second level corresponding to a second percentage of the peak-to-peak voltage of Vsig when Vsig is less than the VTH threshold voltage. The detector of Figure 27 differs from those previously described however in the manner in which the PDAC and NDAC voltages are updated. The detector of Figure 27 updates the PDAC and NDAC voltages by a predetermined amount (e.g., by a predetermined number of bits) after transitions of the Vout voltage and permits the PDAC and NDAC voltages to freely track the positive and negative peaks of the Vsig voltage, respectively. More particularly, the PDAC voltage is decremented by a predetermined amount upon transitions of the Vout voltage of a first polarity and the NDAC voltage is incremented by a predetermined amount upon transitions of the Vout voltage of a second, opposite polarity. With this arrangement, a relatively simple and robust circuit and technique are provided for updating the PDAC and NDAC voltages to ensure that the VTH threshold voltage remains, within a predetermined tolerance, a percentage of the peak-to-peak Vsig voltage, even as the peak-to-peak Vsig voltage varies. Further advantages of the detector of Figure 27 include additional hysteresis which serves to reduce the susceptibiltiy of the circuit to noise, but which is introduced at a time when the switch points defining transitions of the Vout voltage are not affected, as will be described below. Additionally, the detector has a relatively fast response time to changes in the peak-to-peak Vsig voltage, since the PDAC and NDAC voltages are updated at each transition of the Vout voltage. In this way, the detector of Figure 27 provides a Vout signal which accurately reflects changes in the sensed magnetic field and reduces the likelihood that the Vout signal would not switch following significant peak Vsig voltage variations. Further, circuit testing and thus manufacture time and cost is reduced since complicated PDAC and NDAC voltage updating decisions are eliminated by updating the PDAC and NDAC voltage by a predetermined amount after transitions of the Vout signal. It will be appreciated by those of ordinary skill in the art, and is within the scope of this invention, that the various techniques for updating the PDAC and NDAC voltages described herein may be implemented in the form of, or to include, mid-signal detectors in which the threshold voltage is a fixed percentage of the peak-to-peak Vsig voltage (as is determined by computing a fixed percentage of the voltage PDAC - NDAC) or peak- referenced (i.e., slope-activated) detectors. As one example, the peak-to-peak percentage threshold detectors may include a peak-referenced detector such that the detector output signal Vout becomes one binary level when Vsig rises to exceed a held negative peak excursion (the NDAC voltage) by a predetermined amount and another binary level when Vsig falls to below a held positive peak excursion (the PDAC voltage) by a predetermined amount. With the exception of the update controller 125, which is shown and described in conjunction with Figure 28, the detector of Figure 27 is substantially identical to the detector of Figure 13, as is illustrated by the use of like reference numbers for like components. Thus, the counter 114 is a six bit counter which is enabled when the HOLD input (at the output of NOR gate 108) is low. Further the direction of operation of the counter 114 is governed by the p_cnt_up signal. In particular, when the counter 114 is enabled and the p_cnt_up signal is high, the counter 114 counts up and when the counter 114 is enabled and the p_cnt_up signal is low, the counter 114 counts down. Likewise, the counter 130 is enabled when the output of NOR gate 112 is low and is disabled when the output of NOR gate 112 is high. Further, when the counter 130 is enabled and the n_cnt_up signal is high, counter 130 counts up and when the counter 130 is enabled and the n_cnt_up signal is low, the counter 130 counts down. The outputs of the counter 114 are coupled to inputs of PDAC 118, the outputs of which provide the PDAC voltage which is used to generate the VTH threshold voltage. The PDAC voltage is coupled to a buffer 124 and is fed back to the negative input of comparator 100, as shown. The comparator 100, NOR gate 108, counter 114, PDAC 118 and buffer 124 comprise a "positive portion" of the detector circuitry. A "negative portion" of the detector includes like circuitry similarly arranged. Specifically, the output of the NOR gate 112 is coupled to a HOLD input of counter 130 which is responsive to the CLK clock signal, the pndac resn reset signal, and an n_cnt_up control signal provided by the update controller 125 (Figure 28) which controls the count direction. The outputs of the counter 130 are coupled to inputs of NDAC 134 which provides the NDAC voltage which is used, along with the PDAC voltage, to generate the VTH threshold voltage. The NDAC voltage is coupled to buffer 136 and is further fed back to the negative input of comparator 104, as shown. The buffered PDAC and NDAC voltages are coupled to resistor divider 140 which generates the VTH threshold voltage for comparison to the Vsig voltage by a comparator 160. The output of comparator 160 provides the detector output signal, Vout, which is at a first binary, or logic level when the Vsig voltage exceeds the VTH threshold voltage and is at a second binary level when the Vsig voltage is less than the VTH threshold voltage. Further, the VTH threshold voltage is provided with hysteresis in the sense that VTH is at a first level corresponding to a first percentage of the peak-to-peak Vsig voltage when Vsig exceeds the VTH threshold voltage and is at a second level corresponding to a second percentage of the peak-to-peak Vsig voltage when Vsig is less than the VTH threshold voltage. That is, once the Vsig voltage falls below the first level of the VTH threshold voltage, the VTH threshold voltage is increased so that the Vsig voltage has to exceed a second, higher level of the VTH threshold voltage before the Vout signal transitions. Before discussing the update controller 125 of Figure 28, the manner in which the PDAC and NDAC voltages are updated will be described with reference to the waveforms of Figures 29, 29a, 29b, 30 and 30a. The illustrative Vsig voltages of Figures 29 and 30 are shown with the positive peaks changing and with the negative peaks varying, respectively. It will be appreciated that the positive and negative peaks of Vsig may vary as a result of different factors, such as changes in the airgap between the magnetic article and the detector, a chipped gear tooth in the magnetic article, or a piece of metal being stuck between gear teeth. Further, it will be appreciated that both the positive and negative peaks of the Vsig voltage will vary within a given magnetic article revolution. However, the waveform of Figure 29 is shown with only the positive peaks varying and the waveform of Figure 30 is shown with only the negative peaks varying for simplicity of illustration. The PDAC and NDAC voltages are permitted to freely track the positive and negative peaks of the Vsig voltage, respectively. Further, upon each transition of the Vout signal, the PDAC and NDAC voltages are decremented and incremented, respectively, by a predetermined amount. In particular, the PDAC voltage is decremented by a predetermined amount upon each transition of the Vout signal of a first polarity (e.g., positive-going) and the NDAC voltage is incremented by a predetermined amount upon each transition of the Vout voltage of the second, opposite polarity (e.g., negative-going). In the illustrative embodiment, the PDAC and NDAC voltages are decremented and incremented, respectively, by three bits. However, the predetermined amount can be readily varied and, in general, is selected to approximately equal the largest expected voltage variation between successive Vsig peaks. Referring to Figure 29, at time t2, when the Vout voltage transitions to a high level, the PDAC voltage is decremented by three bits. Thereafter, the PDAC voltage tracks the Vsig voltage back up to its next positive peak. The same occurs at times t4, t6, t8, and tl2. However, at time tlO, when the PDAC voltage is decremented by three bits, the PDAC voltage is greater than the next positive peak of the Vsig voltage (occurring between times tlO and ti l) and thus, the PDAC voltage does not track the Vsig voltage again until time tl2. Referring also to Figure 30, at time t21, when the Vout voltage transitions to a low level, the NDAC voltage is incremented by three bits. Thereafter, the NDAC voltage tracks the Vsig voltage back down to its next negative peak. And the same occurs at times t23, t27, t29 and t31. However, at time t25, when the NDAC voltage is incremented by three bits, the NDAC voltage is less than the next negative peak of the Vsig voltage (occurring between times t25 and t26) and thus, the NDAC voltage does not track the Vsig voltage again until time t27. Recall that the VTH threshold voltage is generated by a resistor divider network 140 (Figure 27) coupled between the PDAC and NDAC voltages which includes switches 166, 168 controlled by the Vout and pos_compn control signals such that the VTH threshold voltage is at a first percentage, such as on the order of 40%, of PDAC - NDAC when the Vout voltage is at a first level and is at a second percentage, such as on the order of 60%), of PDAC - NDAC when the Vout voltage is at a second level. In Figures 29 and 30, the VTH voltage is shown by a solid line and the other one of the first and second percentages of PDAC - NDAC is shown by a dotted line. Since the VTH threshold voltage is a function of the PDAC and NDAC voltages, as the PDAC and NDAC voltages are updated, so too is the VTH threshold voltage, as shown. This is best seen in the expanded view of Figure 29b. At time t8, when the PDAC voltage is decremented, the VTH voltage is decremented too. Thereafter, as the PDAC voltage tracks the Vsig voltage to its next positive peak, the VTH threshold voltage moves up concomitantly. As is further apparent from consideration of Figure 29b, the VTH threshold voltage updating scheme implemented with the embodiment of Figures 27 and 28 provides additional threshold voltage hysteresis. For example, at time t8, when the VTH threshold voltage changes from the first percentage of PDAC-NDAC to the second, lower percentage of PDAC-NDAC, as shown by the solid line, the VTH voltage is pulled even lower, so that the Vsig voltage would have to go even lower to cause the Vout voltage to switch. Significantly however, this additional hysteresis is provided at a time t hat does not affect the switch points of Vout voltage. That is, by the time the Vsig voltage crosses the VTH threshold voltage again at time t9, the PDAC voltage has tracked the next positive peak of the Vsig voltage and the additional hysteresis no longer affects the VTH voltage. This is particularly advantageous in systems in which the switch points are required to be close to each other, but noise on the Vsig signal makes it desirable for the switch points to be far apart in order to avoid switching as a result of the noise. Referring also to Figure 28, the update controller 125 includes a positive portion 400 and a negative portion 402. The positive and negative controller portions are structurally substantially identical, but are responsive to different input signals and provide different output signals. In particular, the positive portion 400 is responsive to the Vout signal and generates the p_cnt_up signal for coupling to the NOR gate 108 and to the counter 114 of Figure 27 and the p_cnt_up_n signal for coupling to the counter 114. The negative portion 402 is responsive to the inverse of the Vout signal, Voutn, and generates the n cnt up signal for coupling to the NOR gate 112 and to the counter 130 of Figure 27 and the n cnt up n signal for coupling to the counter 130. The positive portion 400 of the update controller 125 includes a flip-flop 406 clocked by the Vout signal. A dcrp signal is provided at the Q output of the flip-flop 406 and is coupled to the D input to a flip-flop 408. Flip-flop 408 is clocked by the CLKN signal and provides the p_cnt_up signal at its Qbar output, as shown. The p_cnt_up signal is further coupled to an input of a NOR gate 410, the second input which receives the startup signal. The output of the NOR gate 410 is coupled to a reset input of a pair of flip- flops 416, 418 which are connected to form a ring counter 420. The outputs 422, 424 of the ring counter 420 are coupled to inputs of an AND gate 428. A pdone signal provided at the output of the AND gate 428 is coupled to an input of a NOR gate 430, the output of which resets the flip-flop 406. The negative controller portion 402 includes flip-flops 430, 432, flip-flops 434 and 436 forming ring counter 444, and gates 438, 440, and 442 coupled and arranged in the same manner as like elements of the positive portion 400. The operation of the update controller 125 will be illustrated by consideration of Figure 28 in conjunction with the illustrative Vsig waveform of Figure 31 and the Vout, dcrp, CLK, p_cnt_up, ring counter output and pdone signals of Figures 31 a - 31 f . In order to permit the PDAC and NDAC voltages to track the positive and negative peaks of the Vsig voltage, respectively, the update controller 125 enables the counters 114 and 130 at all times except as follows: (1) the counter 114 is disabled when the p_cnt_up signal is high which causes the counter 114 to count up and the PDAC voltage is greater than the Vsig voltage; and (2) the counter 130 is disabled when the n_cnt_up signal is high which causes counter 130 to count up and the NDAC voltage is less than the Vsig voltage. In this way, the PDAC voltage is held at the positive peak of the Vsig voltage and is prevented from exceeding the positive peak of the Vsig voltage and the NDAC voltage is held at the negative peak of the Vsig voltage and is prevented from falling below the negative peak of the Vsig voltage. At all other times, the counters 114 and 130 are enabled. The PDAC voltage is decremented by a predetermined amount upon each positive- going transition of the Vout voltage and the NDAC voltage is incremented by a predetermined amount upon each negative-going transition of the Vout voltage. To this end, when Vout transitions to a logic high level (and once the startup signal transitions, indicating that a certain time has lapsed since power up), the flip-flop 406 is set and causes the dcrp output signal to go high as shown in Figure 31b. Upon the next falling edge of the CLK signal (or rising edge of CLKN), flip-flop 408 causes the p_cnt_up signal to go low, as shown in Figure 3 Id. In its low state, the p_cnt_up signal commands the counter 114 (Figure 27) to count down. Further, the counter 114 is enabled since the p_cnt_up_n signal coupled to the NOR gate 108 (Figure 27) is high. The counter 114 thus counts down once per CLKN edge until the p_cnt_up signal transitions to a high level which this occurs once the ring counter 420 counts to a predetermined value (corresponding to the predetermined amount by which the PDAC voltage is to be decremented). More particularly, in its low state, the p_cnt_up signal causes the reset inputs to the ring counter flip-flops 416, 418 to be released. Thus, upon each rising edge of the CLK signal, the ring counter 420 increments. Once the ring counter 420 counts to three, the pdone output signal of AND gate 428 goes high, thereby causing the flip-flop 406 to be reset which, in turn, causes the dcrp signal to transition to a low level. Upon the next falling edge of the CLK signal, the p_cnt_up signal transitions to a high level, thereby causing the counter 114 to count up. The rising edge of the p_cnt_up signal further causes the ring counter 420 to be reset and thus, the pdone signal to return to its low level, as shown in Figure 31 f. Conversely, when Vout transitions to a logic low level, the flip-flop 430 is set and causes the dcrn signal to go high, as shown in Figure 3 lg. Upon the next rising edge of CLKN, flip-flop 432 causes the n_cnt_up signal to go low, as shown in Figure 31i. In its low state, the n_cnt_up signal commands the counter 130 (Figure 27) to count down. Further, the counter 130 is enabled since the n_cnt_up_n signal coupled to the NOR gate 112 (Figure 27) is high. The counter 130 thus counts down to increment the NDAC voltage until the n_cnt_up signal transitions to a high level which occurs once the ring counter 444 counts to three. More particularly, in its low state, the n_cnt_up signal causes the reset inputs to the ring counter flip-flops 434, 436 to be released. Thus, upon each rising edge of the CLK signal, the ring counter 444 increments. Once the ring counter 444 counts to three, the ndone output signal of AND gate 438 goes high, causing the flip-flop 430 to be reset which, in turn, causes the dcm signal to transition to a low level. Upon the next falling edge of the CLK signal, the n cnt up signal transitions to a high level, causing the counter 430 to count up. The rising edge of the n_cnt_up signal further causes the ring counter 444 to be reset and thus, the ndone signal to return to its low level as shown in Figure 31k. In certain instances, jitter on the Vsig signal may cause concern about unnecessary and inaccurate Vout switching. This scenario could result if noise on the Vsig signal causes the Vsig signal to bounce above and below the VTH threshold voltage. One way to eliminate this possibility is to slow down the CLK signal. In this way, the relatively high frequency noise will not cause the Vout voltage to switch. Another alternative for eliminating the deleterious effects of relatively high frequency Vsig noise is to add an integrator to the comparators 100 and 104 (Figure 27). Referring to Figure 32, illustrative comparator 100 is shown to include an integrator comprising a resistor 103 and a capacitor 105 coupled in feedback relationship with an operational amplifier 107. The output of the operational amplifier 107, Vint, is coupled to a buffer 109 with hysteresis to provide the comparator output signal (which is coupled to an input of NOR gate 108 (Figure 27)). Referring also to Figure 33, when a noisy Vsig is processed by the comparator 100 (Figure 32), the noise is integrated and the Vout voltage does not switch until the Vint voltage exceeds the VTH voltage, as shown. It will be appreciated by those of ordinary skill in the art that the above-presented jitter solutions can be implemented with any of the detector embodiments described herein. Referring to Figure 34, a further magnetic-article detector is shown. The detector of Figure 34 is, in part, identical to the detector of Figure 27 in which like reference numbers refer to like components. Most significantly however, the manner in which the PDAC and NDAC voltages are updated is different than in the detector of Figure 27 and thus, the update controller 510 of Figure 34 is different than the update controller 125 of Figure 27. In the detector of Figure 34, upon each transition of the Vout signal, one of the PDAC and NDAC voltages is updated, or reset to the level of the Vsig voltage, following which the one of the PDAC and NDAC voltages is allowed to follow Vsig up to the next positive peak or down to the next negative peak, respectively. More particularly, the Vout signal becomes a first binary level when Vsig rises to exceed the VTH threshold voltage and a second binary level when Vsig falls to below the threshold voltage. Upon each transition of the Vout signal from the second binary level to the first binary level, the PDAC voltage is updated to the level of the Vsig voltage and is thereafter allowed to follow the Vsig voltage up to the next positive peak value of Vsig. Likewise, upon each transition of the Vout signal from the first binary level to the second binary level, the NDAC voltage is updated to the level of the Vsig voltage and is thereafter allowed to follow the Vsig voltage down to the next negative peak of Vsig. The detector of Figure 34 provides the same advantages as described above in conjunction with the detector of Figure 27. Namely, the detector provides a relatively simple and robust circuit for updating the PDAC and NDAC voltages in order to maintain the VTH threshold voltage at a percentage of the peak-to-peak Vsig voltage, even as the peak-to-peak Vsig voltage varies. Further, additional hysteresis is provided, the detector has a fast response time and, circuit manufacture is simplified as compared to updating the PDAC and NDAC voltages in response to complicated decisions. As is noted above in connection with the other magnetic-article detector embodiments, the techniques and apparatus for updating the PDAC and NDAC voltages described in connection with Figures 34-39 may be implemented in the form of, or to include mid-signal detectors in which the threshold voltage is a fixed percentage of the peak-to-peak Vsig voltage or peak-referenced detectors in which the threshold voltage is a predetermined voltage away from the peaks and valleys of the Vsig voltage. As noted, like reference numbers in the detectors of Figures 27 and 34 refer to like elements, although a few of the signal names differ. In particular, in Figure 34, the counter 114 is enabled by a logic low P_HOLD signal and the count direction is controlled by a P_UPDN signal. The counter 114 counts up when the P UPDN signal is high. The counter 130 is enabled by a logic low N_HOLD signal and counts up in response to a low NJJPDN signal. Both counters are reset by a DAC_RESET_N signal. An inverter 500 inverts the COMP_P output signal of comparator 100 to provide a COMP_P_N signal and an inverter 504 inverts the COMP N output signal of comparator 104 to provide a COMP N N signal, as shown. The resolution of the counters 114, 130 and DACs 118, 134 may be varied to suit a particular application. As one example, the counters 114, 130 and DACs 118, 134 may be 9 bit devices. The update controller 510 is responsive to the Vsig signal and a CLKN signal (the falling edge of a CLK signal provided by an oscillator, such as a 1MHz oscillator). The update controller 510 is further responsive to the POSCOMP signal (i.e., Vout) from comparator 160, the POSCOMP N signal (i.e., Voutn) from inverter 164, the COMP P N signal from inverter 500, and the COMP_N_N signal from inverter 504, as shown. A positive portion of the update controller 510 for generating the P_UPDN and P HOLD signals for the counter 114 is shown in Figure 35 and a negative portion of the update controller for generating the N_UPDN and N HOLD signals for counter 130 is shown in Figure 36. In the illustrative embodiment, the update controller 510 is implemented with a state machine. Before considering the implementation of the update controller 510 shown in Figures 35 and 36, the manner in which the PDAC and NDAC voltages are updated will be described with reference to the waveforms of Figures 38, 38a, 39 and 39a. Figure 38 shows an illustrative Vsig signal along with the PDAC, NDAC, and VTH voltages generated by the detector of Figure 34 in response to the Vsig signal. Figure 38a shows the resulting Vout signal of the detector of Figure 34. Figure 39 shows another illustrative Vsig signal and the resulting PDAC and NDAC voltages and Figure 39a shows the resulting Vout signal from the detector of Figure 34. Upon each transition of the Vout signal, the PDAC and NDAC voltages are decremented and incremented, respectively, to the Vsig voltage level. Stated differently, the PDAC 118 and NDAC 134 are reset to the level of the Vsig signal. In particular, the PDAC voltage is decremented to the value of the Vsig voltage upon each transition of the Vout signal of a first polarity and the NDAC voltage is incremented to the level of the Vsig voltage upon each transition of the Vout signal of the second, opposite polarity. In the illustrative embodiment, the PDAC voltage is decremented to the value of Vsig as labeled 512 upon each positive-going transition of Vout as labeled 517 and the NDAC voltage is incremented to the value of Vsig as labeled 514 upon each negative-going transition of Vout as labeled 513. Thereafter, the PDAC and NDAC voltages are allowed to track the next positive and negative peak, respectively, of the Vsig voltage. That is, beginning at a time labeled 516, the PDAC voltage tracks Vsig up to the next positive peak which occurs at a time labeled 518 and, beginning at a time labeled 515, the NDAC voltage tracks Vsig down to the next negative peak which occurs at a time labeled 519. Recall that the VTH voltage is a percentage of the difference between the PDAC and NDAC voltages. And, in particular, VTH is at a first percentage of the difference between the PDAC and NDAC voltages, such as 60%>(PDAC-NDAC), when the Vsig voltage is lower than the threshold voltage (before time 100 microseconds, for example) and is at a second, lower percentage of the difference between the PDAC and NDAC voltages, such as 40%(PDAC-NDAC), when the Vsig voltage is higher than the threshold voltage (between approximately 110-210 microseconds, for example). Referring also to Figures 38b, 38c, 38d, and 38e, the counter 130 is disabled by the logic high N_HOLD signal until the negative-going transition of the Vout signal labeled 513. Thereafter, the N_HOLD signal goes low enabling the counter 130 until the Vsig signal exceeds the NDAC voltage. Note that the N_UPDN signal transitions to a high level causing the counter 130 to count down once the NDAC voltage reaches the Vsig voltage at time 515. The N_HOLD signal toggles up and down causing the NDAC voltage to step down to track the Vsig signal down to the next negative peak occurring at time 519, as shown. Similarly, the counter 114 is disabled by the logic high P HOLD signal until the positive-going transition of the Vout signal labeled 517. Thereafter, the P_HOLD signal goes low enabling the counter 114 until the Vsig signal exceeds the PDAC voltage. Note that the P_UPDN signal transitions to a high level causing the counter 114 to count up once the PDAC voltage reaches the Vsig voltage at time 516. The P HOLD signal toggles up and down causing the PDAC voltage to step up to track the Vsig voltage to the next positive peak occurring at time 518, as shown. Referring also to the waveforms of Figures 39 and 39a, start up operation of the detector is illustrated. In the illustrative embodiment, the update controller 510 is disabled for a predetermined number of transitions of the Vout signal following start up of the detector in order avoid conflicts between automatic gain control (AGC) and the threshold update scheme. In general, an AGC technique is implemented in which the gain of the amplifier 12 (Figure 34) is adjusted at start up according to the level of the Vsig signal. This is accomplished by resetting the DACs 118, 134. Since the threshold update scheme by which the PDAC and NDAC voltages track the Vsig voltage also cause resetting of the DACs 118, 134, disabling the threshold update scheme during the interval of operation of the AGC avoids conflicting control of the DACs 118, 134. In the illustrative embodiment, the threshold update scheme is disabled for three transitions of the Vout signal. Thus, the PDAC voltage is not updated until the time labeled 530 and the NDAC voltage is not updated until the time labeled 540. Thereafter, and as discussed above in conjunction with Figures 38 and 38a, upon each transition of the Vout signal, one of the PDAC and NDAC voltages is updated to the level of the Vsig signal, following which the updated one of the PDAC and NDAC voltages tracks Vsig back to the next positive peak or negative peak voltage, respectively. This operation occurs for the PDAC voltage at times labeled 530, 532, 534, and 536 and for the NDAC voltage at times labeled 540, 542, 544, 546, and 548. Referring to Figure 37, a first state diagram 550 illustrates operation of the positive portion of the update controller 510 (Figure 35) and a second state diagram 560 illustrates operation of the negative portion of the update controller 510 (Figure 36). The first digit of a particular state (e.g., 0 for state 00, 0 for state 01, 1 for state 10, and 1 for state 11) indicates whether or not the HOLD input to the respective counter 114, 130 (Figure 34) is asserted and the second digit indicates whether or not the UPDN input to the respective counter is asserted. Considering the state diagram 550 for the positive portion of the update controller, in state 00, the counter 114 is enabled and counts down (i.e., the P_UPDN signal is low and the PJHOLD signal is low). The update controller 510 is in this state during time interval 512 in Figure 38. Once the Vsig signal exceeds the PDAC voltage, as occurs at time 516 in Figure 38, the state machine transitions to a state 01 in which the counter 114 is enabled and counts up (i.e., the P UPDN signal is high and the P_HOLD signal remains low). Once the PDAC voltage exceeds the Vsig signal, the state machine transitions to state 11 in which the P_HOLD signal is high, disabling the counter 114 and the P UPDN signal is high. The state machine remains in state 11 and the NDAC voltage is held constant until either the Vsig voltage exceeds the PDAC voltage again or the Vout signal transitions low. If the Vsig voltage exceeds the PDAC voltage, the state machine returns to state 01 and the counter 114 continues to count up. The state machine remains in this loop, transitioning between states 01 and 11, as the PDAC voltage climbs from the level of the Vsig signal to the next positive peak (i.e., between times 516 and 518 in Figure 38). When the Vout signal transitions to a low level, a state 10 is entered in which the P HOLD signal remains high, disabling the counter 114 and the P_UPDN signal is low. Once the Vout signal transitions to a high level, then state 00 is again entered and the counter 114 counts down, thereby resetting the PDAC voltage to the Vsig voltage level. It will be appreciated by those of ordinary skill in the art that state 10 could be omitted (as indicated by the dotted line). In this case, the state machine transitions from state 11 directly to state 00 upon positive-going transitions of the Vout signal. The state diagram 560 for the negative portion of the update controller 510 is symmetrical, but opposite in polarity with respect to the state diagram 550. In particular, in state 00, the counter 130 is enabled and counts up (i.e., the N_UPDN signal is low and the N HOLD signal is low). The update controller 510 is in this state 00 during time interval 514 in Figure 38. Once the Vsig signal is less than the NDAC signal, as occurs at time 515 in Figure 38, the state machine transitions to a state 01 in which the counter 130 is enabled and counts down (i.e., the N_UPDN signal is high and the N HOLD signal remains low). Once the Vsig signal exceeds the NDAC voltage, as occurs at time 519 in Figure 38, the state machine transitions to a state 11 in which the counter 114 is disabled and the N_UPDN signal is high. The state machine remains in state 11 and the NDAC voltage is held constant until either the Vsig voltage again becomes less than the NDAC voltage or the Vout signal transitions high. When the Vsig voltage falls to below the NDAC voltage, the state machine returns to state 01 and the counter 130 continues to count down. The state machine remains in this loop, transitioning between states 01 and 11, as the NDAC voltage falls from the level of Vsig to the next negative peak (i.e., between times 515 and 519 in Figure 38). When the Vout signal transitions to a high level, a state 10 is entered in which the N HOLD signal remains low, disabling the counter 130 and the N_UPDN signal is high. Once the Vout signal transitions to a low level, then state 00 is again entered and the counter 130 counts up, thereby resetting the NDAC voltage to the Vsig voltage level. Here again, it will be appreciated by those of ordinary skill in the art that state 10 could be omitted (as indicated by the dotted line). In this case, the state machine transitions from state 11 directly to state 00 upon negative-going transitions of the Vout signal. It will be appreciated by those of ordinary skill in the art that the state diagrams 550, 560 (or as modified by the dotted lines in state diagrams 550, 560) could be implemented with various circuitry. One suitable implementation is shown in Figure 35 for the state diagram 550 and in Figure 36 for the state diagram 560. In particular, the P_HOLD and P_HOLD_N signals are generated with NAND gates 570, 572, 574, and flip-flop 576. The gate 570 is responsive to the PJJPDN signal and the COMP P N signal provided at the output of inverter 500 (Figure 34). Gate 572 is responsive to the P_HOLD, P_UPDN_N, and POSCOMP_N signals. The outputs of gates 570 and 572 are coupled to inputs of gate 574 which provides, at its output, the D input to flip-flop 576. The flip-flop 576 is clocked by the CLKN signal and reset by a DAC_RESET_N signal upon start up of the detector. The P_UPDN and P_UPDN_N signals are generated by NAND gates 580, 582, 584, and 586, AND gate 588, and flip-flop 590, as shown. In particular, gate 580 is responsive to the P_HOLD and P_UPDN_N signals, gate 582 is responsive to the P_HOLD, COMP P N, and POSCOMP_N signals, and gate 584 is responsive to the P_UPDN_N and COMP_P_N signals, as shown. The outputs of gates 580, 582, and 584 are coupled to inputs of gate 586, the output of which provides an input to an AND gate 588. A further input to the gate 588 is provided by an OUTPUT_CNT4_LATCH signal which enables the positive portion of the update controller following a predetermined number of transitions of the Vout signal following detector start up, as described above. The output of gate 588 provides the D input to flip-flop 590. Flip-flop 576 is clocked by the CLKN signal and is reset by a DAC_RESET_N signal upon start up of the detector. The negative portion of the update controller state machine 510 is shown in Figure 36 and is substantially similar to the positive portion of Figure 35. The components in Figure 36 have like reference numbers, but with a prime, with respect to like components in Figure 35. Having described the preferred embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may be used. It is felt therefore that these embodiments should not be limited to disclosed embodiments, but rather should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incoφorated herein by reference in their entirety. What is claimed is:

Claims: What is claimed is: 1. A magnetic article proximity detector comprising: (a) a magnetic field sensor providing a Vsig voltage proportional to a magnetic field; (b) a comparator for comparing said Vsig voltage to a threshold voltage to generate a detector output voltage that becomes a first binary level when said Vsig voltage exceeds said threshold voltage and a second binary level when said Vsig voltage falls to below said threshold voltage; and (c) a threshold voltage generator operative to generate said threshold voltage, wherein said threshold voltage generator comprises: (i) a first counter responsive to said Vsig voltage for providing a first count signal; (ii) a first digital-to-analog converter coupled to said first counter for converting said first count signal into a PDAC voltage, which PDAC voltage decreases for a first interval beginning at each transition of said detector output voltage from said second binary level to said first binary level and ending when said PDAC voltage reaches said Vsig voltage, and which PDAC voltage increases for a second interval beginning at the end of said first interval and ending when said Vsig voltage reaches a positive peak; (iii) a second counter responsive to said Vsig voltage for providing a second count signal; and (iv) a second digital-to-analog converter coupled to said second counter for converting said second count signal into an NDAC voltage, which NDAC voltage increases for a third interval beginning at each transition of said detector output voltage from said first binary level to said second binary level and ending when said NDAC voltage reaches said Vsig voltage, and voltage NDAC voltage decreases for a fourth interval beginning at the end of said third interval and ending when said Vsig voltage reaches a negative peak. 2. The magnetic article proximity detector of claim 1 wherein said first count signal is incremented when said PDAC voltage is less than said Vsig voltage. 3. The magnetic article proximity detector of claim 1 wherein said second count signal is incremented when said NDAC voltage is greater than said Vsig voltage. 4. The magnetic article proximity detector of claim 1 wherein said threshold voltage generator further comprises a resistor divider coupled between an output of said first digital-to-analog converter and an output of said second digital-to-analog converter. 5. The magnetic article proximity detector of claim 1 wherein said threshold voltage generator is operative to provide said threshold voltage at a first percentage of the peak-to- peak Vsig voltage when said Vsig voltage is greater than the threshold voltage and at a second percentage of the peak-to-peak Vsig voltage when said Vsig voltage is less than the threshold voltage. 6. The magnetic article proximity detector of claim 5 wherein said first percentage of the peak-to-peak Vsig voltage is less than said second percentage of the peak-to-peak Vsig voltage. 7. The magnetic article proximity detector of claim 1 further comprising a state machine for generating a first control signal for coupling to said first counter and a second control signal for coupling to said second counter. 8. A method of detecting passing magnetic articles comprising the steps of: sensing an ambient magnetic field and generating a Vsig voltage proportional to the magnetic field; generating a PDAC voltage as a function of positive peak values of said Vsig voltage; generating an NDAC voltage as a function of negative peak values of said Vsig voltage; generating a threshold voltage as a percentage of the difference between the PDAC and NDAC voltages; generating a detector output voltage that becomes a first binary level when said Vsig voltage exceeds the threshold voltage and a second binary level when said Vsig voltage falls to below the threshold voltage; upon each transition of said detector output voltage from said second binary level to said first binary level, updating said PDAC voltage to the level of said Vsig voltage and allowing said PDAC voltage to follow said Vsig voltage up to the next positive peak value; and upon each transition of said detector output voltage from said first binary level to said second binary level, updating said NDAC voltage to the level of said Vsig voltage and allowing said NDAC voltage to follow said Vsig voltage down to the next negative peak value, wherein said threshold voltage is a first percentage of the difference between the PDAC and NDAC voltages when said Vsig voltage is lower than said threshold voltage and wherein said threshold voltage is a second, lower percentage of the difference between the PDAC and NDAC voltages when said Vsig voltage is higher than said threshold voltage. 9. The method of claim 8 wherein said PDAC voltage generating step comprises the steps of: a) counting with a counter when said PDAC voltage is less than said Vsig voltage to provide a first count signal; and b) converting said first count signal to an analog signal to provide said PDAC voltage. 10. The method of claim 8 wherein said NDAC voltage generating step comprises the steps of: a) counting with a counter when said NDAC voltage is greater than said Vsig voltage to provide a second count signal; and b) converting said second count signal to an analog signal to provide said NDAC voltage.

Inventors: FORREST GLENN A, SCHELLER P KARL, FORREST, GLENN, A., SCHELLER, P., KARL
Applicants: ALLEGRO MICROSYSTEMS INC, ALLEGRO MICROSYSTEMS, INC.
Application year: 2001
Main classification: G01D5
IPC codes: G01D5, G01D18, G01P3, G01R33, G01V9