Offset compensation for magnetic-field sensor with Hall effect device

The invention relates to a magnetic-field sensor with a Hall effect device, a power supply, and an evaluating facility, which can be supplied with a Hall signal from the Hall-effect device and comprises an input amplifier, a storage element, and a signal superposition unit. To improve the accuracy of the magnetic-field sensor, in a first phase, a balancing signal for balancing the measurement-signal path with respect to an interface can be produced with the evaluating facility, the balancing signal being storable in the storage element, and in a second phase, the balancing signal stored in the storage element can be applied through the signal superposition unit to the interface, where it is superimposed on a Hall signal.

FIELD OF THE INVENTION 
The present invention relates generally to magnetic-field sensors and more 
specifically to a magnetic-field sensor comprising a Hall-effect device, a 
power supply, and an evaluating facility which can be supplied with a Hall 
signal from the Hall-effect device. 
BACKGROUND OF THE INVENTION 
Magnetic-field sensors which incorporate Hall-effect devices are well known 
in the art. One example of a magnetic-field sensor is disclosed in 
European Patent application 0 548 391 A1 entitled OFFSET-COMPENSATED HALL 
SENSOR, by S. Mehrgardt et al. published on Jun. 30, 1993, and assigned to 
Deutsche ITT Industries the assignee herein. The magnetic-field sensor 
disclosed in 0 548 391 A1 comprises a Hall-effect device, a power supply, 
and an evaluating facility which can be supplied with a Hall signal from 
the Hall-effect device. The evaluating facility disclosed therein includes 
an input amplifier, a storage element, and a signal superposition unit. A 
magnetic-field sensor such as the one disclosed in 1445 is frequently 
implemented as a monolithic integrated circuit comprising the Hall-effect 
device, the voltage supply, and the evaluating facility. Such a combined 
circuit is generally fabricated using conventional silicon integrated 
circuit processing techniques, such as a bipolar or a MOS. 
The accuracy of such a magnetic-field sensor can be increased by 
compensating for the offset signal component of the Hall-effect device by 
superposition of a first and a second measurement signal. The offset 
signal component of the Hall-effect device is caused by mechanical 
stresses in the environment of the Hall-effect device, i.e., in the 
crystal structure of the monolithic component. Compensation is achieved 
during the determination of a first measurement signal, wherein terminal 
pairs of the Hall-effect device, which are connected to the power supply 
and the evaluating facility, are reversed with respect to the 
determination of a second measurement signal. Through the reversal of the 
terminal pairs, a changeover of the power-supply and the evaluation 
terminals of the Hall-effect device, henceforth called "terminal-pair 
changeover", is achieved. The geometry of the Hall-effect device and the 
terminal pairs causes the resulting useful signal components of the 
measurement signals before and after the terminal-pair changeover to be in 
phase, whereas the resulting offset signal components of the Hall-effect 
device are opposite in phase to one another. By adding the measurement 
signals produced before and after the changeover, which is done in the 
evaluating facility, the offset signal component of the Hall-effect device 
is eliminated. To simulate this effect, a Hall plate is thought of, to a 
first approximation, as a resistance bridge which is balanced in the 
presence of a magnetic field. The offset signal component results from 
resistance changes caused in the crystal of the monolithic component by 
piezoelectric effects and from lithography inaccuracies, etc. 
Although the offset signal component of the Hall-effect device is 
compensated for in such magnetic-field sensors, the accuracy of these 
sensors is still reduced by the offset signal components of the electronic 
components in the evaluating facility. For example, when the evaluating 
facility adds the first and second signals to compensate for the offset 
signal component of the Hall-effect device, the offset signal component of 
the input amplifier of the evaluating facility is added. 
It is, therefore, a primary object of the present invention to provide an 
improved magnetic-field sensor which displays substantially greater 
accuracy when compared with prior art magnetic-field sensors. 
SUMMARY OF THE INVENTION 
The object the present invention is accomplished by providing a 
magnetic-field sensor comprising a Hall-effect device, a power supply, and 
an evaluating facility including an input amplifier, a storage element, 
and a signal superposition unit. The magnetic-field sensor of the present 
invention operates in a first phase, wherein a balancing signal for 
balancing a measurement-signal path with respect to an interface, is 
producible with the evaluating facility, the balancing signal being 
storable in the storage element. The magnetic-field sensor further 
operates in a second phase, wherein the balancing signal stored in the 
storage element is feedable via the signal superposition unit to the 
interface, where it is superimposable on a Hall signal (second Hall 
signal). 
In the arrangement according to the present invention, the offset signal 
components of the evaluating facility, particularly those of the input 
amplifier and the superimposition unit, are additionally eliminated. In 
the first phase, the evaluating facility is offset-nulled, so to speak. 
This improves the measurement accuracy of the magnetic-field sensor.

DETAILED DESCRIPTION OF THE INVENTION 
As described above, the magnetic-field sensor of the present invention 
generally comprises a Hall-effect device, a power supply, and an 
evaluating facility including an input amplifier, a storage element, and a 
signal superposition unit. The magnetic-field sensor of the present 
invention exhibits a substantial increase in measurement accuracy over 
prior art sensors in the following manner. A first Hall signal is feedable 
to an interface in a first phase, and a second Hall signal is producible 
by inverting the first Hall signal. Through this measure, the useful 
signal component of the measurement signal doubles, while the offset 
signal component of the evaluating facility is compensated for. 
Advantageously, the inversion of the Hall signal can be effected by a 
terminal-pair changeover of the Hall-effect device. In that case, the 
Hall-effect device is symmetrical with respect to two orthogonal axes as 
regards its resistance, and has two correspondingly symmetrical terminal 
pairs which are alternately connectable via a switching device, to the 
power supply and the evaluating facility during the first phase and a 
second phase, with the polarity of the Hall-voltage taps being reversed if 
necessary. Thus the offset component of the Hall-effect device itself can 
be eliminated simultaneously with the offset components of the evaluating 
facility, particularly with those of the input amplifier and the 
superimposition unit. 
Referring now to FIG. 1 a block diagram of an exemplary embodiment of the 
magnetic-field sensor according to the present invention is shown. The 
magnetic-field sensor comprises a Hall-effect device 1 with two pairs of 
terminals 2, 3. The terminal pairs 2, 3 can be connected alternately to a 
power supply 5 and an evaluating facility 6. In this embodiment, the power 
supply 5 is a voltage source. The input amplifier and the superposition 
unit of the evaluating facility 6 include a first transconductance 
amplifier 7 and a second transconductance amplifier 8. The 
transconductance amplifiers 7 and 8 are so arranged that their output 
currents flow to common nodes 9 and 10 for forming the difference between 
them. The transconductance amplifiers have differential inputs and 
outputs. This configuration is chosen here to obtain a symmetrical 
arrangement of the evaluating facility 6, which is operated in a push-pull 
mode, i.e., differential mode. This symmetry serves to eliminate any 
interference that may be caused, for example, by external signals. This 
arrangement converts the measurements signals obtained from the 
Hall-effect device 1 in the first and second phases, which contain the 
Hall voltage and the offset voltage of the Hall-effect device, into a 
first current signal and a second current signal by means of the 
transconductance amplifier. In response to the first current signal, a 
voltage is stored in the storage element and converted by the second 
transconductance amplifier 8 back into a current signal. Since the second 
transconductance amplifier 8 supplies a current directed opposite to the 
current of the first transconductance amplifier 7, in the second phase, 
the first current signal and the second current signal are superimposed in 
the common nodes such that the useful signal components of the Hall-effect 
device add, while the offset components of the Hall-effect device and 
particularly of the first transconductance amplifier 7 neutralize. A 
current signal is thus obtained which corresponds to twice the value of 
the useful signal component of the Hall-effect device and is free from 
offset signal components. 
The second transconductance amplifier 8 advantageously has a lower 
transconductance than the first transconductance amplifier 7. To supply 
the same output current, a greater voltage is then necessary at the input 
of the second transconductance amplifier 8, this voltage being stored in 
the capacitors 11, 11'. As a result, a current signal provided by the 
second transconductance amplifier 8 has a correspondingly reduced 
interference susceptibility. Thus, any distortion of the useful signal 
component for determining the magnetic field due to interference signals 
at the second transconductance amplifier 8 or the superposition unit is 
reduced, whereby the accuracy of the magnetic-field sensor is further 
increased. For example, the transconductance of the second 
transconductance amplifier 8 can be lower than that of the first 
transconductance amplifier 7 by a factor of 50, so that the 
interference-signal components at the second transconductance amplifier 8 
will enter into the overall signal with a magnitude reduced by a factor of 
50, and thus will no longer reduce the accuracy of the magnetic-field 
sensor. 
To reduce the effect of such interference signals, it is essential that the 
first measurement signal is stored by the storage unit in amplified form 
and is output by the superposition unit (second transconductance amplifier 
8) in a correspondingly reduced form. Thus, any reduction of the accuracy 
of the magnetic-field sensor due to the storage unit and the superposition 
unit is also prevented in other embodiments of the magnetic-field sensor 
according to the invention in which no transconductance amplifiers are 
used in the input amplifier and the superposition unit. Thus, as shown in 
FIG. 1, connected to each input of the second transconductance amplifier 8 
is at least one capacitor 11, 11' which comprise the storage unit. Each 
capacitor enables the first signal to be stored in a simple manner. 
Between each output of the first transconductance amplifier 7 and the 
respective input of the second transconductance amplifier 8 located on the 
same symmetry side of the evaluating facility 6, a respective switching 
element 12, 12' is provided. The switching elements 12, 12' can be 
switched by a switching device 20. As will be described, the switching 
elements are closed by the switching device 20 during a third phase and 
open during the second phase, the third phase being ended within the first 
phase. The switching elements 12, 12' can be switched by a switching 
device, 20 so that the switching elements 12, 12' cause the first 
measurement signal to be stored in the capacitors 11, 11' in the first 
phase and the first and second measurement signals to be superimposed in 
the second phase. To avoid disturbing transient effects, the switching 
element 20 opens and closes during the first phase. 
The output of the evaluating facility 6 shown in FIG. 1, is formed by a 
comparator 13 in which the voltage U measured between the nodes 9 and 10 
is evaluated. The comparator enables the output signal to be compared with 
a reference signal in a simple manner. This makes it possible to 
positively determine, for example, whether the magnetic switch points, 
thus set, were exceeded. The reference value can be a unipolar signal; 
advantageously, it can also be a difference signal which is fed to the 
comparator 180.degree. out of phase through a symmetrical arrangement in 
the evaluating facility. Symmetrical arrangements operated in this manner 
are used particularly where major external interference has to be avoided 
during evaluation. The evaluating facility can also be designed to output 
the measurement result as an analog measurement signal or a digital data 
word. 
Referring still to FIG. 1, the output of the comparator 13 is connected to 
a holding circuit 14 for storing and passing on an output signal during 
the second phase. In the embodiment of FIG. 1, the holding circuit 14 is 
implemented with a latch. The signal from the evaluating facility is not 
transferred to the latch until the steady state is reached in the second 
phase. This prevents short-time and nonrelevent changes of state from 
affecting the output signal, i.e., from being falsely evaluated as signal 
changes. 
As further shown in FIG. 1, a current source 15 for generating a bias 
signal is applied between the respective terminal pair 2 or 3 which is 
connected to the evaluating facility 6. The magnetic-field then must 
exceed a given threshold value to be detected by the magnetic-field 
sensor. A switching means 16 is connected to the holding circuit 14. Via 
the switching means 16, a bias signal coming from the current source 15 is 
applied between the terminal pair 2 or 3 in response to a signal from the 
holding circuit 14 when the latter has passed on the output signal. Thus, 
the bias voltage is applied across the respective terminal pair when an 
output signal from the evaluating facility 6 has been registered. 
Therefore, the magnetic field must first change by a given value before 
the magnetic-field sensor will respond to this change. A hysteresis curve 
is thus obtained which prevents the magnetic-field sensor from constantly 
changing state, and a stable signal state is produced. 
A correction-signal output 17 of the current source 15 for outputting a 
signal dependent on the temperature of the Hall-effect device is coupled 
to a control input 18 of the voltage source 5 as shown in FIG. 1. In this 
arrangement, measurement errors due to the temperature dependence of the 
sensitivity of the Hall-effect device can be avoided. The signal applied 
from the voltage source 5 to the Hall-effect device must be changed by the 
current source so as to compensate for the temperature dependence of the 
Hall sensitivity. The current source 15 advantageously contains a 
reference resistance equivalent to the Hall-effect device 1. If the 
magnetic-field sensor is a monolithic component, the reference resistance 
equivalent to the Hall-effect device 1 will undergo the same changes due 
to temperature and process variations as the Hall-effect device. Thus, the 
correction signal of the current source 15 can be changed in accordance 
with the changes in the reference resistance, and since the correction 
signal is applied to the control input 18 of the voltage source 5, the 
signal applied between the respective terminal pair of the Hall-effect 
device 1 can be changed accordingly. This makes it possible to very 
reliably correct measurement errors of the magnetic-field sensor which are 
due to temperature and process variations. 
The operation of the magnetic-field sensor according to the invention is as 
follows. Via the switching unit 4, the terminal pairs 2, 3 of the 
Hall-effect device 1 are connected alternately to the voltage source 5 and 
the evaluating facility 6 during a first phase P1 and a second phase P2. 
For example, during the first phase P1, the terminal pair 2 is connected 
to the voltage source 5, and the terminal pair 3 to the evaluating 
facility 6. Then, in the second phase P2, the terminal pair 3 of the 
Hall-effect device is connected via the switching device 4 to the voltage 
source 5, and the terminal pair 1 to the evaluating facility 6. The clock 
signal S1 for switching to the first phase P1 is shown in FIG. 2 along 
with the clock signal S2 for switching to the second phase P2. In the 
first phase P1, the switching elements 12, 12' are closed. The clock 
signal S3 closing the switching elements 12, 12' is also shown in FIG. 2. 
The first measurement signal arriving at the evaluating facility 6 in the 
first phase P1 contains a first useful signal component and a first offset 
signal component of the Hall-effect device 1. This measurement signal is 
converted to a corresponding current signal in the first transconductance 
amplifier 7. The two different inputs of the first transconductance 
amplifier 7 produce mutually inverse signals of the same magnitude. These 
first measurement signals charge the capacitors 11, 11' through the closed 
switches 12, 12' during a phase P3. The clock signal necessary for this is 
S3 as shown in FIG. 2. It is chosen so that the charging time of the 
capacitors 11, 11' lies within the second phase P2, so that a stable state 
of the second phase is ensured. The second transconductance amplifier 8 
supplies a current directed opposite to the current of the first 
transconductance amplifier 7. These two oppositely directed currents are 
superimposed at the nodes 9 and 10 to give a zero total current, since 
they are equal in magnitude. With respect to the resulting differential 
current at the nodes, the balancing can also be referred to as "zero 
adjustment", since the automatic adjustment evaluates the magnitude and 
direction of the differential current as the controlled variable and 
varies the capacitor voltage 11, 11' until the differential current 
becomes zero. 
In the second phase P2, a second measurement signal is applied to the 
evaluating facility 6 via the switching device 4. Since in this phase P2 
the terminal pairs 2, 3 of the Hall-effect device 1 are reversed from the 
first phase P1, the second measurement signal contains a second, equally 
large useful voltage component, which is opposite in phase to the first 
useful voltage component, and a second offset signal component of the 
Hall-effect device, is in phase with and equal in magnitude to the first 
offset signal component of the Hall-effect device. When the second 
measurement signal passes through the first transconductance amplifier 7, 
a second offset signal component of the transconductance amplifier 7 and 
the superposition unit 8 is superimposed on it, this second offset signal 
component being in phase with the first offset signal component of the 
first transconductance amplifier 7 and the superposition unit 8, which was 
superimposed on the first measurement signal. Since the switching elements 
12, 12' are open during the second phase P2 (FIG. 2), the voltages of the 
first measurement signal stored in the capacitors 11, 11' are still 
applied at the second transconductance amplifier 8. Thus, the currents 
flowing from the second transconductance amplifier 8 to the nodes 9, 10 
are determined by the first measurement signal, whereas the currents 
supplied by the first transconductance amplifier 7 are determined by the 
second measurement signal. Since in the first and second phases P1, P2 the 
differential voltage U between the nodes 9, 10 contains the useful signal 
components as an in-phase component, and the offset signal components of 
the Hall-effect device as well as the offset signal components of the 
input amplifier and the superposition unit as antiphase components, the 
offset signal components average out of the overall signal U, whereas the 
useful signal components of the phases P1 and P2 acid together. The 
voltage detected in the comparator 13 thus corresponds to twice the useful 
signal component. A prerequisite for this measurement procedure is that 
the second measurement signal is measured in the presence of an unchanged 
magnetic field, so that the useful signal components are equal in 
magnitude. 
The symmetrical design of the evaluating facility 6 described here, in 
which the two symmetrical branches are traversed in push-pull, serves to 
eliminate frequent interferences. The circuit according to the invention 
can also be operated with only one branch at a time in which the 
respective transconductance amplifier 7, 8 has one current output, and in 
which only one capacitor 11, one switch 12, and a comparator 13 with a 
reference input are used. 
The transconductance amplifiers 7, 8 are chosen so that the second 
transconductance amplifier 8 has a distinctly lower transconductance than 
the first transconductance amplifier 7. This can be implemented by 
suitable choice of the w/1 ratios of the respective transistors. A typical 
ratio of the transconductances of the transconductance amplifiers 7 and 8 
is a factor of 50 as mentioned earlier. In that case, the susceptibility 
of the measurement-signal compensation to interference is reduced. 
When the Hall-effect device 1 senses a magnetic field, the signal presented 
to the comparator 13 changes. This signal is accepted in the fourth phase 
P4 as shown in FIG. 2. The signal of the second phase is passed on to the 
holding circuit 14, which corresponds to a latch. The holding circuit 14 
stores the signal and transfers to the output A. The storage and transfer 
of the signal are controlled by the clock signal S4 shown in FIG. 2. The 
time for which the signal is stored and must have a given value lies in 
the middle range of phase 2. This ensures that a steady state is already 
reached in the second phase, the evaluation phase, so that an actual 
signal is present rather than just short-time variations. 
The current source 15 serves to produce a hysteresis as described earlier. 
It provides a bias signal which is applied between the terminal pair 1, 2 
connected at the respective instant to the evaluating facility 6. This 
bias signal causes a magnetic field sensed by the Hall-effect device 1 to 
be registered only if this magnetic field exceeds a predetermined 
threshold value. The current source 15 is connected to the terminal pair 2 
or 3 via the switching means 16. The switching means 16 is switched by 
means of the signal delivered by the holding circuit 14 when the latter 
has passed on the output signal. As a result, the threshold magnetic field 
at the terminal pair assumes another value as soon as an output signal 
from the holding circuit 14 has been reliably detected. This prevents the 
evaluating circuit from switching to and fro due to small magnetic-field 
variations. 
To prevent errors due to the temperature and process-technology dependence 
of the Hall-effect device 1, which would distort the Hall voltage and thus 
falsify the measured magnetic-field value, the correction signal output 17 
of the current source 15 is coupled to the control input 18 of the power 
supply 5. The current supply 15 may contain a reference resistor made of 
the same material, and having the same temperature dependence and sheet 
resistivity, as the Hall-effect device 1. A change in the value of the 
reference resistor causes a change of the signal at the correction output. 
The magnetic-field sensor shown in the embodiment of FIG. 1 includes one 
Hall-effect device 1. However, in another exemplary embodiment of the 
invention, a second, laterally displaced Hall-effect device may be 
connected in parallel with, but in a direction opposite to, the first 
Hall-effect device with respect to the Hall voltage. The second 
Hall-effect device must then be geometrically identical to, or have the 
same resistance as, the first hall-effect device. Thus, the output signal 
of the two Hall-effect devices will respond only to a magnetic-field 
difference. Thus, the measurement result is not affected by magnetic 
interference fields existing at both Hall-effect devices. Accordingly, the 
magnetic-field sensor can also be used in an environment where external, 
dynamic or steady, relatively large magnetic fields are unavoidable. For 
example, the two Hall-effect devices will respond to a distant field, such 
as magnetic interference field caused by the starter or generator in an 
automobile, in the same manner, so that any response to such field can be 
avoided. Near fields, which are caused, for example, by moving a magnet 
past the Hall-effect devices, are then detected without superposition of 
distant-field components because of the differential arrangement of the 
two Hall-effect devices. This relates to both DC magnetic fields and 
alternating magnetic fields.