Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
1999-08-27
2002-09-10
Strecker, Gerard R. (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C324S11700H, C324S207120, C324S207200, C324S225000, C327S511000
Reexamination Certificate
active
06448768
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a magnetic sensor, and more particularly to a magnetic sensor with a signal processing circuit, which is usable as a sensor such as a proximity switch, current sensor, or encoder.
BACKGROUND ART
As a magnetic sensor with a signal processing circuit, a Hall IC is well known which employs a Hall element as its sensor. As a typical conventional Hall IC, a silicon (Si) monolithic Hall IC (referred to as “Si Hall IC” from now on) has a magnetic sensor section in the form of a Hall element made of silicon (Si), and a signal processing IC section for processing a signal detected by the magnetic sensor section.
This type of the magnetic sensor has a low sensitivity to magnetic field because the magnetic sensor section of the Si Hall IC consists of the Hall element made of Si with a small electron mobility. Therefore, to operate the Hall IC as a magnetic sensor, large magnetic field must be applied to it. In other words, the Hall IC has a problem of a low sensitivity to magnetic field.
In addition, it is known that Si generates some voltage when mechanical stress is applied from outside.
Thus, the Si Hall IC has another problem of varying its sensitivity to magnetic field because of the voltage generated in the Hall element of the magnetic sensor section when external stress is applied.
Such problems must be considered when fabricating highly accurate, highly reliable proximity switches, current sensors or encoders by using the Si Hall IC.
Therefore, a magnetic sensor with a signal processing circuit is desired which can achieve accurate detection of a magnet position or magnetic field strength with a high sensitivity independent of external stress and with stable characteristics. Such a magnetic sensor has not yet been realized because of great difficulty.
On the other hand, various methods are studied to achieve highly accurate detection of the magnet position or magnetic field strength. For example, sensors, which use a Hall element as their magnetic sensor and include a signal processing circuit composed of discrete components such as an operational amplifier or resistors, are applied to the proximity switches, current sensors or encoders.
In these methods, however, users are required to have special technical expertise to understand the characteristics of the sensor, to implement optimum circuit design, and to acquire discrete components and assemble them. In addition, it is unavoidable that their cost and size increase because the sensors are implemented by mounting on a circuit board the magnetic sensor element and the signal processing circuit consisting of the discrete components. The cost and size increase presents a critical problem in the field of sensors that requires low cost and small size.
For example, in a conventional technique as shown in
FIG. 12
of Japanese Patent Application Laid-open No. 38920/1990, the signal processing circuit comprises magnetoresistive elements
60
and
70
constituting a discrete magnetic sensor, resistors
6
,
7
and
7
′ and an operational amplifier
51
. The feedback resistance is composed of the combined resistance of the resistors
6
,
7
and
7
′ with different temperature coefficients to form a feedback loop from the output terminal of the operational amplifier
51
to its inverting input terminal, thereby implementing magnetic characteristics with a desired temperature characteristic. This circuit configuration, however, has the foregoing problem of increasing the cost and size. Besides, the circuit configuration has another problem of decreased yield when obtaining an intended output because of the variations of the output voltage due to the variations in the midpoint potential of the magnetoresistive elements
60
and
70
. Furthermore, since the midpoint potential usually drifts in accordance with temperature, the drift appears in the output voltage of the circuit, and has an adverse effect on the temperature characteristic of the output signal of the sensor. Thus, it is difficult for conventional circuits to obtain the desired temperature characteristic.
In addition, although the configuration of
FIG. 12
can freely use the discrete resistors
6
,
7
and
7
′ with different temperature coefficients to implement the magnetic characteristics with the desired temperature characteristic, it discloses nothing about the implementation of a circuit like the Si monolithic IC.
Still another problem arises in that a common conventional Si Hall IC as shown in
FIG. 14
, which includes a signal processing circuit section
20
a
and a magnetic sensor section
30
a
that are electrically isolated from a substrate
21
a
only through the PN junction, for example, cannot perform stable operation in an ambient temperature above 125° C., and cannot operate at all beyond 150° C.
On the other hand, a technique is known which improves the temperature characteristics by reflecting the temperature dependence of the output resistance of a Hall element to a threshold voltage by employing the output resistance of the Hall element as the input resistance of a Schmitt trigger circuit. Specifically, in a circuit configuration as shown in
FIG. 12
, the threshold voltage Vth of the Schmitt trigger circuit is expressed as Vth=(Vdo−V
1
)·Rho/RF, where V
1
is the potential at the inverting input terminal of the operational amplifier
51
; RF is the feedback resistance; Vdo is the output potential of the amplified output signal
18
of the operational amplifier
51
; and Rho is half the output resistance of the Hall element
4
(Japanese Patent Application Laid-open No. 226982/1986).
Here, the potential V
1
causes a problem. Since the potential V
1
, which is the output potential of the Hall element
4
, is about half the product of the input resistance Rhi of the Hall element
4
and the Hall element driving current Ic, the variations in Rhi causes the variations in V
1
. This in turn causes the variations in Vth, which makes it impossible to establish the threshold voltage exactly at a value designed. This results in the Hall IC with magnetic characteristics different from those designed, thereby reducing the yield.
The potential at the output terminal of the Hall element
4
, which equals about half the input voltage to the Hall element, is referred to as a midpoint potential of the Hall element. The value has certain distribution due to the production variations of the Hall elements. The variations in the midpoint potential also cause the variations of V
1
, resulting in the reduction in the yield.
DISCLOSURE OF THE INVENTION
The inventors of the present invention intensively conducted the research to implement practical magnetic sensors capable of solving the foregoing problems of the magnetic sensor.
We aim to fabricate a highly sensitive, stable operation magnetic sensor with a signal processing circuit by forming the magnetic sensor with a structure of isolating the magnetic sensor section from the signal processing circuit consisting of a Si IC.
To implement a highly sensitive magnetic sensor with a signal processing circuit with stable operation characteristics, we study a magnetic sensor with a signal processing circuit that combines the signal processing circuit with a highly sensitive magnetic sensor composed of a compound semiconductor thin film or a magnetic thin film, which has a higher sensitivity in the magnetic field than the Si Hall element and can provide a stable magnetic sensor output independently of the mechanical external stress.
As a result, the inventors of the present invention invented a hybrid Hall IC which employed the compound semiconductor as the sensor, and combined it with a Si monolithic IC to be packed in a single package.
The present invention can implement a versatile, inexpensive, small size, high performance magnetic sensor with a signal processing circuit that does not require users to have any technical expertise such as special circuit technique, thereby making it possible to achieve detection of a magnet position or magnetic field
Ishibashi Kazutoshi
Shibasaki Ichiro
Asahi Kasei Electronics Co. Ltd.
Pennie & Edmonds LLP
Strecker Gerard R.
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