Magnetic detector for a frequency generator responsive to motor rotation

A magnetic detector, such as is used in a frequency generator, for detecting the rotational rate of a motor includes magnetic resistance elements and an operational amplifier. The magnetic resistance elements have a predetermined resistance ratio and are connected serially between a power source and ground. The common node between the two magnetic resistance elements serves as an output terminal for providing an output voltage. The output voltage from the output terminal being supplied to one of the two input terminals of the amplifier. The voltage from a resistance voltage divider disposed between the power source and ground is supplied to the other input terminal of the amplifier. A change of the resistance ratio between the two magnetic resistance elements is effected by a resistor disposed in series with the elements or in parallel with one of the elements. This change serves to equalize the DC component of the output voltage and the divided voltage and, consequently, common mode noise level effects of the power source are substantially eliminated.

BACKGROUND OF THE INVENTION 
This invention relates to a magnetic detector using a magnetic resistance 
element and, more particularly, the application of such detector as a 
frequency generator incorporated in a motor. 
BACKGROUND OF THE RELATED ART 
Frequency generators housed within a motor are used to detect the 
rotational rate of a motor. One type of such frequency generator comprises 
a ring magnet which is magnetized in such a manner that N and S poles 
occur at an interval alternately in the circumferential direction. The 
ring magnet is provided on a rotor. The frequency generator also includes 
a magnetic detector comprising a magnetic resistance element and an 
amplifier, etc. which outputs signals of the frequency proportionate to 
the rotational rate by detecting the magnetic poles of the above mentioned 
magnet. 
FIG. 9 shows the equivalent structure of a prior art magnetic detector 
circuit. In the figure, two magnetic resistance elements m1 and m2 are 
serially connected between a power source Vcc terminal and a ground 
terminal. Elements m1 and m2 have their respective resistance in a 1:1 
relationship, and are structured as a unilateral bridge. Their common node 
also acts as an output terminal, and the output voltage from this terminal 
is supplied to one of the input terminals of an operational amplifier 11 
via a capacitor C.sub.1. Between the power source Vcc and the ground is 
connected a resistance voltage divider comprising serially connected 
resistances r1 and r2. The voltage from this divider is supplied as a 
reference voltage Vref at the other input terminal of the amplifier 11. 
Resistance Rf sets the amplification of the amplifier 11 and is connected 
between the other input terminal and the output terminal of the amplifier 
11. 
Two magnetic resistance elements m1 and m2 are disposed so as to oppose the 
ring magnet that rotates integrally with a rotor of the motor but in a 
staggering phase in the circumferential direction. When the ring magnet 
rotates, sine wave signals are outputted from the common node of the two 
elements m1 and m2 and are supplied to one of the input terminals of the 
amplifier 11. The signals supplied to the other input terminal of the 
amplifier, on the other hand, are the voltage dividing signals or the 
reference voltages Vref of the resistance voltage divider comprising 
resistances r1 and r2. The amplifier 11 amplifies and provides the sine 
wave signals as an output at the node of the two magnetic resistance 
elements m1 and m2. 
According to the prior art magnetic detectors discussed above, any 
difference between the reference voltage Vref of the amplifier 11 
determined by the voltage ratio of resistances r1 and r2 and the mid-point 
voltage of the magnetic resistance elements m1 and m2 would cause the 
amplifier 11 to amplify the common-mode noises superposed on the power 
source line in correspondence to the difference between the reference 
voltage Vref and the mid-point voltage of the elements m1 and m2 as well 
as with the amplification factor of the amplifier 11. This adversely 
affected the output signals of the amplifier 11. The reasons for this are 
explained below. 
The above mentioned common-mode noises are those having the same phase 
among the noises carried on the voltage that creates the mid-point voltage 
of the elements m1 and m2 and the mid-point voltage Vref between the 
elements m1 and m2, and indicated by the letter e in FIG. 9. The 
common-mode noise e may be represented as superimposed on the power source 
voltage Vcc. Assuming that the common-mode noise appearing at the 
mid-point of the elements m1 and m2 is Km.multidot.e, the difference 
voltage .DELTA.e of the noises between these two mid points is expressed 
by the following equation: 
EQU .DELTA.e=(Kr-Km)e 
wherein 
EQU Km=m2/(m1+m2) 
EQU Kr=r2/(r1+r2) 
The amplification factor of the amplifier 11 is calculated. As shown in 
FIG. 10, when the output resistance of the magnetic resistance element is 
Rim, the amplification A of the amplifier 11 becomes 
EQU A=Rf/(Ri+Rim)(.times.2) 
The amplification A amplifies the above mentioned noise difference voltage 
.DELTA.e. In other words, noises present in the output of the magnetic 
detector are expressed as 
EQU .DELTA.e.multidot.A 
and the noise amplification N as 
EQU N=(.DELTA.e/e).multidot.A(.times.2) 
Noise is thus amplified and affects the output signals of the amplifier 11 
adversely. 
As is clear from the above going explanation, noises are amplified when 
there are differences between the DC component of the mid-point output 
voltage of the magnetic resistance elements m1 and m2 and the mid-point 
voltage of the resistances r1 and r2. When there are no such differences 
in voltage between these midpoints, noises are not amplified. Assuming the 
power source voltage Vcc to be 5 V, 4 V on the upper side is used as an 
effective range since 1 V on the lower side cannot be used. Thus, the 
mid-point voltage of the resistances r1 and r2 as the reference voltage 
Vref of the amplifier 11 is set at 3 V which is the midpoint between 1 V 
and 5 V. As an amplifier 11 is generally constructed of ICs, it is 
difficult to change the reference voltage Vref. On the other hand, the 
resistance ratio of the elements m1 and m2 is set at 1:1 as mentioned 
above in order to obtain the sine wave signals. Thus, the DC component of 
the mid-point output voltage of the elements m1 and m2 becomes 2.5 V or 
the midpoint of 5 V current source voltage Vcc. There is a difference of 
0.5 V between 3 V, the mid-point voltage of the resistances r1 and r2 and 
the DC component 2.5 V of the mid-point voltage of the elements m1 and m2. 
The noises are amplified by this difference in voltage by the above 
mentioned noise amplification N, affecting the output signals of the 
amplifier 22 adversely. The effect is greater if the amplification of the 
amplifier 11 is greater. 
SUMMARY OF THE PRESENT INVENTION 
A primary object of the present invention is to overcome these problems of 
the prior art. The present invention aims to offer a magnetic detector 
which decreases the influences of the common-mode noises superposed on the 
power source line by equalizing the DC component of the mid-point voltage 
of the magnetic resistance elements with the reference voltage of the 
amplifier. 
In accordance with the invention, a magnetic detector comprises two 
magnetic resistance elements, having a predetermined resistance ratio, 
connected serially between a power source and ground, and an output 
terminal connected to a common node of the two magnetic resistance 
elements for providing an output voltage. The output voltage from the 
output terminal is supplied to one of two output terminals of an 
operational amplifier. The voltage from a resistance voltage divider 
disposed between the power source and ground is supplied to the other 
input terminal of the amplifier. Resistance means are included for 
modifying the resistance ratio between the two magnetic resistance 
elements so as to equalize the DC component of the output voltage and the 
divided voltage. In this way, common mode noise level effects of the power 
source are substantially eliminated. 
For a better understanding of the present invention, reference is made to 
the following description and accompanying drawings while the scope of the 
invention will be pointed out in the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The embodiments of magnetic detector according to the present invention are 
now explained by referring to FIGS. 1 through 4. 
FIG. 1 shows a first embodiment wherein two magnetic resistance elements 
Rm1 and Rm2 are serially connected with magnetic resistor Rma between the 
power source Vcc and the ground. The resistances of the two elements have 
the relation of 1:1. Two elements RM1 and Rm2 and the resistor Rma are 
formed on the same chip (Chip MR). However, two elements Rm1 and Rm2 are 
sensitive to magnetism of the magnet disposed opposite to the chip MR, 
whereas the resistor Rma is disposed at a position that would not detect 
the magnetism of the magnet (such as a position not opposing the magnet), 
or at a position where magnetic lines would be formed perpendicular to the 
direction of magnetic lines of the elements Rm1 and Rm2. The two elements 
Rm1 and Rm2 are constructed as a unilateral bridge, using the node 
therebetween as an output terminal and the output voltage of the terminal 
is supplied to one of the input terminals of the operational amplifier 11 
via capacitor C1. On the other hand, there is connected a voltage divider 
comprising serially connected resistances r1 and r2 between the power 
source Vcc and the ground. The voltage of the resistance voltage divider 
is supplied to the other input terminal of the amplifier 11 as a reference 
voltage Vref. Between one of the input terminals and the output terminal 
of the amplifier 11 is connected a resistance R1 which determines the 
amplification factor of the amplifier 11. 
The magnetic resistor Rma is connected between the magnetic resistance 
element Rm2 and ground. The resistor Rma equalizes the DC component of the 
output voltage from the common node of the serially connected resistances 
Rm1 and Rm2 and the divided voltage formed by the resistances r1 and r2. 
Assuming, for instance, that the power source voltage Vcc is 5 V and the 
divided voltage of the reference voltage Vref formed by the resistances r1 
and r2 of the amplifier is 3 V, and assuming further that there exists no 
magnetic resistor Rma, the DC component of the output voltage from the 
node of the elements Rm1 and Rm2 becomes 2.5 V to make the voltage 
difference of 0.5 V. The magnetic resistor Rma is thus serially connected 
to two magnetic resistance elements Rm1 and Rm2 in order to eliminate the 
above mentioned difference in voltage that occurs as a result of a 
disturbed balance at the mid point of Rm1 and Rm2. The relation among 
resistances at that time is expressed as 
EQU Rm1:(Rm2+Rma)=r1:r2 
By connecting in series the two magnetic resistance elements Rm1 and Rm2 
with the magnetic resistor Rma to equalize the DC component of the output 
voltage of the elements Rm1 and Rm2 and the divided voltage of resistances 
r1 and r2, common-mode noises which are superposed on the current source 
line are not amplified by the amplifier 11. As the magnetic resistor Rma 
is formed on the same chip using the same material as the elements Rm1 and 
Rm2, there are no temperature-dependent changes in the resistance ratio 
between the elements Rm1 and Rm2 and the magnetic resistor Rma, and the 
overall precision of the magnetic resistance elements can be maintained at 
a high level. Further advantage of the present invention lies in that the 
magnetic resistor Rma and the magnetic resistance elements Rm1 and Rm2 can 
be formed simultaneously with the same material by the same method on the 
same chip, thus not requiring any increase in costs. 
The level of output signals of the magnetic resistance elements Rm1 and Rm2 
is lowered by the addition of the magnetic resistor Rma, but the signals 
of the same level as that obtained without the magnetic resistor Rma may 
be provided simply by controlling the gains of the amplifier 11. 
A modified version of the first embodiment is now explained. In order to 
equalize the DC component of the output voltage of the magnetic resistance 
elements Rm1 and Rm2 and the divided voltage by the resistances r1 and r2, 
a magnetic resistor Rma may be connected serially on the top side of the 
two elements Rm1 and Rm2 that are serially connected or between the source 
Vcc and the magnetic resistance element Rm.sub.1 as shown in FIG. 2(a). 
The reference voltage Vref which is the voltage ratio of the resistances 
r1 and r2, shown in FIG. 1, may be set lower than 1/2 of the source 
voltage Vcc. In this case, the magnetic resistor Rma may be connected as 
shown in FIG. 2(a) in order to equalize the DC component of the output 
voltage of the elements Rm1 and Rm2 and the divided voltage by the 
resistances r1 and r2. The magnetic resistor Rma is also formed in a not 
magnetically sensitive manner. The relation of resistances among elements 
is expressed by the following equation: 
EQU (Rma+Rm1):Rm2=r1:r2 
The magnetic resistor Rma comprising two magnetic resistance elements Rm1 
and Rm2 connected serially may be provided between the two elements Rm1 
and Rm2 as shown in FIGS. 2(b) and 2(c). The embodiment of FIG. 2(b) shows 
an output terminal provided at a node of the element Rm1 and the resistor 
Rma as the latter is connected to the side of the element Rm2 
substantially similarly to the one shown in FIG. 1. The embodiment of FIG. 
2(c) shows that magnetic resistor Rma is connected to the magnetic 
resistance element Rm1 and an output terminal is provided at the node of 
the element Rm2 and the resistor Rma. It is substantially the same as the 
example in FIG. 2(a). In any event, the resistor Rma is disposed in a 
non-magnetically sensitive manner as in the above mentioned embodiments. 
Embodiment 2 is now explained. In this embodiment, the magnetic resistor 
may be connected in parallel with at least one of the two magnetic 
resistance elements Rm1 and Rm2 for equalizing the DC component of the 
output voltage of the magnetic resistance elements Rm1 and Rm2 and the 
voltage divided by the resistances r1 and r2. FIG. 3 shows such an example 
wherein the magnetic resistor Rmb is connected in parallel with one of the 
magnetic resistance elements Rm1. The relation among resistances is 
expressed by the equation: 
EQU Rm1.multidot.Rmb/(Rm1+Rmb):Rm2=r1:r2 
In the case of FIG. 3(b), the magnetic resistor Rmb is connected in 
parallel with the other magnetic resistance element Rm2. The relation 
among resistances in this embodiment is expressed by the following 
equation: 
EQU Rm1:Rm2.multidot.Rmb/(Rm2+Rmb)=r1:r2 
In the embodiments shown in FIG. 3, the magnetic resistor Rmb is disposed 
on the same chip as the elements Rm1 and Rm2 but in a non-magnetically 
sensitive manner. 
When a magnetic resistor is to be connected serially to two magnetic 
resistance elements Rm1 and Rm2, it may be connected on two sides, the 
side of Rm1 and that of Rm2. Similarly, when a resistor is to be connected 
in parallel to the two elements Rm1 and rm2, the magnetic resistor may be 
connected respectively in parallel to the two magnetic resistance elements 
Rm1 and Rm2. At any rate, it suffices so long as the connection of the 
magnetic resistor will equalize the DC component of the output voltage of 
the elements Rm1 and Rm2 and the divided voltage of the resistances r1 and 
r2. 
According to Embodiments 1 and 2 discussed above, the magnetic resistor 
disposed in a non-magnetically sensitive manner is connected serially to 
two magnetic resistance elements, or in parallel to one of the two 
elements, but the object of the present invention can be achieved without 
adding the magnetic resistor which is not magnetically sensitive. In sum, 
the resistance balance of the above mentioned two magnetic resistance 
elements should be broken to equalize the DC component of the output 
voltage provided at the node of the two magnetic resistance elements or an 
input of the amplifier and the divided voltage from the voltage divider or 
the other input of the amplifier. 
FIG. 4 shows a third embodiment of the present invention wherein two 
magnetic resistance elements Rm1 and Rm2 are formed in a manner sensitive 
to the magnetism of the magnet on a chip MR. There is formed no magnetic 
resistor which is not magnetically sensitive. The ratio of resistances of 
the two elements Rm1 and Rm2 is not 1:1. The balance between two elements 
Rm1 and Rm2 is broken so as to equalize the direct current component of 
the output voltage taken out of the node of two elements Rm1 and Rm2 with 
the divided voltage caused by the voltage divider resistances r1 and r2. 
In other words, the resistances may be set as in the following equation: 
EQU Rm1:Rm2=r1:r2 
By referring to FIG. 5, the fourth embodiment of the magnetic detector 
according to the present invention is now explained. 
In FIG. 5, there are connected in series two magnetic resistance elements 
Rm1 and Rm2 and the resistance Ra between the current source Vcc terminal 
and the grounding terminal. The resistances of the two elements Rm1 and 
Rm2 assume the relation of 1:1. The two elements Rm1 and Rm2 are formed on 
the same chip (Chip MR). The two elements Rm1 and Rm2 are constructed as 
the unilateral bridge where the contact therebetween is used as an output 
terminal and the output voltage of the output terminal is supplied to one 
of the input terminals of the amplifier 11 via the capacitor C1. On the 
other hand, there is connected a voltage divider comprising two serially 
connected resistances r1 and r2 between the power source Vcc and the 
ground. The divided voltage of the divider is supplied as the reference 
voltage Vref into the other input terminal of the amplifier 11. There is 
connected a resistance R.sub.1 between one of the input terminal and the 
output terminal of the amplifier 11 to determine the amplification 
thereof. 
The above mentioned resistor Ra is connected between the magnetic 
resistance element Rm2 and the ground. The resistor Ra is for equalizing 
the DC component of the output voltage from the node of the magnetic 
resistance elements Rm1 and Rm2 which are serially connected and the 
divided voltage by the resistances r1 and r2. Assuming, for instance, that 
the power source voltage Vcc is 5 V and the voltage divided by the 
resistances r1 and r2 which is the reference voltage Vref of the amplifier 
11 is 3 V, and if there is no resistor Ra, the DC component of the output 
voltage from the node of the elements Rm1 and Rm2 becomes 2.5 V, 
generating the difference of 0.5 V in voltage. In order to eliminate this 
difference by breaking the balance at the midpoint of the elements Rm1 and 
Rm2, the resistor Ra is connected serially to the two magnetic resistance 
elements Rm1 and Rm2. The relation among resistances is expressed by the 
following equation: 
EQU Rm1:(Rm2+Ra)=r1:r2 
By serially connecting the resistor Ra to the two magnetic resistance 
elements Rm1 and Rm2, the DC component of the output voltage of the 
elements Rm1 and Rm2 and the voltage divided by the resistances r1 and r2 
are equalized, the common-mode noises superposed on the power source line 
are no longer amplified by the amplifier 11, and the effects of such 
common-mode noises are reduced. As it is only necessary to attach the 
resistor Ra externally and there is no need to change the magnetic 
resistance elements Rm1, Rm2 or the circuit of the amplifier 11, the 
design efficiency is advantageously improved. 
The level of the output signals from the magnetic resistance elements Rm1, 
Rm2 is lowered for the amount caused by addition of the resistor Ra. By 
regulating the gains of the amplifier 11, it is possible to provide output 
signals of the level equal to the case where no resistor Ra has been 
added. 
In order to equalize the DC component of the output voltage of the magnetic 
resistance elements Rm1 and Rm2 and the voltage divided by the resistances 
r1 and r2, the resistor Ra may be connected serially above the serially 
connected two elements Rm1, Rm2 as shown in FIG. 2. In other words, the 
resistor Ra may be connected between the current source Vcc terminal and 
the magnetic resistance element Rm1. The reference voltage Vref which is 
the voltage ratio divided by the resistances r1 and r2 shown in FIG. 5 may 
be lower than 1/2 of the source voltage Vcc. In this case, connection of 
the resistor Ra as in the case of FIG. 6 will equalize the DC component of 
the output voltage of the elements Rm1 and Rm2 and the voltage divided by 
the resistances r1 and r2. 
Embodiment 5 is also explained by referring to FIG. 7. The resistance to 
equalize the DC component of the output voltage of the magnetic resistance 
elements Rm1 and Rm2 and the voltage divided by the resistances r1 and r2 
may be connected in parallel to at least one of the two magnetic 
resistance elements. FIG. 7 shows an example thereof wherein the 
resistance Rb is connected in parallel to one of the elements Rm1. The 
relation among resistances at that time is expressed by the following 
equation: 
EQU Rml.multidot.Rb/(Rm1+Rb):Rm=r1:r2 
FIG. 8 shows a sixth embodiment wherein resistances are connected in 
parallel to a magnetic resistance element; the resistor Rb is connected in 
parallel to the other element Rm2. 
When connecting the resistance serially to two elements Rm1, Rm2, it may be 
connected to the side of the element Rm1 and that of the element Rm2. 
Similarly, when the resistance is connected in parallel to two elements 
Rm1 and Rm2, it may be connected respectively in parallel to both the 
elements Rm1 and Rm2. At any rate, connection of the resistance should 
achieve equalization of the DC component of the output voltage of the 
elements Rm1 and Rm2 and the voltage divided by the resistances r1 and r2. 
According to the present invention, in a magnetic detector comprising two 
magnetic resistance elements serially connected between the current source 
terminal and the grounding terminal, an output terminal connected to the 
point where the two elements are connected, the output voltage of the 
output terminal being supplied to one of the input terminals of the 
amplifier and the divided voltage of the voltage divider provided between 
the power source and the ground supplied to the other input terminal of 
the amplifier, magnetic resistances disposed in a non-magnetically 
sensitive manner are connected serially to said two elements or in 
parallel to at least one of the two elements, or by causing the loss of 
resistance balance of the two elements, the DC component of the output 
voltage at the node of the two magnet resistance elements or an input of 
the amplifier, and the voltage divided by the voltage divider or another 
input for the amplifier are equalized and the common-mode noises 
superposed on the current source line are prevented from becoming 
amplified by the amplifier and the effect of such noises is lessened. As 
the magnetic resistances connected in series or in parallel to the 
magnetic resistance elements may be formed on the same chip, the effect of 
temperature characteristics on the magnetic resistance elements can be 
advantageously cancelled. When the balance in resistance of the two 
elements is disturbed, the effects of the temperature characteristics may 
also be cancelled. 
By connecting a resistance with the two magnetic resistance elements in 
series or in parallel to at least one of the two magnetic resistance 
elements, the DC component of the output voltage at the node of the two 
elements acting as an input from the amplifier and the voltage divided by 
the voltage divider acting as another input for the amplifier are 
equalized. Thus the common-mode noises superposed on the current source 
line are not amplified by the amplifier and their effects are reduced. As 
the resistance can be attached externally, there is no need to change the 
magnetic resistance elements or the amplifier circuit, thus improving the 
design efficiency. 
While the foregoing description and drawings represent the preferred 
embodiments of the present invention, it will be obvious to those skilled 
in the art that various changes and modifications may be made therein 
without departing from the true spirit and scope of the present invention.