Abstract:
In a motor rotation rate detecting circuit incorporating a conventional differentiating circuit, raising the upper limit of the differentiating operation frequency of the differentiating circuit to cope with an increased range of the motor rotation rate causes the differentiating circuit to amplify high-frequency noise when the motor is rotating at a low rate. To solve this problem, a motor rotation rate detecting circuit embodying the invention incorporates a differentiating circuit that produces a rotation rate detection signal by differentiating a plurality of alternating signals having different phases that are output according to the rotation position of the motor, and the frequency response of the differentiating operation of the differentiating circuit is switchable according to the voltage of the rotation rate detection signal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a motor rotation rate detecting circuit that produces a rotation rate detection signal by differentiating a plurality of alternating signals having different phases that are output according to the rotation position of a motor. The present invention also relates to a motor driving device incorporating such a motor rotation rate detecting circuit.  
         [0003]     2. Description of the Prior Art  
         [0004]     Japanese Patent Applications Laid-Open Nos. H9-127140 and S57-76456 disclose conventionally known examples of motor rotation rate detecting circuits that produce a rotation rate detection signal by differentiating two-phase sinusoidal signals output from an encoder coupled to a motor.  
         [0005]     The motor rotation rate detecting circuit disclosed in Japanese Patent Application Laid-Open No. H9-127140 includes an inverting circuit that inverts the polarities of two-phase sinusoidal signals output from an encoder coupled to a motor, a switching circuit that selects one among the four sinusoidal signals, namely the two-phase sinusoidal signals output from the encoder coupled to the motor and their respective inverted signals, a differentiating circuit that differentiates the signal output from the switching circuit, a logic selecting circuit that controls the timing of the switching performed by the switching circuit, and a differentiation error eliminating circuit that eliminates a differentiation error component from the differentiated signal output from the differentiating circuit to output a rotation rate detection signal.  
         [0006]     Differentiating circuits of various configurations can be used in a motor rotation rate detecting circuit. However, a signal having a frequency higher than that corresponding to the maximum motor rotation rate is a high-frequency noise signal, and therefore it is customary to use a differentiating circuit that does not perform differentiating operation on a signal having a frequency higher than that corresponding to the maximum motor rotation rate.  
         [0007]      FIG. 12  shows an example of the configuration of a differentiating circuit that does not perform differentiating operation on a signal having a frequency higher than that corresponding to the maximum motor rotation rate. The differentiating circuit shown in  FIG. 12  is composed of a capacitor C 3 , resistors R 9  and R 10 , and an operational amplifier OP 3 . One end of the capacitor C 3  serves as the input end of the differentiating circuit. The other end of the capacitor C 3  is connected through the resistor R 9  to the inverting input terminal of the operational amplifier OP 3 . The non-inverting input terminal of the operational amplifier OP 3  is grounded. The operational amplifier OP 3  receives negative feedback through the resistor R 10 . The node at which the output terminal of the operational amplifier OP 3  and the resistor R 10  are connected together serves as the output end of the differentiating circuit. Here, let the capacitance of the capacitor C 3  be C 3  [F] and the resistance of the resistor R 10  be R 10  [Ω]. Then, the differentiating operation frequency of the differentiating circuit shown in  FIG. 12  is lower than 1/(2×π×C 3 ×R 10 ) [Hz]. The differentiating operation frequency denotes the frequency at which the differentiating circuit offers differentiating operation. Accordingly, the capacitance C 3  and the resistance R 10  are so determined that the maximum value of the differentiating operation frequency is equal to the frequency corresponding to the maximum motor rotation rate.  
         [0008]     The motor rotation rate detecting circuit described above is used to detect the rotation rate of various types of motor. For example, it is used to detect the rotation rate of a spindle motor used in an optical disk apparatus.  
         [0009]     In an optical disk apparatus, as the disk rotation rate becomes increasingly high, the range of the frequency of the output signal of the encoder coupled to the spindle motor becomes increasingly wide. As the range of the frequency of the output signal of the encoder becomes wider, the maximum value of the differentiating operation frequency of the differentiating circuit shown in  FIG. 12  needs to be made higher.  
         [0010]     However, raising the maximum value of the differentiating operation frequency of the differentiating circuit causes the differentiating circuit to perform differentiating operation on signals in a high-frequency range even when such signals are not needed because the motor is rotating at a low rate. As a result, when the motor is rotating at a low rate, quite inconveniently, the differentiating circuit amplifies high-frequency noise.  
       SUMMARY OF THE INVENTION  
       [0011]     An object of the present invention is to provide a motor rotation rate detecting circuit that is capable of wide-range detection with satisfactory resistance to high-frequency noise, and to provide a motor driving device incorporating such a motor rotation rate detecting circuit.  
         [0012]     To achieve the above object, according to the present invention, a motor rotation rate detecting circuit is provided with a differentiating circuit that produces a rotation rate detection signal by differentiating a plurality of alternating signals having different phases that are output according to the rotation position of a motor, and the frequency response of the differentiating operation of the differentiating circuit is switchable according to the voltage of the rotation rate detection signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     This and other objects and features of the present invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanying drawings in which:  
         [0014]      FIG. 1  is a diagram showing an example of the configuration of a motor rotation rate detecting circuit embodying the invention;  
         [0015]      FIG. 2  is a diagram showing an example of the configuration of the inverting circuit provided in the motor rotation rate detecting circuit of  FIG. 1 ;  
         [0016]      FIG. 3  is a diagram showing an example of the configuration of the switching circuit and the logic selecting circuit provided in the motor rotation rate detecting circuit of  FIG. 1 ;  
         [0017]      FIG. 4  is a diagram showing the waveforms of the signals observed at relevant points in the motor rotation rate detecting circuit of  FIG. 1 ;  
         [0018]      FIG. 5  is a diagram showing an example of the configuration of the selecting circuit provided in the motor rotation rate detecting circuit of  FIG. 1 ;  
         [0019]      FIG. 6  is a diagram showing an example of the configuration of the differentiating circuit provided in the motor rotation rate detecting circuit of  FIG. 1 ;  
         [0020]      FIG. 7  is a diagram showing another example of the configuration of the differentiating circuit provided in the motor rotation rate detecting circuit of  FIG. 1 ;  
         [0021]      FIG. 8  is a diagram showing another example of the configuration of the differentiating circuit provided in the motor rotation rate detecting circuit of  FIG. 1 ;  
         [0022]      FIG. 9  is a diagram showing another example of the configuration of the differentiating circuit provided in the motor rotation rate detecting circuit of  FIG. 1 ;  
         [0023]      FIG. 10  is a diagram showing another example of the configuration of the differentiating circuit provided in the motor rotation rate detecting circuit of  FIG. 1 ;  
         [0024]      FIG. 11  is a diagram showing another example of the configuration of the differentiating circuit provided in the motor rotation rate detecting circuit of  FIG. 1 ; and  
         [0025]      FIG. 12  is a diagram showing an example of the configuration of the differentiating circuit provided in a conventional motor rotation rate detecting circuit. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     Hereinafter, embodiments of the present invention will be described with reference to the drawings.  FIG. 1  shows an example of a motor rotation rate detecting circuit embodying the invention. The motor rotation rate detecting circuit shown in  FIG. 1  is fed with two-phase sinusoidal signals A and B 90° out of phase with each other that are output from hole elements or the like provided in a motor or from an encoder coupled to a motor.  
         [0027]     The sinusoidal signal A is fed to an inverting circuit  1 , which inverts the polarity of the sinusoidal signal A and outputs a sinusoidal signal A overscored. The sinusoidal signal B is fed to an inverting circuit  2 , which inverts the polarity of the sinusoidal signal B and outputs a sinusoidal signal B overscored.  
         [0028]     The four sinusoidal signals, namely A, B, A overscored, and B overscored, are fed to a switching circuit  3 , which selects one among those four signals and feeds it to a differentiating circuit  5 . Three of the sinusoidal signals, namely A, B, and B overscored, are fed to a logic selecting circuit  4 , which controls the timing of the switching performed by the switching circuit.  
         [0029]     The differentiating circuit  5  differentiates the signal output from the switching circuit  3 , and feeds the differentiated signal to a differentiation error eliminating circuit  6 . The differentiation error eliminating circuit  6  eliminates a differentiation error component from the differentiated signal output from the differentiating circuit  5 , and outputs a rotation rate detection signal. According to the rotation rate detection signal output from the differentiation error eliminating circuit  6 , a selecting circuit  7  turns on and off an analog switch (not shown) provided within the differentiating circuit  5 .  
         [0030]      FIG. 2  shows an example of the configuration of the inverting circuits  1  and  2 . The inverting circuit shown in  FIG. 2  is composed of resistors  21  and  23  and an operational amplifier  22 . One end of the resistor  21  serves as an input end of the inverting circuit. The other end of the resistor  21  is connected to the inverting input terminal of the operational amplifier  22 . The non-inverting input terminal of the operational amplifier  22  is grounded. The operational amplifier  22  receives negative feedback through the resistor  23 . The node at which the operational amplifier  22  and the resistor  23  are connected together serves as the output end of the inverting circuit.  
         [0031]      FIG. 3  shows an example of the configuration of the switching circuit  3  and the logic selecting circuit  4 . The switching circuit  3  is composed of analog switches  31  to  34 . The analog switch  31  receives at one end thereof the sinusoidal signal A, the analog switch  32  receives at one end thereof the sinusoidal signal B, the analog switch  33  receives at one end thereof the sinusoidal signal A overscored, and the analog switch  34  receives at one end thereof the sinusoidal signal B overscored. The analog switches  31  to  34  are at the other end thereof connected together to serve as the output end of the switching circuit  3 . The switching circuit  3  outputs at the output end thereof a signal SC.  
         [0032]     The logic selecting circuit  4  is composed of operational amplifiers  41  and  42 , inverter circuits  43  and  44 , and NOR circuits  45  to  48 . The operational amplifier  41  receives at the non-inverting input terminal thereof the sinusoidal signal B, and receives at the inverting input terminal thereof the sinusoidal signal A. The operational amplifier  41  outputs a rectangular signal that is high when the sinusoidal signal B is greater than the sinusoidal signal A and low when the sinusoidal signal B is not greater than the sinusoidal signal A. The operational amplifier  42  receives at the non-inverting input terminal thereof the sinusoidal signal A, and receives at the inverting input terminal thereof the sinusoidal signal B overscored. The operational amplifier  42  outputs a rectangular signal that is high when the sinusoidal signal A is greater than the sinusoidal signal B overscored and low when the sinusoidal signal A is not greater than the sinusoidal signal B overscored.  
         [0033]     The rectangular signal output from the operational amplifier  41  is fed to the inverter circuit  43 , which inverts and outputs the signal. The rectangular signal output from the operational amplifier  42  is fed to the inverter circuit  44 , which inverts and outputs the signal.  
         [0034]     The NOR circuit  45  receives the output signal of the operational amplifier  41  and the output signal of the operational amplifier  42 , and feeds a signal S 1  obtained by inverting the OR of those two signals to the control terminal of the analog switch  31  provided in the switching circuit  3 . The NOR circuit  46  receives the output signal of the operational amplifier  41  and the output signal of the inverter circuit  44 , and feeds a signal S 2  obtained by inverting the OR of those two signals to the control terminal of the analog switch  32  provided in the switching circuit  3 . The NOR circuit  47  receives the output signal of the inverter circuit  43  and the output signal of the inverter circuit  44 , and feeds a signal S 3  obtained by inverting the OR of those two signals to the control terminal of the analog switch  33  provided in the switching circuit  3 . The NOR circuit  48  receives the output signal of the operational amplifier  42  and the output signal of the inverter circuit  43 , and feeds a signal S 4  obtained by inverting the OR of those two signals to the control terminal of the analog switch  34  provided in the switching circuit  3 .  
         [0035]     Configured as described above, the logic selecting circuit  4  produces control signals S 1  to S 4  as shown in  FIG. 4 . Accordingly, the signal SC output from the switching circuit  3  has a waveform as shown in  FIG. 4  which is a combination of the waveform of that portion of each of the sinusoidal signals A, B, A overscored, and B overscored which extends from 45° before to 45° after their respective rising zero-cross point.  
         [0036]     The differentiating circuit  5  differentiates the signal SC, and outputs a differentiated signal SD that has a waveform as shown in  FIG. 4 . The differentiation error eliminating circuit  6  eliminates a differentiation error component from the differentiated signal SD output from the differentiating circuit  5 , and outputs a rotation rate detection signal SE. The voltage of the differentiated signal SD is proportional to the angular velocity of the sinusoidal signals A and B, and this makes it possible to detect the motor rotation rate on the basis of the voltage of the rotation rate detection signal SE.  
         [0037]     Next, the differentiating circuit  5  and the selecting circuit  7 , which characterize the present invention, will be described in more detail.  FIG. 5  shows an example of the configuration of the selecting circuit  7 . The selecting circuit shown in  FIG. 5  is composed of an operational amplifier  71 , a constant voltage source  72 , and an inverter circuit  73 . The non-inverting input terminal of the operational amplifier  71  is connected to the output end of the differentiation error eliminating circuit  6  (see  FIG. 1 ) so as to receive the rotation rate detection signal SE. The inverting input terminal of the operational amplifier  71  is connected to the positive side of the constant voltage source  72  so as to receive a constant voltage. The negative side of the constant voltage source  72  is grounded.  
         [0038]     The output terminal of the operational amplifier  71  is connected to the differentiating circuit  5  (see  FIG. 1 ) so as to feed the differentiating circuit  5  with a control signal CTL 1  that is high when the rotation rate detection signal SE is greater than the constant voltage and low when the rotation rate detection signal SE is not greater than the constant voltage. The output terminal of the operational amplifier  71  is also connected to the input terminal of the inverter circuit  73 . The output terminal of the inverter circuit  73  is connected to the differentiating circuit  5  (see  FIG. 1 ) so as to feed the differentiating circuit  5  with a control signal CTL 1  overscored, which is the inversion of the control signal CTL 1 . That is, the selecting circuit  7  feeds the differentiating circuit  5  with the control signals CTL 1  and CTL 1  overscored. The control signal CTL 1  is high when the motor rotation rate is equal to or higher than a predetermined value and low when the motor rotation rate is lower than the predetermined value.  
         [0039]      FIG. 6  shows an example of the configuration of the differentiating circuit  5 . The differentiating circuit shown in  FIG. 6  is composed of a capacitor C 1 , resistors R 1  to R 4 , analog switches SW 1  to SW 4 , and an operational amplifier OP 1 . One end of the capacitor C 1  serves as the input end of the differentiating circuit. The other end of the capacitor C 1  is connected to one end of the resistor R 1  and to one end of the resistor R 3 . The other end of the resistor R 1  is connected through the analog switch SW 1  to the inverting input terminal of the operational amplifier OP 1 , and the other end of the resistor R 3  is connected through the analog switch SW 3  to the inverting input terminal of the operational amplifier OP 1 .  
         [0040]     Also connected to the inverting input terminal of the operational amplifier OP 1  are one end of the resistor R 2  and one end of the resistor R 4 . The other end of the resistor R 2  is connected through the analog switch SW 2  to the output terminal of the operational amplifier OP 1 , and the other end of the resistor R 4  is connected through the analog switch SW 4  to the output terminal of the operational amplifier OP 1 . The non-inverting input terminal of the operational amplifier OP 1  is grounded. The node at which the output terminal of the operational amplifier OP 1  and the analog switches SW 2  and SW 4  are connected together serves as the output end of the differentiating circuit.  
         [0041]     The analog switches SW 1  and SW 2  are controlled by the control signal CTL 1  (see  FIG. 5 ) output from the selecting circuit  7 , and the analog switches SW 3  and SW 4  are controlled by the control signal CTL 1  overscored (see  FIG. 5 ) output from the selecting circuit  7 .  
         [0042]     Now, the operation of the differentiating circuit configured as described above will be described. First, how it operates when the motor rotation rate is lower than a predetermined value will be described. When the motor rotation rate is lower then a predetermined value, the control signal CTL 1  is low, and the control signal CTL 1  overscored is high. Accordingly, the analog switches SW 1  and SW 2  are off, and the analog switches SW 3  and SW 4  are on. Thus, let the capacitance of the capacitor C 1  be C 1  [F], the resistance of the resistor R 3  be R 3  [Ω], and the resistance of the analog switch SW 3  when it is on (hereinafter referred to as the on-state resistance) be R SW3  [Ω]. Then, the differentiating operation frequency of the differentiating circuit shown in  FIG. 6  is lower than 1/( 2×π×C   1 ×(R 3 +R SW3 )) [Hz]. Moreover, let the resistance of the resistor R 4  be R 4  [Ω] and the on-state resistance of the analog switch SW 4  be R SW4  [Ω]. Then, the alternative-current gain of the differentiating circuit shown in  FIG. 6  is (R 4 +R SW4 )/(R 3 +R SW3 ).  
         [0043]     Next, how the differentiating circuit operates when the motor rotation rate is equal to or higher than a predetermined value will be described. In this case, the control signal CTL 1  is high, and the control signal CTL 1  overscored is low. Accordingly, the analog switches SW 1  and SW 2  are on, and the analog switches SW 3  and SW 4  are off. Thus, let the resistance of the resistor R 1  be R 1  [Ω] and the on-state resistance of the analog switch SW 1  be R SW1  [Ω]. Then, the differentiating operation frequency of the differentiating circuit shown in  FIG. 6  is lower than 1/( 2×π×C   1 ×(R 1 +R SW1 )) [Hz]. Moreover, let the resistance of the resistor R 2  be R 2  [Ω] and the on-state resistance of the analog switch SW 2  be R SW2  [Ω]. Then, the alternative-current gain of the differentiating circuit shown in  FIG. 6  is (R 2 +R SW2 )/(R 1 +R SW1 ).  
         [0044]     Here, the resistances R 1  and R 3  of the resistors R 1  and R 3  are so set as to fulfill R 1 &lt;R 3 . Thus, when the motor rotation speed is lower than the predetermined value, the maximum value of the differentiating operation frequency is small, and, when the motor rotation speed is equal to or higher than the predetermined value, the maximum value of the differentiating operation frequency is great. This prevents the differentiating circuit from amplifying high-frequency noise when the motor is rotating at a low rate, but nevertheless permits the motor rotation rate to be detected when the motor is rotating at a high rate. In this way, it is possible to realize a motor rotation rate detesting circuit that is capable of wide-range detection with satisfactory resistance to high-frequency noise.  
         [0045]     Moreover, the resistances R 1  to R 4  of the resistors R 1  to R 4  and the on-state resistances R SW1  to R SW4  of the analog switches SW 1  to SW 4  are so set as to fulfill (R 4 +R SW4 )/(R 3 +R SW3 )=(R 2 +R SW2 )/(R 1 +R SW1 ). This makes it possible to keep the alternating-current gain constant irrespective of the motor rotation rate. Thus, the gain of the rotation rate detection signal SE remains constant irrespective of the motor rotation rate, and this saves the motor driving circuit, i.e., the circuit that controls the motor according to the rotation rate detection signal SE output from the motor rotation rate detecting circuit, from performing extra operation in addition to the operation it conventionally performs.  
         [0046]     Incidentally, in a case where the on-state resistances RSW SW1  to R SW4  of the analog switches SW 1  to SW 4  are made extremely low relative to the resistances R 1  to R 4  of the resistors R 1  to R 4  as by the use of analog switches with low on-state resistances, the differentiating operation frequency and the alternating-current gain can be calculated accurately even if the terms in the respective formulae which relate to the on-state resistances of the analog switches are omitted.  
         [0047]     FIGS.  7  to  11  show other examples of the configuration of the differentiating circuit  5 . With any of these differentiating circuits, the resistances of the resistors and the on-state resistances of the analog switches are so set that, when the motor rotation rate is lower than a predetermined value, the maximum value of the differentiating operation frequency is small and that, when the motor rotation rate is equal to or higher than the predetermined value, the maximum value of the differentiating operation frequency is great. Moreover, the resistances of the resistors and the on-state resistances of the analog switches are so set that the alternating-current gain remains constant irrespective of the motor rotation rate. In a case where the on-state resistances of the analog switches are made extremely low relative to the resistances of the resistor as by the use of analog switches with low on-state resistances, the on-state resistances of the analog switches may be ignored.  
         [0048]     Now, the configuration of the differentiating circuit shown in  FIG. 7  will be described. Here, such circuit elements as are found also in the differentiating circuit shown in  FIG. 6  are identified with the same reference numerals and symbols, and their detailed explanations will not be repeated. In the differentiating circuit shown in  FIG. 7 , the analog switches SW 3  and SW 4  used in the differentiating circuit shown in  FIG. 6  are omitted, and instead the resistor R 3  is connected directly to the inverting input terminal of the operational amplifier OP 1  and the resistor R 4  is connected directly to the output terminal of the operational amplifier OP 1 . Correspondingly, it is advisable to use a selecting circuit in which the inverter circuit  73  used in the selecting circuit shown in  FIG. 5  is omitted. This helps achieve a simpler circuit configuration than in  FIG. 6 .  
         [0049]     Next, the configuration of the differentiating circuit shown in  FIG. 8  will be described. The differentiating circuit shown in  FIG. 8  is composed of a capacitor C 2 , resistors R 5  and R 6 , analog switches SW 5  and SW 6 , and an operational amplifier OP 2 . One end of the capacitor C 2  serves as the input end of the differentiating circuit. The other end of the capacitor C 2  is connected to one end of the resistor R 5 , to one end of the resistor R 6 , and to the non-inverting input terminal of the operational amplifier OP 2 . The other end of the resistor R 5  is grounded through the analog switch SW 5 , and the other end of the resistor R 6  is grounded through the analog switch SW 6 . The analog switch SW 5  is controlled by the control signal CTL 1  (see  FIG. 5 ) output from the selecting circuit  7 , and the analog switch SW 6  is controlled by the control signal CTL 1  overscored (see  FIG. 5 ) output from the selecting circuit  7 . The inverting input terminal and output terminal of the operational amplifier OP 2  are connected together directly, and the node at which they are connected together serves as the output end of the differentiating circuit.  
         [0050]     Next, the configuration of the differentiating circuit shown in  FIG. 9  will be described. Here, such circuit elements as are found also in the differentiating circuit shown in  FIG. 8  are identified with the same reference numerals and symbols, and their detailed explanations will not be repeated. In the differentiating circuit shown in  FIG. 9 , the differentiating circuit shown in  FIG. 8  is further provided with resistors R 7  and R 8 . Here, the inverting input terminal of the operational amplifier OP 2  is grounded through the resistor R 7 . Moreover, the inverting input terminal and output terminal of the operational amplifier OP 2  are connected together not directly but through the resistor R 8 , and the node at which the output terminal of the operational amplifier OP 2  and the resistor R 8  are connected together serves as the output end of the differentiating circuit. This makes it possible to obtain a higher gain and thereby achieve sure detection even when the error component is small.  
         [0051]     Next, the configuration of the differentiating circuit shown in  FIG. 10  will be described. Here, such circuit elements as are found also in the differentiating circuit shown in  FIG. 8  are identified with the same reference numerals and symbols, and their detailed explanations will not be repeated. In the differentiating circuit shown in  FIG. 10 , the analog switch SW 6  used in the differentiating circuit shown in  FIG. 8  is omitted, and instead the resistor R 6  is grounded directly. Correspondingly, it is advisable to use a selecting circuit in which the inverter circuit  73  used in the selecting circuit shown in  FIG. 5  is omitted.  
         [0052]     Next, the configuration of the differentiating circuit shown in  FIG. 11  will be described. Here, such circuit elements as are found also in the differentiating circuit shown in  FIG. 9  are identified with the same reference numerals and symbols, and their detailed explanations will not be repeated. In the differentiating circuit shown in  FIG. 11 , the analog switch SW 6  used in the differentiating circuit shown in  FIG. 9  is omitted, and instead the resistor R 6  is grounded directly. Correspondingly, it is advisable to use a selecting circuit in which the inverter circuit  73  used in the selecting circuit shown in  FIG. 5  is omitted.  
         [0053]     The embodiment described above deals with a case in which a differentiating circuit is used that switches the maximum value of the differentiating operation frequency in two steps by switching analog switches. It is also possible, however, to use a differentiating circuit that switches the maximum value of the differentiating operation frequency in three or more steps.  
         [0054]     A motor rotation rate detecting circuit embodying the present invention may be configured in any other manner than shown in  FIG. 1 . For example, it may be configured like the motor rotation rate detecting circuit disclosed in Japanese Patent Application Laid-Open No. S57-76456, provided that its differentiating circuit is so configured as to permit the maximum value of the differentiating operation frequency to be switched in a plurality of steps and that a selecting circuit for controlling the switches provided in that differentiating circuit is additionally provided. The voltage of the rotation rate detection signal SE may be detected in a plurality of steps, with the switches provided in the differentiating circuit so configured as to offer the maximum value of the differentiating operation frequency in a plurality of steps. In the embodiment described above, the control signal CTL 1  overscored is produced within the selecting circuit. It is also possible, however, to produce the control signal CTL 1  overscored, whenever it is necessary, within the differentiating circuit by inverting the control signal CTL 1 .  
         [0055]     In the embodiment described above, the signals output from the encoder coupled to the motor are sinusoidal signals. However, the signals output from the encoder coupled to the motor may be signals having alternating waveforms of any other type so long as, by differentiating them, it is possible to extract a component that indicates the motor rotation rate.  
         [0056]     In any of the differentiating circuits shown in FIGS.  8  to  11 , the analog switches SW 5  and SW 6  may be replaced with switching circuits using transistors. For example, it is possible to provide, instead of the analog switch SW 5 , an n-channel MOSFET that has its drain connected to the resistor R 5 , that has its source connected to ground, and that receives at its gate the control signal CTL 1 , and provide, instead of the analog switch SW 6 , an n-channel MOSFET that has its drain connected to the resistor R 6 , that has its source connected to ground, and that receives at its gate the control signal CTL 1  overscored.  
         [0057]     A motor driving device embodying the present invention is provided with a motor rotation rate detecting circuit as described above and a motor. A motor driving device embodying the present invention includes a motor and a motor rotation rate detecting circuit that produces a rotation rate detection signal by differentiating a plurality of alternating signals having different phases that are output according to the rotation position of the motor. Here, the frequency response of the differentiating operation of the differentiating circuit provided in the motor rotation rate detecting circuit is switched according to the voltage of the rotation rate detection signal. As a result, it is possible to make the maximum value of the differentiating operation frequency small when the motor is rotating at a low rate and thereby prevent the differentiating circuit from amplifying high-frequency noise when the motor is rotating at a low rate. On the other hand, it is possible to make the maximum value of the differentiating operation frequency great when the motor is rotating at a high rate and thereby achieve sure detection of the motor rotation rate even when the motor is rotating at a high rate. In this way, it is possible to realize a motor driving device incorporating a motor rotation rate detecting circuit that is capable of wide-range detection with satisfactory resistance to high-frequency noise over the entire range.  
         [0058]     In an optical disk apparatus, as the disk rotation rate becomes increasingly high, the range of the frequency of the output signal of the encoder coupled to the spindle motor becomes increasingly wide. Thus, a motor driving device embodying the present invention is suitable as a motor driving device for driving a spindle motor in an optical disk apparatus.  
         [0059]     Motor driving devices embodying the present invention find application not only in optical disk apparatuses but in electric appliances of any kind provided with a motor wherein the range of the rotation rate of the motor is wide and the rotation rate of the motor needs to be controlled.