Abstract:
A voltage detection circuit includes: an amplifier which amplifies a voltage difference between first and second input signals input into non-inverting and inverting input terminals of the amplifier via first and second input portions; a first signal line which connects the first input portion to the amplifier; a second signal line which connects the second input portion to the amplifier; a first capacitor connected in parallel to the first signal line; a second capacitor connected in parallel to the second signal line; a first filter element which has an inductor component and a resistor component and is connected in series to the first signal line between the first capacitor and the amplifier; and a second filter element which has an inductor component and a resistor component and is connected in series to the second signal line between the second capacitor and the amplifier.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a voltage detection circuit capable of detecting a voltage difference between two input signals. 
       BACKGROUND ART 
       [0002]    A chip bead may be used in an electronic circuit such as a voltage detection circuit as a filter element for removing noise included in an input signal or the like. A chip bead is an electronic component through which a required signal is passed. The chip bead absorbs noise in a high frequency band of the signal, and discharges the absorbed noise in the form of heat. 
         [0003]    Japanese patent application No. 2005-365378 discloses a technique of removing noise included in a signal by disposing chip beads in an electronic circuit built into a remote controller for earphones connected to a portable telephone. 
       SUMMARY OF INVENTION 
       [0004]    However, a certain degree of variation exists in an impedance characteristic of a chip bead due to manufacturing errors and the like. 
         [0005]    Therefore, in a voltage detection circuit  200  that amplifies and outputs a voltage difference between two input signals, as shown in  FIG. 2 , when chip beads  51 ,  52  are disposed on signal lines  10 ,  20  further toward the side of input terminals  1 ,  2  than capacitors  41 ,  42 , variation occurs in a voltage drop implemented on the input signals by the chip beads  51 ,  52  due to variation in the impedance characteristics of the chip beads  51 ,  52 . As a result, the voltage difference between the two signals input into a differential amplifier  60  (an amplifier) varies, leading to a reduction in a precision of a voltage value output from the differential amplifier  60 . 
         [0006]    An object of the present invention is to provide a voltage detection circuit with which a reduction in a precision of a voltage value output from an amplifier can be suppressed. 
         [0007]    According to an aspect of the present invention, a voltage detection circuit includes: a first input portion into which a first input signal is input; a second input portion into which a second input signal is input; an amplifier having an inverting input terminal and a non-inverting input terminal, which determines a voltage difference between the first input signal, which is input into the non-inverting input terminal via the first input portion, and the second input signal, which is input into the inverting input terminal via the second input portion, and amplifies the determined voltage difference; a first signal line which connects the first input portion to the non-inverting input terminal of the amplifier; a second signal line which connects the second input portion to the inverting input terminal of the amplifier; a first capacitor connected in parallel to the first signal line; a second capacitor connected in parallel to the second signal line; a first filter element which has an inductor component and a resistor component and is connected in series to the first signal line between the first capacitor and the non-inverting input terminal of the amplifier; and a second filter element which has an inductor component and a resistor component and is connected in series to the second signal line between the second capacitor and the inverting input terminal of the amplifier. 
         [0008]    Embodiments and advantages of the present invention will be described in detail below with reference to the attached figures. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a schematic view showing a configuration of a voltage detection circuit according to an embodiment of the present invention. 
           [0010]      FIG. 2  is a schematic view showing a configuration of a voltage detection circuit serving as a reference example of the related art. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0011]    Referring to  FIG. 1 , a voltage detection circuit  100  according to an embodiment of the present invention will be described. 
         [0012]    The voltage detection circuit  100  includes a first input terminal  1 , a second input terminal  2 , a differential amplifier  60  (an amplifier) having an inverting input terminal and a non-inverting input terminal, a first signal line  10  connecting the first input terminal  1  to the non-inverting input terminal of the differential amplifier  60 , and a second signal line  20  connecting the second input terminal  2  to the inverting input terminal of the differential amplifier  60 . 
         [0013]    The first signal line  10  and the second signal line  20  are connected by a first connecting line  30  and a second connecting line  40 . 
         [0014]    A resistor  31  having a resistance value of several [mΩ] is disposed on the first connecting line  30 . Further, a first capacitor  41  positioned close to the first signal line  10  and a second capacitor  42  positioned close to the second signal line  20  are disposed on the second connecting line  40 , which is provided further toward the differential amplifier  60  side than the first connecting line  30 . The second connecting line  40  is grounded in a position between the first capacitor  41  and the second capacitor  42 . Capacitors having a capacitance of several thousand [pF] are used as the first capacitor  41  and the second capacitor  42 . 
         [0015]    Hence, the first capacitor  41  is provided such that one end is connected (connected in parallel) to the first signal line  10  and the other end is grounded, while the second capacitor  42  is provided such that one end is connected (connected in parallel) to the second signal line  20  and the other end is grounded. 
         [0016]    A rectangular wave voltage signal is input into the first input terminal  1  as a first input signal. Accordingly, a voltage corresponding to the voltage signal input into the first input terminal  1  is applied to one end of the resistor  31 . A voltage at another end of the resistor  31  takes a voltage value dropped by an amount corresponding to the resistance value of the resistor  31 . In other words, a rectangular wave voltage signal obtained by dropping the voltage of the rectangular wave voltage signal on the first input terminal  1  side by the resistance value of the resistor  31  is input into the second input terminal  2  as a second input signal. 
         [0017]    In addition to the first capacitor  41  and the second capacitor  42  described above, the voltage detection circuit  100  is provided with a first chip bead  51  and a second chip bead  52  for removing noise from the first input signal and the second input signal. 
         [0018]    The first chip bead  51  is disposed on (connected in series to) the first signal line  10  between the first capacitor  41  and the differential amplifier  60 , while the second chip bead  52  is disposed on (connected in series to) the second signal line  20  between the second capacitor  42  and the differential amplifier  60 . 
         [0019]    The first chip bead  51  and the second chip bead  52  are electronic components serving as noise filter elements having an inductor component and a resistor component, which pass signals in a predetermined frequency band while removing noise in a high frequency band. Chip beads exhibiting a maximum impedance (several hundred [Ω]) at a frequency of several hundred [MHz] are used as the first chip bead  51  and the second chip bead  52 . It should be noted that the impedance of a chip bead typically takes a smaller value than an input impedance of an operational amplifier such as a differential amplifier. 
         [0020]    Of the noise included in the signal supplied to the differential amplifier  60  from the first input terminal  1 , noise in a predetermined low frequency band is removed by the resistor component of the first chip bead  51  and the first capacitor  41 , while noise in a predetermined high frequency band is removed by the inductor component and the resistor component of the first chip bead  51 . 
         [0021]    Similarly, of the noise included in the signal supplied to the differential amplifier  60  from the second input terminal  2 , noise in the predetermined low frequency band is removed by the resistor component of the second chip bead  52  and the second capacitor  42 , while noise in the predetermined high frequency band is removed by the inductor component and the resistor component of the second chip bead  52 . 
         [0022]    The voltage detection circuit  100  thus configured leads a voltage difference between the first input signal and the second input signal (a voltage difference at the respective ends of the resistor  31 ) to the differential amplifier  60 , where the voltage difference is amplified, and then outputs the amplified voltage difference from an output terminal  70 . 
         [0023]    Next, a relationship between a voltage drop occurring in the first chip bead  51  and the second chip bead  52  and a precision of the voltage value output from the differential amplifier  60  will be described by comparing the voltage detection circuit  100  according to this embodiment with the voltage detection circuit  200  serving as a reference example of the related art. 
         [0024]    In the voltage detection circuit  200  shown in  FIG. 2 , members exhibiting identical functions to their counterparts in the voltage detection circuit  100  have been allocated identical reference numerals. The voltage detection circuit  200  serving as the reference example and the voltage detection circuit  100  according to this embodiment are configured substantially identically, but differ from each other in the disposal positions of the first chip bead  51  and the second chip bead  52 . In the voltage detection circuit  200 , the first chip bead  51  is disposed on the first signal line  10  upstream of the first capacitor  41 , and the second chip bead  52  is disposed on the second signal line  20  upstream of the second capacitor  42 . 
         [0025]    In the voltage detection circuit  200 , when the first input signal and the second input signal constituted by rectangular wave voltage signals are input, charging/discharging is performed in the first capacitor  41  and the second capacitor  42  on the basis of the input signals. At this time, the voltage of the first input signal is dropped on the basis of the impedance of the first chip bead  51 , and the voltage of the second input signal is dropped on the basis of the impedance of the second chip bead  52 . 
         [0026]    The impedance of the first chip bead  51  and the second chip bead  52  varies due to manufacturing errors and the like, and therefore variation also occurs in the voltage drop applied to the input signals in the first chip bead  51  and the second chip bead  52 . As a result, variation occurs in the voltage difference between the two signals input into the differential amplifier  60 , leading to a reduction in the precision of the voltage value output from the differential amplifier  60 . 
         [0027]    In the voltage detection circuit  100  according to this embodiment, on the other hand, the chip beads are not disposed upstream of the first capacitor  41  and the second capacitor  42 , and therefore, even when the first input signal and the second input signal are input and charging/discharging is performed in the first capacitor  41  and the second capacitor  42 , a voltage drop is not caused by the chip beads. As a result, the voltage corresponding to the first input signal is applied as is to one end of the first capacitor  41 , and the voltage corresponding to the second signal is applied as is to one end of the second capacitor  42 . 
         [0028]    In the voltage detection circuit  100 , the first chip bead  51  and the second chip bead  52  are disposed downstream of the first capacitor  41  and the second capacitor  42 . Moreover, an input impedance of the differential amplifier  60  is considerably larger than that of the first chip bead  51  and the second chip bead  52 . Therefore, a current passing through the first chip bead  51  and the second chip bead  52  of the voltage detection circuit  100  is smaller than a current passing through the first chip bead  51  and the second chip bead  52  of the voltage detection circuit  200 . 
         [0029]    Hence, the voltage drop applied to the input signals in the first chip bead  51  and the second chip bead  52  is small, and therefore the effect of the variation in the impedance of the respective chip beads  51 ,  52  can be reduced. As a result, variation in the voltage difference between the two signals input into the differential amplifier  60  can be suppressed. 
         [0030]    With the voltage detection circuit  100  according to this embodiment, therefore, a reduction in the precision of the voltage value output from the differential amplifier  60  can be suppressed. 
         [0031]    An embodiment of the present invention was described above, but the above embodiment merely illustrates a part of examples of applications of the present invention, and the technical scope of the present invention is not limited to the specific configurations described in the embodiment. 
         [0032]    The voltage detection circuit  100  according to this embodiment may be a circuit that detects the voltage difference between the first input signal input from the first input terminal  1  and the second input signal input from the second input terminal  2  without the use of the resistor  31 . 
         [0033]    The present application claims priority based on Japanese Patent Application No. 2012-127835, filed with the Japan Patent Office on Jun. 5, 2012, the entire contents of which are incorporated into this specification by reference.