Abstract:
Provided is a chopper amplifier circuit capable of reducing an offset voltage of a sensor bridge and temperature characteristics of the offset voltage. An offset adjusting voltage generation circuit for generating a voltage equal to an offset voltage of a sensor bridge and an offset temperature characteristics adjusting voltage generation circuit for generating a voltage having temperature characteristics equal to those of the offset voltage are provided. These output voltages are chopper-modulated and subtracted from a chopper-modulated output signal of the sensor bridge.

Description:
This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. JP2005-229501 filed Aug. 5, 2005, the entire content of which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a chopper amplifier circuit and a semiconductor device having the chopper amplifier circuit. 
   2. Description of the Related Arts 
     FIG. 2  is a block diagram showing a conventional chopper amplifier circuit. In the conventional chopper amplifier circuit  20 , multipliers  11  and  12  are provided in front and back stages of an amplifier circuit  1 , respectively. The multipliers  11  and  12  are controlled with a modulation signal CLK in a square wave with a frequency fc. The output of the chopper amplifier circuit  20  is connected to a low-pass filter  22  (see P. Allen and D. R. Holberg, CMOS Analog Circuit Design, P490-494, Saunders College Publishing, 1987). 
     FIGS. 3A to 3F  show frequency characteristics of an input signal in each portion of the conventional chopper amplifier circuit. It is assumed that an input signal has frequency characteristics as shown in  FIG. 3A  at input terminals  5  of the chopper amplifier circuit  20 . Further, it is assumed that the amplifier circuit  1  has an input conversion noise and an offset voltage Vn of the frequency characteristics as shown in  FIG. 3C . After the input signal passes through the multiplier  11 , the input signal is modulated to a frequency that is an odd multiple of the frequency fc of the modulation signal CLK.  FIG. 3B  shows frequency characteristics of the input signal at this time. The modulated input signal is amplified and output after being supplied with the input conversion noise and the offset voltage Vn of the amplifier circuit  1 .  FIG. 3D  shows frequency characteristics of the input signal at the output of the amplifier circuit  1 . The input signal output from the amplifier circuit  1  is modulated to an original frequency band (i.e., a low-frequency region including a DC) through the multiplier  12 . On the other hand, the noise component and the offset voltage Vn at the input of the amplifier circuit  1  are modulated to a frequency of an odd multiple of the frequency fc of the modulation signal CLK.  FIG. 3E  shows frequency characteristics of an input signal at the output of the multiplier  12 . Further, the input signal output from the multiplier  12  passes through the low-pass filter  22 , whereby a high frequency component of the modulation signal CLK is removed. Thus, as shown in  FIG. 3F , only an input signal component can be amplified without amplifying the noise and offset voltage of the amplifier circuit  1 . 
   Further, in another conventional chopper amplifier circuit, an input signal is subjected to double chopper modulation using modulation signals with two different frequencies, whereby the noise and the offset voltage of an amplifier circuit used in a chopper amplifier circuit are further reduced (see U.S. Pat. No. 6,262,626, Bakker, et al., Jul. 17, 2001). 
   However, the conventional chopper amplifier circuit has the following problems. When the conventional chopper amplifier circuit is used, for example, for amplifying the output voltage of a sensor bridge using a piezoresistor, the offset voltage of the sensor bridge cannot be cancelled due to incomplete matching of the piezoresistor. Therefore, the offset voltage of the sensor bridge is amplified to be output. 
   Further, the offset voltage of the sensor bridge using the piezoresistor has temperature characteristics, and the temperature characteristics of the offset voltage appear in the output voltage of the chopper amplifier circuit. 
   SUMMARY OF THE INVENTION 
   In order to solve the above-mentioned problems, the present invention has a configuration in which a circuit for generating a voltage equal to an offset voltage of a sensor bridge, i.e., an offset adjusting voltage generation circuit is provided, and the output voltage of the offset adjusting voltage generation circuit is chopper-modulated by a multiplier, whereby the output signal of the sensor bridge is subtracted from the chopper-modulated signal. 
   Further, the present invention has a configuration in which a circuit for generating a voltage having temperature characteristics equal to those of the offset voltage of the sensor bridge, i.e., an offset temperature characteristics adjusting voltage generation circuit is provided, and the output voltage of the circuit is chopper-modulated and subtracted from the output signal of the sensor bridge subjected to chopper modulation. 
   In the chopper amplifier circuit configured as described above, the offset voltage of the sensor bridge and the output voltage of the offset adjusting voltage generation circuit cancel each other, whereby the offset voltage of the sensor bridge can be cancelled. 
   Further, the temperature characteristics of the offset voltage of the sensor bridge and the temperature characteristics of the output voltage of the offset temperature characteristics adjusting voltage generation circuit cancel each other, whereby the temperature characteristics of the offset voltage of the sensor bridge can be cancelled. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a block diagram of a chopper amplifier circuit of a first embodiment according to the present invention; 
       FIG. 2  is a block diagram of a conventional chopper amplifier circuit; 
       FIGS. 3A-3F  are waveform diagrams of the conventional chopper amplifier circuit; 
       FIG. 4  is a circuit diagram of a chopper amplifier circuit of the first embodiment according to the present invention; 
       FIG. 5  is a block diagram of a chopper amplifier circuit of a second embodiment according to the present invention; 
       FIG. 6  is a circuit diagram of the chopper amplifier circuit of the second embodiment according to the present invention; and 
       FIG. 7  shows an example of temperature characteristics of an offset voltage of a sensor bridge. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
     FIG. 1  is a block diagram of a chopper amplifier circuit of a first embodiment according to the present invention. 
   A chopper amplifier  40  receives sensor signals output from a sensor bridge  21  at input terminals  5 , and outputs amplified signals from output terminals  6 . The sensor signal input to the input terminals  5  is chopper-modulated with a modulation signal CLK in a multiplier  11 , and then amplified in an amplifier circuit  1 . An offset adjusting voltage generation circuit  31  generates an offset adjusting voltage that is equal in magnitude to and has a polarity opposite to an offset voltage of the sensor bridge  21 . The offset adjusting voltage is chopper-modulated with the modulation signal CLK in the multiplier  13  to be amplified in the amplifying circuit  3 . Adders  25  and  26  add the above-mentioned sensor signal to the offset adjusting voltage, thereby canceling the offset voltage of the sensor bridge  21  in the sensor signal. Further, the sensor signal is amplified in the amplifying circuit  2 , and chopper-modulated with the modulation signal CLK in the multiplier  12  to be modulated to an original frequency band (i.e., low frequency region including a DC). 
   Herein, the reason for using two adders  25  and  26  is to handle differential outputs of the amplifying circuits  1  and  3 . 
   According to the above-mentioned method of canceling the offset voltage of the sensor bridge, the offset adjusting voltage was used, which is equal in magnitude to and has a polarity opposite to the offset voltage of the sensor bridge  21  generated by the offset adjusting voltage generation circuit  31 . However, the offset adjusting voltage generation circuit  31  may be allowed to generate an offset adjusting voltage that is equal in magnitude to and has a polarity equal to the offset voltage of the sensor bridge  21 , and the offset modulation voltage may be chopper-modulated with a signal obtained by shifting the phase of the modulation signal CLK by 180° in the multiplier  13 . 
   In order to allow the offset adjusting voltage generation circuit  31  to generate a voltage that is equal in magnitude to the offset voltage of the output of the sensor bridge  21 , a high-frequency component may be removed by connecting a low-pass filter to the output terminal  6 , and the output of the offset voltage adjusting circuit  31  may be adjusted so that the potential of the output of the low-pass filter becomes 0. 
     FIG. 4  shows a circuit diagram of the chopper amplifier circuit of the present invention. The multipliers  11 ,  12 , and  13  are respectively composed of four switches. The switches can be realized by an N-channel MOS transistor or a CMOS transistor (in which the N-channel MOS transistor and the P-channel MOS transistor are connected in parallel). The amplifiers  1  and  3  have an instrumentation amplifier configuration, whereby an input impedance can be set to be significantly high. Therefore, a sensor bridge using a piezoresistor is not influenced. 
   The adders  25  and  26  shown in  FIG. 1  are integrated with the amplifying circuit  2  to be realized as a part of an adder amplifier circuit  10 . The adder amplifier circuit  10  is composed of a plurality of resistors and operational amplifiers. 
   In a configuration of the offset adjusting voltage generation circuit  31 , as an example, a fixed resistor and a variable resistor are used. A resistor  61  represents a fixed resistor, and a resistor  62  represents a variable resistor. By changing the resistor  62 , the output voltage of the offset adjusting voltage generation circuit  31  can be changed by changing the resistor  62  so that the offset voltage of a sensor bridge can be cancelled. 
   The circuit shown in  FIG. 4  is an example obtained by realizing the circuit configuration shown in  FIG. 1 , and the present invention is not limited to the circuit configuration shown in  FIG. 4 . 
   Second Embodiment 
     FIG. 5  is a block diagram of a chopper amplifier circuit of a second embodiment according to the present invention. 
   In general, an offset voltage of a sensor bridge has temperature characteristics. Herein, as an example, it is assumed that the offset voltage of the sensor bridge has temperature characteristics as shown in  FIG. 7 . 
   In the second embodiment, in addition to the chopper amplifier circuit of the first embodiment, an offset temperature characteristics adjusting voltage generation circuit  32  is provided. The offset temperature characteristics adjusting voltage generation circuit  32  outputs an offset temperature characteristics adjusting voltage having characteristics equal to the temperature characteristics of the offset voltage of the sensor bridge. The offset temperature characteristics adjusting voltage is chopper-modulated by the multiplier  14  and amplified by an amplifying circuit  4 , and then, added to a chopper-modulated sensor output signal by the adders  25  or  26 . At this time, the output of the offset temperature characteristics adjusting voltage generation circuit  32  of the sensor bridge is also added to the sensor output signal chopper-modulated by the adders  25  and  26  through the multiplier  13  and the amplifying circuit  3 . Thus, the offset voltage of the sensor bridge and the temperature characteristics of the offset voltage can be cancelled. 
     FIG. 6  shows a circuit diagram of the chopper amplifier circuit of the second embodiment according to the present invention. 
   The multipliers  11 ,  12 ,  13 , and  14  are respectively composed of four switches. The switches can be realized by an N-channel MOS transistor or a CMOC transistor (in which the N-channel MOS transistor and the P-channel MOS transistor are connected in parallel). The amplifying circuits  1 ,  3 , and  4  have an instrumentation amplifier configuration, whereby an input impedance can be set to be significantly high. Therefore, a sensor bridge using a piezoresistor is not influenced. The adder amplifier circuit  10  can be configured using a resistor and an operational amplifier. As an example of the configuration of the offset temperature characteristics adjusting voltage generation circuit  32 , a circuit using two kinds of the resistors  63  and  64  having different resistance temperature coefficients is shown. In a case of forming a resistor of polysilicon, not only a resistance but also a resistance temperature coefficient changes due to the concentration of an impurity contained in polysilicon. Therefore, by allowing the polysilicon resistor  63  and the resistor  64  to have different impurity concentrations, they can have different resistance temperature coefficients. 
   In  FIG. 6 , by shifting the phase of a modulation clock of the multiplier  14  by 180°, the same effect as that obtained by inverting the tilt of the offset voltage temperature characteristics generated in the offset temperature characteristics adjusting voltage generation circuit  32  can be obtained. Thus, by appropriately selecting the phase of the modulation clock of the multiplier  14 , the same effect can be obtained irrespective of whether the tilt of the offset voltage temperature characteristics of the sensor bridge is positive or negative. 
   The circuit shown in  FIG. 6  is an example obtained by realizing the circuit configuration in  FIG. 5 , and the present invention is not limited to the circuit configuration shown in  FIG. 6 .