Abstract:
An input offset voltage corrector used in an operational amplifier includes a switch unit, a register unit, an offset voltage correction unit and a micro control unit. The micro control unit sets the register unit to control the switch unit to switch the input offset voltage corrector to different operating modes. If an input offset voltage corrector is set to the offset voltage correcting mode, the offset voltage correction unit will adjust the input offset voltage of the operational amplifier to output an exact signal. Furthermore, the input offset voltage corrector can adjust the input offset voltage anytime according to the operating conditions to maintain the best characteristic of the operational amplifier.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an apparatus for correcting an input offset voltage, and more particularly to an input offset voltage corrector used for adjusting the input offset voltage of an operational amplifier. 
   2. Description of Related Art 
   As an input offset voltage is generally used for setting a static DC output voltage to zero or to a certain predetermined value, it is necessary to add the input offset voltage to the DC voltage at an input terminal. If the input stage of an operational amplifier is perfectly symmetrical and the transistor is fully compliant, then the input offset voltage will be equal to zero, but the geometric structure and doping of the chip may have discrepancies due to the change of fabrication process. Thus all operational amplifiers need a small voltage to be added between an inverting input terminal and a non-inverting input terminal to compensate for the incompliance of the chips. 
   In general, the input offset voltage is one of the important factors we should take into consideration for applying the characteristics of a comparator or an operational amplifier. The magnitude of the input offset voltage will affect the operating point of DC amplification and the maximum amplification factor for the application, or an error may occur after the signal is amplified. Referring to  FIG. 1  showing a circuit diagram of a non-inverting amplify circuit, an operational amplifier  10  is used as an application circuit of an amplifier. If a first resistor R 1  of the operational amplifier  10  is a 1 kΩ resistor and a second resistor R 2  is a 99 kΩ resistor, then the amplification factor of the non-inverting amplify circuit will be equal to 100. The output Vo of the operational amplifier  10  is equal to the sum of the input voltage Vin and the input offset voltage Vio multiplied by 100 (Vo=(Vin+Vio)×100). From this formula, we know that the input voltage Vin is amplified 100 times, and the input offset voltage Vio is also amplified 100 times. Therefore, it is necessary to consider the magnitude of the input offset voltage Vio for the circuit design and try to minimize the input offset voltage Vio, so as to avoid an excessively large error of the output Vo of the operational amplifier  10  caused by an excessively high input offset voltage Vio or to avoid saturating the operational amplifier  10 . 
   Referring to  FIG. 2  showing a circuit diagram of a current detection circuit, a comparator is used as an application circuit of an operational amplifier  10 . If the current I detected by the circuit falls in the range of 0 A˜20 A, an analog-to-digital converter  21  will be used for reading a current signal into a micro control unit  31 . Since the maximum detection current I can be up to 20 A, a 10 mΩ/5 W resistor is used as the current detection resistor R 3  to lower power consumption. Therefore, the magnitude of the voltage at the current detection resistor R 3  is equal to 0.2V; the voltage at the current detection resistor R 3  is amplified 25 times after going through a 1 kΩ fourth resistor R 4  and a 24 kΩ fifth resistor R 5 ; and the voltage at an output terminal of the operational amplifier  10  is equal to 0˜5V. However, if the input offset voltage Vio of the operational amplifier  10  is equal to ±30 mV, the error of the current of the micro control unit  31  read by the analog-to-digital converter  21  will be up to ±3 A (30 mV/10 mΩ), and the error percentage is equal to 15%. Therefore, the magnitude of the input offset voltage Vio of the operational amplifier  10  will affect the accuracy of the current read by the micro control unit  31 . 
   In a general semiconductor fabrication process for the micro control unit, the input offset voltage of the operational amplifier is approximately equal to ±30 mV. If it is necessary to design a low input offset comparator or amplifier, a trimming method or a chopper method is generally used for the design. However, if the input offset voltage is corrected by the trimming method, the correction result only can guarantee the performance under the correct conditions, since the input offset voltage will shift with different operating voltages, operating temperatures and input voltages. If the input offset voltage is corrected by the trimming method and the operating conditions are changed, the characteristic of the operational amplifier will deteriorate and a larger error will result. If the chopper method is adopted, the circuit requires additional clock circuit and filter, thus causing a higher cost for the circuit. Furthermore, the operating bandwidth and circuit response speed cannot be enhanced to meet the requirements of the application of a high-speed comparator or amplifier. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing shortcomings of the prior art, the present invention uses a micro control unit to set up a register to control the startup of a related switch circuit, such that the operational amplifier can correct and minimize the input offset voltage of an operating point and correct the input offset voltage anytime to maintain the best characteristic according to the operating conditions. 
   The present invention provides an input offset voltage corrector for correcting an input offset voltage of an operational amplifier, and the apparatus comprises a register unit, a micro control unit, a switch unit and an offset voltage correction unit. The register unit is used for temporarily storing an operating mode switching signal, and the micro control unit sets up the operating mode switching signal. An output terminal of the operational amplifier is connected to the micro control unit. The switch unit is installed between a signal input terminal and an operational amplifier for switching an operating mode of the input offset voltage corrector according to the operating mode switching signal. The foregoing operating mode includes a normal operating mode and an offset voltage correcting mode, and the offset voltage correcting mode further includes a positive terminal correcting mode and a negative terminal correcting mode. Each operating mode is determined by the action of each switch in the switch unit, and the action of each switch in the switch unit is operated according to the operating mode switching signal in the register unit. Finally, the offset voltage correction unit is used for correcting the input offset voltage of the operational amplifier, when the input offset voltage corrector is operated at an offset voltage correcting mode. 
   The input offset voltage corrector of the present invention uses a micro control unit to correct the input offset voltage anytime to improve and correct the input offset voltage by using the trimming method. Under different operating conditions, the characteristic of the operational amplifier will deteriorate, and the error will become larger. With the design of the offset voltage correction unit (which increases the number of correction bits) to minimize the input offset voltage more accurately and effectively enhance the application of the characteristic of the operational amplifier. 
   To make it easier for our examiner to understand the innovative features and technical content, we use a preferred embodiment together with the attached drawings for the detailed description of the present invention, but it should be pointed out that the attached drawings are provided for reference and description but not for limiting the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram of a non-inverting amplify circuit; 
       FIG. 2  is a circuit diagram of a current detection circuit; 
       FIG. 3  is a functional block diagram of an input offset voltage corrector in accordance with the present invention; 
       FIG. 3A  is a schematic view of an offset voltage correction unit in accordance with a preferred embodiment of the present invention; 
       FIG. 4  is a functional block diagram of an input offset voltage corrector in accordance with a preferred embodiment of the present invention; 
       FIG. 5  is a functional block diagram of an input offset voltage corrector in accordance with a preferred embodiment of the present invention; and 
       FIG. 6  is a functional block diagram of an input offset voltage corrector in accordance with a preferred embodiment of the present invention 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 3  showing a functional block diagram of an input offset voltage corrector in accordance with the present invention, the input offset voltage corrector comprises a register unit  32 , a micro control unit  31 , a switch unit  34  and an offset voltage correction unit  33  for correcting an input offset voltage of the operational amplifier  30 . The signal inputted by the signal input terminal  35  is sent to the operational amplifier  30  by the switch unit  34 , and the processed signal is outputted from the operational amplifier  30  to the micro control unit  31  for the application. To overcome the influence caused by the input offset voltage of the operational amplifier  30 , the micro control unit  31  sets an operating mode switching signal in the register unit  32  to control the switch unit  34  to switch the input offset voltage corrector between a normal operating mode and an offset voltage correcting mode. If the input offset voltage corrector is at an offset voltage correcting mode, then the offset voltage correction unit  33  will correct and minimize the input offset voltage of the operational amplifier  30 . 
   Referring to  FIG. 3A  showing a schematic view of an offset voltage correction unit in accordance with a preferred embodiment of the present invention, a control circuit  331  disposed between a position input terminal and a negative input terminal of the operational amplifier is used for controlling a control switch S 1 , S 2 , S 3  . . . Sn to correct the input offset voltage of the operational amplifier  30  and minimize the input offset voltage. 
   However, the larger number of bits of the offset voltage correction unit  33 , the smaller the range is for correcting the input offset voltage, and the less impact the application characteristic of the input offset voltage has upon the operational amplifier  30 . Furthermore, the foregoing offset voltage correcting mode includes a positive terminal correcting mode and a negative terminal correcting mode provided for the design of the operational amplifier  30 . In a switch between a positive terminal correcting mode and a negative terminal correcting mode which is similar to a switch between a normal operating mode and an offset voltage correcting mode, the micro control unit  31  sets up an operating mode switching signal in the register unit  32 , and the switch unit  34  executes the corresponding switch according to the operating mode switching signal in the register unit  32 , such that the input offset voltage corrector will be operated at an operating mode set by the micro control unit  31 . 
   To prevent the correction result that can only guarantee the performance under the correct conditions, the micro control unit  31  further includes a timer module (not shown in the figure) for providing a correcting cycle, and the operating mode switching signal is set with a specific cycle in the register unit  32  to start the input offset voltage corrector at an offset voltage correcting mode and execute the input offset voltage correction. 
   Furthermore, the register unit  32  also can be installed in a register of the micro control unit  31  for temporarily storing an operating mode switching signal which is set by the micro control unit  31  to control the switch unit  34 . 
   Referring to  FIG. 4  for a functional block diagram of an input offset voltage corrector in accordance with a preferred embodiment of the present invention, the signal input terminal  35  includes a positive signal input terminal and a negative signal input terminal. The switch unit  34  includes a first switch  341 , a second switch  342  and a third switch  343 . The register unit  32  includes a first register  321  and a second register  322 . A terminal of the first switch  341  is connected to a positive signal input terminal, and another terminal of the first switch  341  is connected to a positive terminal of the operational amplifier  30 . A terminal of the second switch  342  is connected to a negative signal input terminal, and another terminal of the second switch  342  is connected to a negative terminal of the operational amplifier  30 . A terminal of the third switch  343  is connected to the first switch  341  and a positive terminal of the operational amplifier  30 , and another terminal of the third switch  343  is connected to the second switch  342  and a negative terminal of the operational amplifier  30 . 
   In the foregoing description for  FIG. 3 , the register unit  32  temporarily stores an operating mode switching signal set by the micro control unit  31 , and the operating mode switching signal includes an operating mode switching signal temporarily stored in the first register  321  for switching a normal operating mode and an offset voltage correcting mode, and an operating mode switching signal temporarily stored in the second register  322  for switching a positive terminal correcting mode and a negative terminal correcting mode. The first switch  341 , second switch  342  and third switch  343  are operated according to an operating mode switching signal temporarily stored in the first register  321  and the second register  342  to start the input offset voltage corrector at a normal operating mode or an offset voltage correcting mode, or even at a positive terminal correcting mode or a negative terminal correcting mode of an offset voltage correcting mode. If the offset voltage corrector is started at the offset voltage correcting mode, the offset voltage correction unit  33  will be used for correcting the input offset voltage of the operational amplifier  30  to maintain a better application of the characteristic of the operational amplifier  30 . 
   If each switch in the switch unit  34  sets the first switch  341  and second switch  342  to ON and the third switch  343  to OFF according to the operating mode switching signal, then the operating mode of the input offset voltage corrector will be a normal operating mode. If each switch in the switch unit  34  sets the first switch  341  and third switch  343  to ON and the second switch  342  to OFF according to the operating mode switching signal, then the operating mode of the input offset voltage corrector will be a positive terminal correcting mode. If each switch in the switch unit  34  sets the second switch  342  and third switch  343  to ON and the first switch  341  to OFF according to the operating mode switching signal, then the operating mode of the input offset voltage corrector will be a negative terminal correcting mode. A user can switch the mode which is set by the micro control unit  31  to make an appropriate correction of the input offset voltage according to the requirements of the application circuit. 
   Referring to  FIG. 5  for a functional block diagram of an input offset voltage corrector in accordance with a preferred embodiment of the present invention, the embodiment is used for the input offset voltage of the operational amplifier  30 , and its correcting mode is applied for correcting the negative terminal. The related actions and principles are the same as the embodiment as shown in  FIG. 4  and thus will not be described here. The method of using the switch unit  34  to switch an operating mode in accordance with this embodiment will be described as follows. 
   In the embodiment as shown in  FIG. 5 , the switch unit  34  includes a first switch  341  and a third switch  343 . A terminal of the first switch  341  is connected to a positive signal input terminal, and another terminal of the first switch  341  is connected to a positive terminal of the operational amplifier  30 . A terminal of the third switch  343  is connected to the first switch  341  and a positive terminal of the operational amplifier  30 , and another terminal of the third switch  343  is connected to a negative signal input terminal and a negative terminal of the operational amplifier  30 . The negative signal input terminal is connected directly to a negative terminal of the operational amplifier  30 . 
   After the micro control unit  31  sets an operating mode switching signal in the register unit  32 , if the first switch  341  is switched to ON and the third switch  343  is switched to OFF according to the operating mode switching signal temporarily stored in the register unit  32 , the operating mode of the input offset voltage corrector will be a normal operating mode. If the first switch  341  is switched to OFF and the third switch  343  is switched to ON according to the operating mode switching signal, then the operating mode of the input offset voltage corrector will be a negative terminal correcting mode. 
   Therefore, the micro control unit  31  sets a signal temporarily stored in the register unit  32  to control the action of the switch unit  34 , so as to switch the input offset voltage corrector between a normal operating mode and a negative terminal correcting mode. 
   Referring to  FIG. 6  for a functional block diagram of an input offset voltage corrector in accordance with a preferred embodiment of the present invention, the embodiment is used for an input offset voltage of the operational amplifier  30 , and the correcting mode is applied for correcting the positive terminal. The related actions and principles are the same as the embodiment as shown in  FIG. 4  and thus will not be described here. The method of using the switch unit  34  to switch an operating mode in accordance with this embodiment will be described as follows. 
   In an embodiment as shown in  FIG. 6 , the switch unit  34  includes a second switch  342  and a third switch  343 . A terminal of the second switch  342  is connected to a negative signal input terminal, and another terminal of the second switch  342  is connected to a negative terminal of the operational amplifier  30 . A terminal of the third switch  343  is connected to a positive signal input terminal and a positive terminal of the operational amplifier  30 , and another terminal of the third switch  343  is connected to a negative signal input terminal and a negative terminal of the operational amplifier  30 . The positive signal input terminal is connected directly to a negative terminal of the operational amplifier  30 . 
   After the micro control unit  31  sets an operating mode switching signal in the register unit  32 , if the second switch  342  is set to ON and the third switch  343  is set to OFF according to an operating mode switching signal temporarily stored in the register unit  32 , then the operating mode of the input offset voltage corrector will be a normal operating mode. If the second switch  342  is set to OFF and the third switch  343  is set to ON according to the operating mode switching signal, then the operating mode of the input offset voltage corrector will be a positive terminal correcting mode. 
   Therefore, the signal temporarily stored in the register unit  32  set by the micro control unit  31  can be used for controlling the action of the switch unit  34 , so as to switch the input offset voltage corrector between a normal operating mode and a positive terminal correcting mode. 
   In summation of the description above, the input offset voltage corrector of the present invention can be used for correcting an input offset voltage of the operational amplifier  30  effectively, such that if the operational amplifier  30  is applied to the circuit of an amplifier or a comparator, the influence caused by the input offset voltage to the application circuit can be reduced significantly. 
   Although the present invention has been described with reference to the preferred embodiments thereof, it will be understood that the present invention is not limited to the details thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the present invention as defined in the appended claims.