Abstract:
An improved dual slope integrating analog-to-digital converter for use in a Digital Multi-Meter includes an input portion, an A/D core portion, and a digital portion. The A/D core portion includes an auto zero function capability for automatically compensating for any existing offset voltages in various measurement modes such as voltage, resistance, or current measurement modes. The improved A/D converter of the present invention operates at high speed and with high resolution.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an A/D converter for converting an analog signal into a digital signal. Particularly, an A/D converter with a high resolution of &#34;X10 Mode&#34; which is capable of providing a Ratio Metric Reading by cancelling an offset voltage in response to an application of a &#34;Auto-Zero function. 
     2. Description of the Art 
     In general, a resolution of a dual slope integrating A/D converter employed in a Digital Multi-Meter (DMM) is 31/2 digits. But, in a dual slope A/D converter having a 31/2 digit resolution, a residual electric charge zero crossing can be charged by a capacitor of predetermined capacitance to multiply a voltage by ten times. The multiplied value is deintegrated again such that the resolution of the dual slope A/D converter is 41/2 digits or more. 
     The above mentioned mode is called &#34;X10 Mode&#34;. A conventional dual slope integrating A/D converter applying the X10 Mode is shown in FIG. 1A. This converter comprises input portion 1, A/D core portion 2, and digital portion 3. 
     For a voltage measurement, input portion 1 will include mainly two switches S 1  and S 2  and a reference voltage ±V REF  as shown in FIG. 1B. For a resistance measurement, input portion 1 will include mainly a supply power source, switches S 3  to S 8 , a reference resistor R fef , a capacitor C x , and a measured resistor R x  as shown in FIG. 1C. 
     A/D core portion 2 comprises a buffer BF, and operational amplifier (OP AMP), a comparator CM, switches S 9  to S 11 , a resistor R 1 , and a capacitor C 1  to C 3 . 
     As shown in FIG. 1D, digital portion 3 comprises a clock signal input portion 4, a zero crossing detecting portion 5, a logic control portion 6, a decoder/resistor 8, and a drive/display portion 9. 
     The conventional dual slope A/D converter comprising the above-mentioned constituent elements for performing a voltage measurement operation, connects the portion measuring voltage between a power source terminal V and a common terminal Acom in FIG. 1B. Reference voltage ±V REF  is employed as the voltage for discharging the electric charge charged by integrating capacitor C 1  of A/D core portion 2 shown in FIG. 1A by way of the common terminal. In this manner, the discharged electric charge quantity can be regulated. 
     If a measured voltage C IN  has a negative polarity, a reference voltage ±V REF  is supplied, and if the measured Voltage V IN  is a positive polarity, a reference voltage -V REF  is supplied. 
     For performing a resistance measurement, input portion 1 resembles a ratio metric reading circuit. A supply power source V s , a reference resistor R ref , a resistance measuring portion (that is, resistor R x ), and common terminal A COM  are all connected in series as shown in FIG. 1C. As a current flows through this series circuit, a voltage drop across reference resistor R ref  will be represented as V ref  and a voltage drop across resistor R x  will be represented as V x . 
     Reference resistor R ref  is connected to capacitor C x  through switches S 3  and S 4 . One terminal of capacitor C x  is connected to an output terminal LO through switch S 5 . The other terminal of capacitor C x  is connected to an output terminal HI through switch S 6 . 
     As input terminal R is connected to output terminal HI through switch S 7  and terminal A com  is connected to output terminal LO through switch S 8 , the voltage charge in capacitor C x  is deintegrated. 
     On the other hand, the conversion operation is performed by a conversion cycle defined as shown in FIG. 2. The conversion cycle includes a zero integration phase Z1, an integration phase INT, a first deintegration phase DE1, a REST phase, X10 phase, and a second deintegration phase DE2. 
     In order to display the difference between the value converting the offset voltage and the value converting the following input V IN  (the measured voltage), first the power source terminal V must be shorted at the common terminal (that is, the zero reading). Then it can receive the input V IN . 
     Let us define the operating time during zero integration phase ZI as T ZI . During a voltage measurement operation, if switches S 9  and S 10  of A/D core portion 2 in FIG. 1A are closed during zero integration phase Z1, the voltage V o  at node P during zero integration phase Z1 can be expressed by the following equation. ##EQU1## Therefore, you will find that the voltage V o  at node P varies with a change in T zI . 
     Let us now define the operating time during integration phase INT as T INT . 
     If switch S 1  in FIG. 1B is closed during integration phase INT, a current will flow through buffer BF, resistor R 1  and capacitor C 1  of A/D core portion 2 in FIG. 1A. As a result, the voltage V o  at node P is changed. At this time, the change (ΔV o ) is expressed by the following equation: ##EQU2## The first deintegration phase DE1 is operated from the application of reference voltage -V REF  till the beginning of the zero crossing. 
     Let us define the operating time in first deintegration phase DE1 as T DE1 . The change (ΔV o ) of voltage V o  at node P can be expressed by the equation: ##EQU3## The amount of voltage change at the moment a zero crossing becomes &#34;0&#34; can be expressed by the equations: 
     
         V.sub.o |T.sub.INT +V.sub.o |T.sub.DE1 =0 (4) 
    
     and ##EQU4## In Eq. 5, if reference voltage (V REF ) is regulated and V REF  +V os1  -V os2  equals V REF&#39; , Eq. 5 can be expressed by the equation: ##EQU5## Therefore, if a zero reading is performed first and ##EQU6## the offset voltage can be cancelled by T DE1  -T DE  (zero reading). 
     That is, T DE  is expressed by the equation. ##EQU7## where V REF&#39;  equals V REF  +V os2 , and T INT  is a fixed time. 
     Next, let us define the operating time during the REST phase as T REST . If switch S 9  of A/D core portion 2 in FIG. 1A is closed during the REST phase, the residual electric charge stored in capacitor S 3  will be kept during the REST phase. 
     At this time, the voltage across capacitor C 1  becomes the same voltage as that across capacitor C 3 . 
     During the X10 phase, switch S 10  of A/D core portion 2 is closed and switch S 11  is opened. Then, the output of comparator CM is fedback to buffer BF and operational amplifier OP AMP as the residual electric charges in capacitor C 3  is transferred to capacitor C 2 . The multiplied voltage at node P varies with the capacitance of capacitors C 2  and C 3 . 
     As the capacitance of capacitor C 3  reaches a value ten times greater than the capacitance of capacitor C 2 , the voltage applied across capacitor C 2  becomes ten times greater than that across capacitor C 3 . 
     The Second integration phase DE2 discharges the multiplied voltage V o  at node P and counts the time for zero crossing to begin. As the counted time and time T DE1  for first deintegration phase DE1 is calculated, a resolution of X10 can be obtained. 
     The operating condition of the switches in each conversion cycle is shown by the following table 1. 
     
                       TABLE 1______________________________________The operating condition of theswitches during each conversion cycle:DrawingsFIG. 1A      FIG. 1B  FIG. 1CSwitchPhase  S.sub.9        S.sub.10              S.sub.11                  S.sub.1                      S.sub.2                           S.sub.3                                S.sub.4                                    S.sub.5                                        S.sub.6                                             S.sub.7                                                 S.sub.8______________________________________ZI     C     C     C   O   O    C    C   O   O    O   OINT    O     O     C   C   O    C    C   O   O    C   CDE1    O     O     C   O   C    O    O   C   C    O   OREST   C     O     C   O   O    C    C   O   O    O   OX10    O     C     O   O   O    C    C   O   O    O   ODE2    O     O     C   O   C    O    O   C   C    O   O______________________________________ C represents a switch is closed, and O represents a switch is opened. 
    
     Up to now we have described the operation for a voltage measurement operation. A conversion cycle for a resistance measurement operation progresses very much like that for a voltage measurement operation. 
     For a resistance measurement operation, during the zero integration phase Z1, the measuring register R x  is connected between input terminal R and common terminal ACOM as shown in FIG. 1C such that supply power source V s  is applied through reference resistor R ref . The current I s  through measuring resistor R x  is therefore expressed by the following equation: ##EQU8## 
     Switches S 5  and S 6  are closed during the integration phase INT such that voltage V x  applied across measuring resistor R x  is integrated. 
     Voltage V REF  applied across reference resistor R REF  in FIG. 1C is expressed by the equation: ##EQU9## 
     Switches S 5  and S 6  are closed during the first deintegration phase DE1 such that voltage V o  at node P integrated by voltage V x  is discharged again. 
     The first deintegration phase DE1 is operated until the zero crossing begins. At this point, T DE1  is calculated during the first deintegration phase DE1. T DE1  is expressed by the equation: ##EQU10## It should be noted that ##EQU11## varies in response to a change of measuring resistor R x . Therefore, although it provides the zero reading, the offset voltage V os1-V   os2  is not cancelled. 
     The conventional dual slope A/D converter has the following problems. 
     The conventional dual slope A/D converter operating as mentioned above is in need of a zero reading step, therefore the conversion time during each conversion cycle is lengthened. As a result, its operating speed is slow. Further, as the offset voltage is not completely cancelled in a ratio metric reading, it is very difficult to obtain high resolution. 
     SUMMARY OF THE INVENTION 
     The object of the present invention which overcomes the above-mentioned problems of the conventional dual slope A/D converter is to provide a dual slope integrating A/D converter which has a fast operating speed and a resolution of 41/2 digits and up. 
     To achieve the above objects, according to the present invention, a dual slope integrating A/D converter comprises an input portion 11; and A/D core portion 12 which includes a buffer BF and an operational amplifier (OP AMP) for respectively receiving an output of input portion 11, a resistor R1 connected to the output terminal of the buffer BF, an auto zero capacitor C AZ  connected between resistor R1 and operational amplifier OP AMP, a capacitor C 1  for charging an offset voltage and integrating the input voltage using resistor R1, a capacitor C 2  for amplifying the voltage V o  at node P with a residual electric charge, a capacitor C 3  for charging the residual electric charge, a comparator CM, and a switch S A  to S c  and S9 to S 11  ; a digital portion 13 for inputting the output of A/D core portion 12 in which swithces S A  to S D  and S 1  to S 11  are driven by generated control signals AZ, INT, DE1, REST, Z10, X10, REX, and DE2. 
     Here, the constitution of input portion 11 includes the same circuits as that of the conventional input portion 1 shown in FIGS. 1B and 1C for voltage and resistance measurement operations, respectively. The constitution of the digital portion 13 of the present invention also includes the same circuit as that of the conventional digital portion 13 shown in FIG. 1D. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above object and feature of the present invention will be apparent from the following description of the preferred embodiment with reference to the accompanying drawings. 
     FIG. 1A is a circuit diagram of a conventional dual slope integrating A/D converter. 
     FIG. 1B is a detailed circuit diagram of an input portion for performing voltage measurements in a dual slope integrating A/D converter. 
     FIG. 1C is a detailed diagram of an input portion for performing resistance measurements in a dual slope integrating A/D converter. 
     FIG. 1D is a schematic block diagram of a digital portion for a dual slope integrating A/D converter. 
     FIG. 2 is a waveform chart showing an output signal of a conventional dual slope integrating A/D converter. 
     FIG. 3 is a circuit diagram of a dual slope integrating A/D converter of the present invention. 
     FIG. 4 is a waveform chart of a switch controlling signal. 
     FIG. 5 is a waveform chart illustrating an output signal of the present invention. 
     FIG. 6 is a block diagram illustrating a preferred embodiment of applying the present invention to a Digital Multi-Meter (DMM). 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The constitution, function and effect of the present invention will be explained in detail with reference to the accompanying drawings. 
     FIG. 3 is a circuit diagram showing a constitution of the dual slope integrating A/D converter of the present invention. FIG. 4 is a waveform diagram showing the controlling signal regulating the switches shown in FIG. 3. FIG. 5 is a waveform diagram showing the output signal of the dual slope integrating A/D converter of the present invention. 
     An offset voltage is charged by auto zero capacitor C AZ  and capacitor C 1  in A/D core portion 12 during an auto zero phase AZ. Input voltage V IN  is integrated through resistor R1 and capacitor C1 during an integration phase INT. 
     The charged voltage is discharged as reference voltage ±V REF  during a first deintegration phase DE1. Thereafter, the residual electric charge is charged to capacitor C 3  and proceeds to capacitor C 2  during a REX phase and a REST phase such that the residual electric charge serves to amplify voltage V o  at node P. 
     First, the operation and constitution of the circuit during the auto zero phase will be explained. Input terminal HI of A/D core portion 12 is connected to common terminal A COM  by switch S A . The switch S B  is opened, and the switch S 11  is closed. The output of comparator CM is also feedback to the negative terminal of buffer BF by switch S c . At such point, capacitor C 3  is separated from the positive input of comparator CM such that capacitor C 3  will have no influence on the circuit. 
     Voltage V A  at node A at time T AZ  is about -Vos 1 . Vos 1  represents the offset voltage of buffer BF and a current flows through resistor R 1 . At that instant, voltage V o  at node P is expressed by the equation: 
     
         Vo|T.sub.AZ =(V.sub.os2 +v.sub.03)                (A) 
    
     Also, during the integration phase INT, switch S 1  shown in FIG. 1B is closed such that the measured input voltage V IN  is integrated. At this point, switches S A , S B , and S C  are opened by switch controlling signals AZ and REX shown in FIG. 4, and voltages V A  and V o  at nodes A and P, respectively during the auto zero phase AZ are as follows: 
     
         V.sub.A =V.sub.os1, and V.sub.o =V.sub.os2 +V.sub.os3 
    
     When an input voltage V IN  is applied, the voltage applied across resistor R 1  is (V IN  -V os1 )-V A  =V IN  -V os1  -(-V os1  =V IN , and the offset voltage V os1  is thus cancelled. 
     At this point, if the change of voltage V o  at node P is to be represented as ΔV o , ΔV o  can be expressed by the equation: ##EQU12## the voltage at node P being as follows: ##EQU13## 
     Switch S 2  shown in FIG. 1B is closed by switch controlling signal DE1 during the first deintegration phase DE1. As a result, the electric charge charged to the reference voltage V REF  is discharged and a zero crossing is performed. 
     At this point, the zero crossing voltage becomes the voltage at the auto zero phase (V o  |T AZ  =V os2  +V os3 ) somewhat the polarity of the reference voltage V REF  becomes the opposite of the polarity of the input voltage V REF , so that the change V o  of the voltage V o  at the node P is expressed by the equation: ##EQU14## voltage V o  at node P in the zero crossing is expressed by the equation: 
     
         V.sub.o |T.sub.INT +T.sub.DE1 =V.sub.o |T.sub.AZ =V.sub.os2 +V.sub.0s3.                                    (E) 
    
     Therefore, Eq. (E) can be expressed by the equation: ##EQU15## 
     Thus, if the time of T DE1  satisfying V o  |T INT  +T DE1  =V O  T AZ  is obtained, T DE1  equals ##EQU16## Therefore, the offset voltage is completely cancelled. The offset voltage is cancelled irrespective of the strength of the input voltage V IN  or that of the reference voltage V REF , which is applicable to the ratio metric reading, that is, the resistance measurement operation. 
     And, during the REST phase, switches S9, S11, and S B  are closed by the switch controlling signals REST, X10, and REX shown in FIG. 4, such that the residual electric charge is charged by the output voltage at node P after the zero crossing. 
     During the X10 phase, the switch controlling signals X10 and X10 shown in FIG. 4 close the switch S10 and open the switch S11. However, since the capacitance of capacitor C3 is far more voluminous than that of capacitor C2, the electric charge C3 is far more voluminous than that of capacitor C2, the electric charge of capacitor C3 is transferred to capacitor C2, and the voltage Vo at the node P is multiplied according to the capacitance value of capacitors C2 and C3. 
     In order to multiply by ten the voltage Vo at the node P due to the residual electric charge, the capacitance of capacitor C 3  must be ten times in comparison with that of the capacitor C 2 . That is, C 3  must be equal 10×C 2 . 
     When the voltage V o  at the node P is swung by the combination of the capacitors C 3  and the auto zero capacitor C AZ  during the X10 phase, the swung voltage is again applied to the voltage V A  at the node A and produces the gain error. 
     In order to prevent this occurrence, switch S B  is closed by the switch controlling signal REX shown in FIG. 4 only for the REST phase and the X10 phase and the REX phase is operated. 
     Also, during the second integration phase DE2, the voltage V o  multiplied by the reference voltage V REF  is deintegrated again so that the resolution may be improved. 
     The operating condition of the switches during a conversion cycle are shown in Table 2. 
     
                                           TABLE 2__________________________________________________________________________The operating condition of the switches in aconversion cycle:DrawingsFIG. 3               FIG. 1B                     FIG. 1CSwitchPhase    S9 S10     S11        SA SB SC                S1                  S2 S3                       S4                         S5                           S6 S7                                S8__________________________________________________________________________AZ  O  O  C  C  O  C O O  C C O O  O OINT O  O  C  O  O  O C O  C C O O  C CDEI O  O  C  O  O  O C C  O O C C  O OREST    C  O  C  O  C  O O O  C C O O  O OREX O  O  C  O  C  O O O  C C O O  O OX10 O  C  O  O  C  O O O  C C O O  O O ##STR1##O  O  C  O  C  O                 O                   O  C                        C                          O                            O  O                                 ODE2 O  O  C  O  O  O O C  O O C C  O O__________________________________________________________________________ C represents that a switch is closed. O represents that a switch is opened. 
    
     FIG. 6 shows a preferred embodiment of the present invention. That is, in FIG. 6 is shown the present dual slope integrating A/D converter applied to a DMM of the MICOM type. The reduction function (not shown) is connected to input portion 21 so as to perform the auto range function. 
     The constitution of input portion 21 changes according to the selected operating measurement mode (i.e., a voltage V, a resistance R, a current I and so on). The MICOM 24 recognizes the function selection information and changes the constitution of the input portion 21. Here, the circuit must function such that the voltage for converting the common feature in measuring voltage V, resistance R or current I into the appropriate voltage may be output to the A/D core portion 22 and converted in the A/D core portion 22. 
     The A/D converter, which also includes a reference voltage V REF  generator, provides the reference voltage V REF  to the A/D core portion 22 and is regulated with an external minute value. 
     Also, the A/D core portion 22 converts the inputted voltage and transmits the zero crossing time to the MICOM interface 23. 
     The MICOM interface 23 transmits all control signals of the MICOM 24 to the input portion 21 and the A/D core portion 22 as well as those signals generated by the A/D core portion 22 to the MICOM 24. 
     The MICOM 24 generates the respective conversion cycle according to the constitution of the software, receives and calculates the zero crossing signal, and generates the digital signal or the signal performable display by driving LCD 25. 
     Therefore, the present system has the advantage that there can be added digital functions of memory including the specific functions of calculating minimum, maximum, and mean values by classifying a conversion cycle into its respective measuring mode or by utilizing an operation function. 
     As explained above, the dual slope integrating A/D converter of the present invention improves the operating speed of the circuit by obviating use of zero reading steps. 
     Further, the offset voltage is removed during an auto zero phase such that the dual slope integrating A/D converter of the present invention may be used in a DMM without difficulty. 
     Also, the present invention is such that the auto zero function can be added with little circuit modification to conventional A/D converter circuits with 31/2 digit resolution to serve as an A/D converter with 41/2 -digit resolution or more. 
     The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.