Abstract:
An automatic self-calibration system for digital teraohmmeter wherein a built-in standard resistor is provided. The system is able to calibrate the values of the key components by taking the standard resistor as reference. The calibration procedure of the invention is controlled by a computer and executed automatically. The system is used to measure high-resistance standards up to 10 15  Ω where the standard uncertainty does not exceed 100 ppm after a traceable calibration.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an automatic self-calibration system for a digital teraohmmeter, and especially to an automatic self-calibration device and method applicable to integrating high-resistance measuring apparatus. The automatic self-calibration system of this invention may execute the calibration automatically at any time before the measurement procedure by the digital teraohmmeter. The automatic self-calibration system for the digital teraohmmeter of this invention is able to measure high-resistance standards up to 10 15  Ω where the standard uncertainty does not exceed 100 ppm after a traceable calibration. 
     BACKGROUND OF THE INVENTION 
     The integrator approach is one of the most popular approaches in the field of high-resistance measurement. When applied to the measurement of high-resistance, this approach avoids several innate drawbacks of high-resistance devices. 
     The integrator approach employs the principle of an analog integrator by recording the time during which the level of the output terminal of the integrator changes to another level as the &#34;integrating time&#34; and using the integrating time as a basis to give the value of a device having a high-resistor. The resistance value is then displayed on the screen of the teraohmmeter. 
     FIG. 1 is illustrates a block diagram of a conventional teraohmmeter. In this figure, 1 represents a DC voltage source which supplies DC voltage to the resistor, 2 is a circuit for providing reference voltages and a test voltage, 3 is an operational amplifier, 4 is a level comparator which compares the test and reference voltages or levels, 5 is a counter, 6 is a standard frequency generator which generates a standard frequency, and 7 is a gate. R X  represents the test resistor which is a device with high resistance to be measured. The voltage as provided by the DC voltage 1 is divided by circuit 2 into a reference voltages V 1  and V 2  and a test voltage V T  at the ratio 2. The test voltage V T  is transmitted through the test resistor R X  and integrated at the operational amplifier 3, and reaches the level comparator 4. Relay RL 1  is used to start/stop the integrating operation. C is an integrating capacitor. 
     Under normal conditions, the relay RL 1  is closed and the capacitor C is precharged. During measurement, the relay RL 1  is open and the integrating process is started. The level comparator 3 compares the voltage V 0  and the counter 5 counts the integrating time dt of the voltage V 0 , as the reference it changes from V 1  to V 2 , according to the signals generated by the standard frequency generator 6. The integrating time is then provided to a calculation device (not shown). The calculation device calculates the resistance value of the resistor R X  according to the equation as described hereinafter. 
     The calculation of the resistance R X  includes the following equation: 
     Given that R X  represents the resistance value of the resistor, i R  represents the current of the output terminal of the resistor R X , V T  is the voltage of the input terminal of the resistor R X , the capacitance of the capacitor C is C, the voltage difference of the operational amplifier 3 is dV 0  and the integrating time is dt, then: 
     According to the principle of the operational amplifier: ##EQU1## 
     The value of R X  may be given from the value of C, dt and the voltage ratio V T  /dV 0 . 
     In the conventional teraohmmeter as described above, the resistance value R X  is a function of the integrating capacitance C. As a result, the accuracy of the measurement of the resistor R X  is dependent on the accuracy and stability of the integrating capacitor C. When a drift happens in the integrating capacitor C, accurate measurement of the resistor R X  may not be achieved. 
     OBJECTS OF THE INVENTION 
     It is thus an object of this invention to provide a digital teraohmmeter wherein self-calibration and in particular calibration of the capacitance used as the standard for calculating a resistance by integrating may be carried out automatically. 
     Another object of this invention is to provide a digital teraohmmeter with self-calibration functions but without the need for complicated circuitry. 
     SUMMARY OF THE INVENTION 
     According to this invention, a digital teraohmmeter is provided wherein a built-in standard resistor is taken for reference in calibrating the capacitance used in resistance calculating and a special self-calibration procedure is used to calibrate the digital teraohmmeter automatically. 
     The above and other objects and advantages of this invention may be clearly understood according to the detailed specification of the invention by referring to the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, 
     FIG. 1 is a the block diagram of a conventional digital teraohmmeter. 
     FIG. 2 is a block diagram of an automatic self-calibration system for the digital teraohmmeter of the preferred invention. 
     FIG. 3 is a flow chart of the self-calibration method for the digital teraohmmeter of the preferred invention. 
     Table I records the self-calibration process and the results of calibration of the automatic self-calibration method for digital teraohmmeter of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following is a detailed description of the automatic self-calibration system and method for digital teraohmmeter of this invention. 
     FIG. 2 is the block diagram of the automatic self-calibration system for the digital teraohmmeter of the preferred invention. In the figure, components which appear in FIG. 1 are labeled with the same numbers. 
     As shown in FIG. 2, therefore, the automatic self-calibration system for the digital teraohmmeter of this invention comprises a DC voltage source 1, a circuit for providing reference voltages V 1  and V 2  and a test voltage V T  2, an operational amplifier 3, a level comparator 4, a counter 5 and a capacitor C settable to three different values; 900, 100 and 8100 pF. The operational amplifier 3 and the capacitor C constitute an integrator. The device of this invention further comprises a first switch SW 1  and a second switch SW 2 , a first amplifier α, a second amplifier β and a third amplifier γ, and a reference standard device whose resistance is R S . R X  represents the resistor to be measured. 
     In this embodiment of the invention, the gain ratio of the first amplifier α is: α1:α2:α3=1/9:1:9; of the second amplifier β is: β1:β2:β3=1/90:1/10: 9/10; and of the third amplifier γ is ±1. 
     In the measurement procedure, the input voltage V T  is provided by the DC voltage source 1. The first switch SW 1  is set at the &#34;Meas&#34; position in FIG. 2. The resistance R X  will be measured according to the process as described above. 
     The automatic self-calibration system for the digital teraohmmeter of this invention not only measures the resistance R X  but also the capacitance of the capacitor C. The capacitance of the capacitor C may then be used to calibrate the digital teraohmmeter. 
     The following is a description of the self-calibration method of this invention: 
     A. External Calibration 
     In the automatic self-calibration system for a digital teraohmmeter of this invention, a built-in standard resistor R S  is provided. The standard resistor R S  comprises nine 10 MΩ resistors. When the external calibration is operated, the nine 10 MΩ resistors are connected in parallel and the total resistance is equal to 1.1 MΩ. After the external calibration, the nine resistors are connected in series to make the total resistance 90 MΩ, for self-calibration. The external calibration of the standard resistor generally takes place once a year at a 1 MΩ level. 
     B. Self-Calibration. 
     The self-calibration of the teraohmmeter is operated before the measurement of the resistor R X  takes place. The automatic self-calibration system of this invention enables the calibration of the capacitor C and the linearity of the teraohmmeter from time to time. 
     During the self-calibration, the second switch SW 2  is set at the &#34;V cal  &#34; position and the first switch SW 1  at the &#34;Cal&#34; position. The testing voltage V T  is set equal to V REF  and is converted into a 1 V voltage at the second operational amplifier β. The 1 V voltage is then applied to the standard resistor R S  (90 MΩ) and an input current of 11 nA is generated. 
     The testing voltage V REF  reaches the operational amplifier 3 via the second switch SW 2 , the second amplifier β, the standard resistor R S  and the first switch SW 1 . The integrating operation takes place at the output terminal of the operational amplifier 3 and the actual capacitance C of the capacitor C may be calculated according to the integrating time dt of the operational amplifier 3. 
     In the embodiment of this invention, the standard resistor R S  is a 9×10 MΩ resistor. 
     C. Calibration Procedure 
     FIG. 3 is a flow chart of the automatic self-calibration method for digital teraohmmeter of this invention. Table I records the self-calibration procedure and the results. The following is a description of the procedure according to Table I and FIG.3 
     In the illustrated process, the standard resistor is a 90 MΩ resistor. First, at 101, the second switch SW 2  is set at the &#34;R cal  &#34; position and the first switch SW 1  at the &#34;Cal&#34; position. The 11 nA current is provided from V REF  to the teraohmmeter (1 V/90 MΩ). At 102, the gain ratio of the first operational amplifier is set to 1/9 (α=1/9). An Integrating operation is processed at the operational amplifier 3, with the capacitance set at 900 pF. and the initial parameters as shown at 1 of Table I are recorded. 
     At 103, the first switch SW 1  and the second switch SW 2  are kept at their original positions and the 11 nA current is provided from V REF . The gain ratio of the first amplifier is set to 1 (α=1) and the linearity of the measurement system is checked. At 104, all of the other conditions are maintained, but the gain ratio of the first amplifier is accordingly set to 9 (α=9). The linearity of the measurement system is then checked again. 
     At 105, the gain ratio of the second amplifier β is switched from 1/10 to 9/10 (β=9/10) and the 11 nA current is provided from V REF  (1 V/90 MΩ). The capacitor C is then switched from 900 pF to 8100 pF and the integrating operation at the output terminal of the integrator is processed. The integrating time is counted as shown at 4 of Table I. 
     At 106 the gain ratio of the second amplifier β is switched from 9/10 to 1/10 but the other factors are kept unchanged. The integrating time dt is counted to make sure that although the slew rate (slew rate=dV 0  /dt) is reduced, the operation of the device remains accurate. The result is shown at 5 of Table I. At 107 the gain ratio of the second amplifier β is set to 9/10 and the capacitance of the capacitor C is set to 900 pF. the integrating time to check the slew rate is measured again with the being result shown at 6 of Table I. Finally, at 108, the capacitance of the capacitor C is set to 100 pF and the integrating time is measured to make sure that the capacitance is 100 pF. The result is shown at 7 of Table I. 
     After all the above steps, the functionality of all the key components are verified and the values of dt under different conditions are obtained. The system may use the dt values to calibrate the capacitance of the capacitor C in accordance with Equation I, as follows: ##EQU2## 
     The calibration process is similar to that of the measurement process. 
     In Equation I, the value of V T  is V REF , dV 0  is 1 * V REF  or 2 * V REF . As a result, the errors in the measurement of V REF  may be counterbalanced in the equation. Since R X  is known and dt is known, we have: ##EQU3## 
     The self-calibration process as described above may be controlled by a computer program. The calibration may be operated at any time during the measurement process of the teraohmmeter. 
     EFFECTS OF THE INVENTION 
     As described above, in the automatic self-calibration system for a digital teraohmmeter of the preferred invention, a built-in standard resistor is provided and the self-calibration of the system may be controlled by a computer. Accurate measurement of high-resistance is obtained. 
     It is to be understood that the above-described embodiment is merely an illustrative example of the invention and that numerous variations may be made by those skilled in the art without departing from the scope of the invention. It is, intended that all Such variations be included within the scope of the invention to the extent defined by the appended claims. 
     
                                           TABLE I__________________________________________________________________________No.   I.sub.T (nA)  C(pF)      dV.sub.0 (v)          dt(s)             dV.sub.0 /dt(v/s)                    Remarks__________________________________________________________________________1  11  900 18.0          1.47             12.2   Calibrate with R.sub.S2  11  900 2.00          0.16             12.2   Check linearity3  11  900 0.22          0.02             12.2   Check linearity4  100 8100      18.0          1.46             12.3   Find C.sub.8100 /C.sub.9005  11  8100      18.0          13.2             1.36   Check slew rate6  100 900 18.0          0.16             111    Check slew rate7  11  100 18.0          0.16             110    C.sub.100 /C.sub.900 determination__________________________________________________________________________