Abstract:
A voltmeter having multiple voltage ranges comprises substantially linear elements such that the transfer functions of the elements can be independently measured and logically combined to derive the transfer function of a given combination of elements configured to obtain a given voltage range.

Description:
BACKGROUND &amp; SUMMARY OF THE INVENTION 
     The calibration of voltmeters has previously been accomplished by measuring the outputs obtained upon the application of two known reference voltages and adjusting the transfer function thereof. Typically one of the reference voltages is ground and the other reference voltage is selected so as to produce a near full scale output. The drawback of this method is that separate full scale reference voltages are required for each voltage range, many calibration adjustments are required and `drift` requires periodic calibration to retain full accuracy. Since reference voltages are inherently expensive, especially in the case of high voltage references, it is desirable to calibrate a multirange voltmeter using less than a reference voltage for each range. 
     It is therefore an object of the present invention to calibrate a voltmeter having multiple ranges without the need for a separate reference voltage for each range. 
     It is a further object of the present invention to achieve a high degree of accuracy in a voltmeter with a minimum of high precision components and adjustments. 
     These objects have been accomplished in accordance with the preferred embodiment of the present invention by independently measuring the transfer functions of circuit elements in the voltmeter and logically deriving the transfer functions of the combinations of the circuit elements corresponding to the circuit configurations of the voltmeter for the respective voltage ranges. This transfer function is then logically applied to the output obtained upon the application of an unknown signal so as to produce a calibrated output. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the preferred embodiment of the present invention. 
     FIG. 2 is a circuit illustrating the prior art technique for calibrating a linear voltmeter. 
     FIG. 3 is a detailed schematic of the D.C. Preamplifier 10 illustrating especially the measurement of the input offset errors (-X o ,n) for the first three voltage ranges. 
     FIG. 4 is a detailed schematic of the Operational Attenuator 20 illustrating especially the measurement of the input offset errors (X o ,n) for the fourth and fifth voltage ranges. 
     FIG. 5 illustrates the gain error measurement for the third (10V) range. 
     FIG. 6 illustrates the gain error measurement for the second (1V) range. 
     FIG. 7 illustrates the second offset error measurement for the second (1V) range. 
     FIG. 8 illustrates the gain error measurement for the fourth (100V) range. 
     FIG. 9 illustrates an autocalibrating system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a block diagram of the preferred embodiment of the present invention, a voltmeter is illustrated having five voltage ranges. A first range (0.1V) is achieved by closing relay K1 and FET switch Q11 and selecting a gain of 100 from D.C. Preamplifier 10. A second range (1V) is achieved by selecting a gain of 10 from D.C. Preamplifier 10. A third range (10V) is achieved by selecting a gain of 1 from D.C. Preamplifier 10. A fourth range (100V) is achieved by closing relay K2 and selcting a gain of 0.1 from the Operational Attenuator 20 by closing FET switch Q16 and selecting a gain of 1 from the D.C. Preamplifier 10. A fifth range (1000V) is achieved by selecting a gain of 0.01 from the Operational Attenuator 20 and by closing FET switch Q18. Note that the full scale voltage applied to the Analog to Digital (A/D) Converter 30 is always ˜10 volts. 
     Note also that the reference voltage, V REF , can be applied to either the D.C. Preamplifier 10 by closing FET switch Q17, or to Operational Attenuator 20 by closing relay K3 and FET switch Q9. A ground input can similarly be applied to D.C. Preamplifier 10 by closing FET switch Q13, or to the Operational Attenuator 20 by closing relay K3 and FET switch Q10 and for the combined calibration of D.C. Preamplifier 10 in series with the Operational Attenuator 20 as will be discussed in detail below. 
     The prior art techniques for calibration of a linear voltmeter in the prior art is illustrated in FIG. 2. The reading of meter V will be: 
     
         X.sub.i = (V.sub.i + V.sub.o) G                            (1) 
    
     where V o  is the offset error of the voltmeter and G is the gain of the voltmeter. If switch S2 is closed such that V i  = 0, the reading of meter V will be: 
     
         X.sub.o = V.sub.o G.                                       (2) 
    
     if switch S1 is closed such that V i  = V REF , the reading of meter V will be: 
     
         X.sub.REF = (V.sub.REF + V.sub.o) G.                       (3) 
    
     combining equations (1), (2) and (3) gives the transfer function of the voltmeter when switch S3 is closed: 
     
         V.sub.IN = V.sub.REF (X.sub.IN - X.sub.o,n)/(X.sub.REF - X.sub.o,D) (4) 
    
     where X o ,n is the offset of the voltmeter with a grounded input and X o ,D is the offset of the voltmeter in a configuration prior to application of the reference voltage V REF , in this case X o  = X o ,n = X o ,D. 
     By utilizing a voltmeter comprising highly linear elements, and further comprising logic for computing the transfer functions of the elements when configured for the various voltage ranges from the transfer functions of convenient calibration configurations, the present invention calibrates five voltage ranges with only a single reference voltage and a single precision resistive divider. 
     Referring to FIG. 3, a detailed schematic of the D.C. Preamplifier 10, the input offset error measurements (X o ,n) for the first three voltage ranges are made with the input of the D.C. Preamplifier 10 grounded through a 100 K ohm resistor by FET switch Q13. A separate measurement is made for each range to include the respective configurations of FET switches Q31, Q32, Q23 and Q29 appropriate for each voltage range. These measurements will subsequently be referred to as X o ,n 0 .1,X o ,n 1  ; and X o ,n 10 . 
     Referring to FIG. 4, a detailed schematic of the Operational Attenuator 20, the input offset errors for the fourth and fifth (100V and 1000V) ranges are made with the input of Operational Attenuator 20 grounded through a 100 K ohm resistor by relay K3 and FET switch Q10. A separate measurement is made for each range. The fourth range is selected by closing FET switches Q22 and Q18. The fifth range is selected by closing FET switches Q21, and Q16. These measurements will be referred to as X o ,n 100  and X o ,n 1000 . 
     Referring now to FIG. 5, the gain error measurement for the third (10V) range is made by applying the internal precision reference voltage, V REF , (+10 VDC) to the input of D.C. Preamplifier 10 through FET switch Q17. This measurement will be referred to as X REF   10 . The calibration of the third (10V) range is now determined by the relationship: 
     
         V.sub.IN.sup.10 = V.sub.REF (X.sub.IN - X.sub.o,n.sup.10)/(X.sub.REF.sup.10 - X.sub.o,n.sup.10)                                       (5) 
    
     since for this configuration X o ,n 10  = X o ,D 10 . 
     Referring to FIG. 6, the gain error measurement for the second (1V) range is made by applying the internal precision reference voltage, V REF , (+10 VDC) to the precision ten-to-one divider (900K/100K) by closing FETs Q25 and Q20. The one volt output is applied to D.C. Preamplifier 10 by FET switch Q16. The gain of the D.C. Preamplifier 10 is set to X10 to give a full scale output. This measurement will be referred to as X REF   1 . Referring to FIG. 7, a second offset, error measurement is made on the 1 VDC range with the input of the D.C. Preamplifier 10 grounded through the precision ten-to-one divider by FET switch Q16. This measurement is made to include offset errors which may be present during the gain error measurement and will be subsequently referred to as X o ,D 1 . The calibration of the second (1 VDC) range is therefore determined by the relationship: ##EQU1## 
     A separate gain error is not made for the first (0.1 VDC) range since only difference between the circuit configuration of the 1 VDC range and the 0.1 VDC range is a precise gain of 10 as determined by the precision ten-to-one divider. The calibration for the first range thus becomes: ##EQU2## 
     Referring to FIG. 8, for the 100 VDC range gain error measurement the internal precision voltage reference (10 VDC) is applied to the input of Operational Attenuator 20 through FET switch Q9 and relay K3. The attenuator gain is set to 0.1 by FET switch Q22. The output of the Operational Attenuator 20 is applied to the input of the D.C. Preamplifier 10 by FET switch Q18. The D.C. Preamplifier 10 is set to a gain of 10 to provide a 10 VDC full-scale output. This measurement will subsequently be referred to as X REF   100 . A second offset measurement is made on the 100 VDC range to include offsets which may be present during the reference voltage measurement. This measurement is identical to the measurement of X o ,n 100  described above and illustrated in FIG. 4 except that the D.C. Preamplifier 10 gain is set to X 10 . This measurement will be subsequently referred to as X o ,D 100 . Since the full-scale measurement was made with an X10 D.C. Preamplifier 10 gain configuration and an X1 D.C. Preamplifier 10 gain configuration is used in the 100 VDC range, the transfer function is corrected by dividing by the X10 gain and multiplying by the X1 gain. The calibration for the fourth (100 VDC) range therefore is determined by the relationship: ##EQU3## 
     A separate gain error measurement is not required for the 1000 VDC range. Since the only difference between the 100 VDC and the 1000 VDC gain errors is a precise attenuation of 10 resulting from the precision ten-to-one divider. The calibration for the 1000 VDC range therefore becomes: ##EQU4## 
     FIG. 9 shows the autocalibrating system. The input voltage and the reference voltages are selectively applied to the input of a selected configuration of circuit elements SC. A detector 10, responsive to the output of the selected configuration, SC, loads selected outputs into a memory M. These selected outputs defines the transfer function of the selected configuration and can be logically combined with other transfer functions to determine the transfer function of selected configurations not directly measured. A processor, P, then combines the appropriate transfer function with the detected output when an unknown input voltage is applied and provides an output normalized for the transfer function of the configuration of circuit elements used for the measurement. 
     The objects of the present invention have been accomplished by the use of only two precision elements, a precise internal 10 VDC reference and a precise resistive divider. By having the processor periodically sample the correction factors during measurements and applying the corrective transfer functions to the output obtained from a measurement of an unknown voltage, an extremely accurate mode of voltage determination is accomplished, which constantly corrects for drifting components and requires a minimum amount of operator intervention for calibration.