Apparatus for and method of internally calibrating an electrical calibrator

• US 5,012,181 A
• Filed: 01/29/1987
• Issued: 04/30/1991
• Est. Priority Date: 01/29/1987
• Status: Expired due to Term

• Current Assignee: John Fluke Mfg. Co., Inc. (Fortive Corp.)
• Original Assignee: John Fluke Mfg. Co., Inc. (Fortive Corp.)
• Inventors: Eccleston, Larry E.
• Primary Examiner(s): Baker, Stephen M.
• Application Number:
  US07/008,234
• Time in Patent Office: 1,552 Days
• Field of Search: 324/74, 324/115, 341/120, 364/482, 364/483, 364/571.01
• US Class Current: 324/74
• CPC Class Codes: G01R 15/125 for digital multimeters
• CPC Class Codes: G01R 35/005 Calibrating; Standards or reference devices, e.g. voltage or resistance standards, "golden" references G01R33/0035, G01R35/002 take precedence
• CPC Class Codes: H03G 1/04 Modifications of control circuit to reduce distortion caused by control modifications to reduce influence of variations of internal impedance of amplifying elements caused by control H03F1/08

Abstract

A calibrator supplying electric parameters (viz voltage, current and resistances) over plural ranges to a meter to be calibrated comprises internal components for deriving each parameter over the plural ranges, an internal voltage reference, a DAC, a memory, an analog comparator and analog-to-digital converter. The internal components are connected in circuit with the internal voltage reference, analog-to-digital converter, comparator and memory for writing data signals into the memory indicative of calibration correction factors for the internal components. The internal components are connected in circuit with the memory and DAC so the DAC responds to the stored data signals indicative of the calibration correction factors to enable the value of the parameter supplied to the device by the internal components to be modified and corrected in accordance with the calibration correction for the parameter.

Claims

1. A calibrated source for supplying a calibrated electric parameter over plural ranges to a device to be calibrated comprising

internal components for deriving the parameter over the plural ranges, an internal voltage reference, digital-to-analog converter means, a memory, analog comparison means, and analog-to-digital converter means,

first means for connecting the internal components in circuit with the internal voltage reference, analog-to-digital converter means, comparison means and memory for writing data signals into the memory indicative of calibration correction factors for the internal components as applicable to the internal components, and

second means for connecting the internal components in circuit with the memory and digital-to-analog converter means so the digital-to-analog converter means responds to the stored data signals indicative of the calibration correction factors to modify the value of the parameter supplied to the device by the internal components so the supplied parameter is corrected in accordance with the calibration correction for the parameter.

2. The source of claim 1 wherein the parameter is voltage to be derived over plural ranges, the internal components being variable voltage changing means, the first connecting means including means for connecting:

(a) the voltage changing means to the internal voltage reference means so that different voltages applicable to the plural ranges are derived at different times, (b) the voltages derived by the voltage changing means and an analog output signal of the digital-to-analog converter means, as corrected for calibration correction factors of the converter means and the internal reference by a stored signal from the memory to the comparison means so the comparison derives an error signal, (c) the comparison means to the analog-to-digital converter means so the analog-to-digital converter means derives a signal indicative of the error, and (d) the converter means to the memory so the memory stores a digital signal representing a correction factor for each range, determined by the error indicating signal.

3. The source of claim 2 wherein the variable voltage changing means includes voltage divider means having different voltage division factors.

4. The source of claim 3 wherein the variable voltage changing means includes amplifier means having different gain factors.

5. The source of claim 2 wherein the variable voltage changing means includes amplifier means having different gain factors.

6. The source of claim 2 wherein the voltage to be measured is DC and the internal voltage reference means is a DC source, and the first connecting means includes means for coupling a DC voltage from the DC source to the comparison means.

7. A calibrated source for supplying a calibrated electric parameter over plural ranges to a device to be calibrated comprising

internal components for deriving the parameter over the plural ranges, an internal voltage reference, digital-to-analog converter means, a memory, analog comparison means, and analog-to-digital converter means,

first means for connecting the internal components in circuit with the internal voltage reference, digital-to-analog converter means, analog-to-digital converter means, comparison means and memory for writing data signals into the memory indicative of calibration correction factors for the internal components,

second means for connecting the internal components in circuit with the device to be calibrated, and

means for activating the memory to read out the stored data signals indicative of the calibration correction factors while the second means connects the internal components with the device to be calibrated to enable the value of the parameter supplied to the device by the internal components to be modified to enable the supplied parameter to be corrected in accordance with the calibration correction factors for the internal components as applicable to the parameter.

8. The source of claim 7 wherein the parameter is an impedance to be coupled to the external device over plural ranges, the internal components including a reference impedance having a reference value and impedance means having different values for each of the ranges, the first connecting means including means for connecting:

(a) the internal voltage reference means in circuit with the reference impedance and the impedance means to derive a first analog signal having a magnitude indicative of a reference value for the impedance means in one of the impedance ranges, a different magnitude being derived for each of the impedance ranges, (b) the digital-to-analog converter means to the memory to derive a second analog signal, (c) the first and second analog signals to the comparison means so that the comparison means derives an analog error signal, (d) the comparison means to the analog-to-digital converter means so the analog-to-digital converter means derives a digital signal indicative of the error, and (e) the memory to the analog-to-digital converter means so the memory stores a digital signal representing an impedance calibration correction factor for each range, determined by the error indicating signal.

9. The source of claim 7 wherein the parameter is a current to be measured over plural ranges, the internal components including a constant current generator for deriving a different current for each of the ranges and impedance means having different values for each of the ranges, the first connecting means including means for connecting:

(a) the internal voltage reference means in circuit with the constant current generator and the impedance means to derive a first analog signal having a magnitude indicative of a reference value for the current in each of the current ranges, a different magnitude being derived for each of the current ranges, (b) the digital-to-analog converter means to the memory to derive a second analog signal, (c) the first and second analog signals to the comparison means so that the comparison means derives an analog error signal, (d) the comparison means to the analog-to-digital converter means so the analog-to-digital converter means derives a digital signal indicative of the error, and (e) the memory to the analog-to-digital converter means so the memory stores a digital signal representing a current calibration correction factor for each range, determined by the error indicating signal.

10. A method of calibrating an electric measuring instrument for an electric parameter in each of several ranges, the calibration being performed with an instrument including:

internal components associated with measuring the parameter over the plural ranges, an internal voltage reference, digital-to-analog converter means, a memory and analog comparison means, the method comprising;

simultaneously connecting;

(a) an external reference voltage source to the comparator, (b) the internal voltage reference source as a power supply to the converter means while the converter means responds to a digital signal representing the magnitude of the external reference voltage so the converter means derives a first analog signal having a value equal to the magnitude of the external voltage reference as modified by inaccuracies of the internal reference and the converter means and (c) the converter means to the comparator so the comparator derives an output signal indicative of a first error between the first analog signal and the external voltage reference,

in response to the first error, storing an indication of inaccuracies of the internal reference and the converter means,

then calibrating the internal components by simultaneously connecting;

the internal reference supply to power the internal components and the converter means while the converter means responds to a series of stored digital signals representing the values of responses from the converter means for different configurations of the internal components, the converter responding to the series of digital signals to derive a series of analog signals having values adjusted by the first error so they are equal to calibrated values for analog signals derived by the internal components, connecting the converter means and the internal components to the comparator while the components are in the different configurations and the converter is deriving the series of analog signals so the comparator derives a series of error signals representing the errors between the series of analog signals and further signals derived from the internal components while they are in the different configurations,

storing the error signals representing the magnitudes of the errors between the series of analog signals and the further signals to represent calibration correction factors for each of the different configurations,

connecting the internal components to the instrument being calibrated so the instrument being calibrated is responsive to a series of analog signals, one for each of the ranges,

while the instrument is responsive to the series of analog signals, reading out the calibration correction factor associated with the analog signal being supplied to the instrument by the memory, and modifying the input to the instrument to account for the read-out calibration correction factor.

11. Method of deriving calibrating voltage and/or current and/or resistance parameters over multiple ranges for calibrating voltage and/or current and/or resistance responses of instruments over multiple ranges, the calibrating parameters being derived with a calibrator having:

(a) input terminals responsive to an external standard source of at least one of said parameters having a precise value, (b) switch means, (c) a DC reference voltage source, (d) digital-to-analog converter means, (e) a DC comparator, (f) an analog-to-digital converter, (g) memory means, and (h) circuit means for deriving calibrating output signals over multiple ranges for at least one of said parameters;

the method comprising;

(A) from time to time connecting an external DC voltage standard having a precise magnitude to said input terminals while activating said switch means to connect simultaneously (a) one of said input terminals to an input of said comparator, (b) said DC reference source and a signal from said memory means as inputs to said digital-to-analog converter means, a digital output indicative of said magnitude of the DC reference source and the signal from the memory means causing the digital-to-analog converter means to derive a DC signal nominally having a predetermined magnitude relative to the magnitude of the voltage standard, (c) said digital-to-analog converter means to an input of said comparator, (d) an output of said comparator indicative of the magnitude of the deviation of said inputs thereof to said analog-to-digital converter means, and (e) an output of said analog-to-digital converter means to said memory means;

the output of the analog-to-digital converter means supplied to said memory means being indicative of a calibrating error of the digital-to-analog converter means while supplied with the voltage of the DC reference voltage source;

(B) from time to time, after step A has been performed at least once, activating said switch means to connect simultaneously (a) said DC reference source to an input of said comparator, (b) said DC reference source and a signal from said memory means as inputs to said digital-to-analog converter means, a digital output indicative of said magnitude of the DC reference source and the signal from the memory means causing the digital-to-analog converter means to derive a DC signal nominally having a predetermined magnitude relative to the magnitude of the DC reference source, the memory also supplying to the digital-to-analog converter means with a signal indicative of the calibrating error derived during step A, (c) said digital-to-analog converter means to an input of said comparator, (d) an output of said comparator indicative of the magnitude of the deviation of said input thereof to said analog-to-digital converter means, and (e) an output of said analog-to-digital converter means to said memory means;

the output of the analog-to-digital converter means supplied to said memory means being indicative of a calibrating error of the DC reference source;

(C) from time to time, after step B has been performed at least once, activating the switch means to connect simultaneously (a) said DC reference source and said memory means as inputs to said digital-to-analog converter means, and (b) said DC reference source, said comparator, said digital-to-analog converter means output and said circuit means to each other while the digital-to-analog converter means responds to calibrating error signals derived during steps A and B so that the comparator derives a signal indicative of a calibrating error for the circuit means; and

(D) from time to time, after step C has been performed at least once, deriving a sequence of at least one of said calibrating parameters by;

(i) activating the switch means to connect simultaneously (a) said DC reference source and said memory means to inputs of said digital-to-analog converter means, (b) the output of said digital-to-analog converter means to said circuit means, and (ii) with the connection of (i) established and the digital-to-analog converter means being responsive to the calibrating errors derived during steps (A) and (C), activating the memory means to supply signals representing a gamut of values of the at least one parameter of the digital-to-analog converter means so that the circuit means derives a gamut of analog values, one for each range, of the at least one parameter, each of the analog values in the gamut being calibrated for errors derived during steps A and C are suitable for calibrating the instrument.

12. The method of claim 11 wherein the parameter is resistance and further comprising from time to time after step A has been performed connecting a precise external four terminal resistor to said input terminals while activating the switch means to connect (a) an internal resistor having about the same value as the external resistance in circuit with the input terminals, and (b) a terminal between said resistor and an output of the digital-to-analog converter means to said comparator while said resistors are energized by a source inside of the calibrator and said digital-to-analog converter means is supplied by the memory with a signal indicative of the calibration error of the DC reference voltage source so that the comparator derives a signal having a magnitude indicative of a calibration error for the internal resistor.

13. The method of claim 12 wherein the internal and external resistors have values much greater than one ohm, said switch means being activated to connect:

(a) the DC reference voltage across said resistors, and (b) the memory to the digital-to-analog converter means so that the digital-to-analog converter means is supplied with signals indicative of the value of the expected voltage drop across said internal resistor and the calibration error of the DC reference voltage and the digital-to-analog converter so that the comparator derives a signal having a magnitude representing the calibration error of the internal resistor.

14. The method of claim 13 wherein the internal and external resistors have values on the order of one ohm, the circuit means including a constant current generator adapted to be powered by the DC voltage reference source, said switch means being activated to connect (a) the constant current generator to be powered by the DC voltage reference source so the generator derives a predetermined current magnitude, (b) the resistors to be responsive to the predetermined current magnitude derived by the constant current generator, and (c) the memory to the digital-to-analog converter means so that the digital-to-analog converter means is supplied with signals indicative of the value of the expected voltage drop across said internal resistor and the calibration error of the DC reference voltage and the digital-to-analog converter so that the comparator derives a signal having a magnitude representing the calibration error of the internal resistor.

15. The method of claim 14 wherein the circuit means includes an instrument amplifier having:

complementary first and second input terminals and a switch having;

(a) first and second armatures respectively connected to said first and second input terminals, and (b) contacts connected to one terminal of the internal resistor, one terminal of the external resistor and a common terminal for the internal and external resistors, activating said switch means to connect an output of said instrument amplifier to an input of the comparator while the comparator is responsive to an output of the digital-to-analog converter means while the digital-to-analog converter means is supplied by the memory with a signal indicative of the calibration error of the DC reference source;

with the switch means so activated, activating said switch so that during a first interval;

the first and second terminals of the instrument amplifier are respectively connected to said one terminal of the internal resistor and the common terminal; and

during a second interval;

the first and second terminals of the instrument amplifier are respectively connected to the common terminal and said one terminal of the external resistor;

the switch means being activated so that signals having values representing the magnitudes of the comparator output during the first and second intervals are supplied to first and second locations in the memory in response to outputs of the analog-to-digital converter means;

16. The method of claim 11 wherein the parameters and responses are voltage, the circuit means including voltage divider means,

step C comprising activating the switch means to connect the voltage divider means in circuit with the DC reference source, the comparator, the digital-to-analog converter means, the analog-to-digital converter means and the memory so that the comparator derives analog signal having a magnitude indicative of the calibration error of the voltage divider means and the analog-to-digital converter means supplies the memory with a signal indicative of the voltage divider calibration error, and

step D comprising activating the switch means so the voltage divider means is connected to be responsive to the gamut of analog signals derived from the digital-to-analog converter means while the digital-to-analog converter means is responsive to the signal in the memory indicative of the calibration error of the voltage divider means.

17. The method of claim 16 wherein the parameter is resistance, determining the calibration error of an internal reference resistor by connecting the internal reference resistor in circuit with an external calibrated resistor connected to said input terminals, storing the determined calibration error in the memory, and then activating the switch means to connect simultaneously (a) the internal reference resistor in circuit with the DC reference voltage sand resistors of a chain of resistors to be calibrated, (b) a tap on the resistor chain and the analog output of the digital-to-analog converter means to the comparator, and (c) the memory to the digital-to-analog converter means while the memory is supplying the digital-to-analog converter means with a calibration correction signal for the internal resistor and signals representing nominal voltage drops for resistors in the chain so that the comparator derives signals having magnitudes indicative of calibration errors of resistors in the chain.

18. The method of claim 16 wherein the parameter is resistance, storing a determined calibration error for an internal reference resistor in the memory, and then activating the switch means to connect simultaneously (a) the internal reference resistor in circuit with the DC reference voltage and a resistor to be calibrated, (b) the voltage across the resistor to be calibrated and the analog output of the digital-to-analog converter means to the comparator, and (c) the memory to the digital-to-analog converter means while the memory is supplying the digital-to-analog conveter means with a calibration correction signal for the internal resistor and a signal representing the nominal voltage drop across the resistor to be calibrated so that the comparator derives a signal having a magnitude indicative of the calibration error of the resistor to be calibrated.

19. The method of claim 16 wherein the parameter is current, the circuit means including a constant current source and a resistor having a predetermined value and a determined calibration error stored in the memory, activating the switch means to connect simultaneously:

a constant current derived by the constant current source to said resistor (b) the voltage developed across the resistor and the output of the digital-to-analog converter means to said comparator and (c) the memory to the digital-to-analog converter means so that the digital-to-analog converter means is supplied with stored signals indicative of the nominal voltage across the resistor and the calibration error of the resistor, whereby the comparator derives a signal having a magnitude representing the calibration correction error for the constant current generator.

20. The method of claim 19 wherein step D of claim 16 is performed by connecting resistors having different values and determined calibration errors to the output terminal of the constant current generator as the constant current generator is adjusted to derive currents having differing nominal values.

21. The method of claim 16 wherein the voltage divider means includes a resistive voltage divider having a tap, the switch means being activated so the voltage divider means is connected across the reference source and the tap is connected to an input of the comparator.

22. The method of claim 21 wherein the voltage divider means includes a further resistive voltage divider having a tap, the tap of the further resistive voltage divider having a greater voltage division factor than the first named tap, the switch means being activated so the further voltage divider means is connected across the reference source and the tap of the further resistive voltage divider is connected to an input of the comparator.

23. The method of claim 22 wherein the voltage divider means includes a power amplifier and the reference source includes a tap, the switch means being activated to connect (a) an input of the power amplifier to the tap of the reference source, and (b) opposite terminals of the further voltage divider across outputs of the power amplifier and the digital-to-analog converter means, the tap of the further resistive voltage divider to an input of the comparator while the digital-to-analog converter means is responsive to a signal from the memory indicative of a calibrating error of the further resistive voltage divider so that the comparator derives a signal determined by the calibrating error of the power amplifier.

24. The method of claim 23 wherein the voltage divider means includes an additional resistive voltage divider having a tap having a voltage division factor much greater than that of the further voltage divider, the switch means being activated so (a) inputs and outputs of the power amplifier are respectively connected to the reference source and the additional divider, and (b) the comparator is responsive to the voltage at the tap of the additional voltage divider and the output of the digital-to-analog converter means while the digital-to-analog converter means is responsive to a stored signal in the memory indicative of the power amplifier calibration error.

25. The method of claim 21 wherein the voltage divider means includes an amplifier, the switch means being activated to connect (a) the resistive voltage divider to be responsive to an output of the digital-to-analog converter means, (b) an input of the amplifier to be responsive to the voltage at the tap, and (c) the comparator to be responsive to the reference voltage and an output of the amplifier so that the comparator derives a signal determined by the calibration error of the amplifier;

step D comprising activating the switch means to the resistive voltage divider is cascaded with the amplifier so the resistive voltage divider and the amplifier are connected to be responsive to the gamut of analog signals derived from the digital-to-analog converter means while the converter means is responsive to the signal in the memory indicative of the calibration errors of the resistive voltage divider and the amplifier.

26. The method of claim 25 wherein the voltage divider means includes another resistive voltage divider having a tap with a greater voltage division factor than the first named resistive voltage divider, the reference source including a tap, the connecting means being activated so (a) the another resistive voltage divider is connected across the tap of the reference source, (b) the tap of the another resistive voltage divider is connected to an input of said amplifier, and (c) said comparator is responsive to output signals of said amplifier and said digital-to-analog converter means to derive an analog signal having a magnitude indicative of the calibration correction error for the another resistive voltage divider, whereby said calibration correction error for the another resistive voltage divider is stored in the memory;

and step D comprising activating the switch means so the another resistive voltage divider is connected to be responsive to the gamut of analog signals derived from the digital-to-analog converter means while the converter means is responsive to the signal in the memory indicative of the calibration error of the another resistive voltage divider; and

the magnitude of the voltage at the tap of the another resistive voltage is supplied to the instrument to be calibrated.

27. Method of deriving calibrating resistance parameters over multiple ranges for calibrating resistance responses of instruments over multiple ranges, the calibrating parameters being derived with a calibrator having:

(a) switch means, (b) a first resistor having a known calibration error, (c) plural second resistors having unknown calibration, (d) a DC reference voltage source having a known calibration error, (e) digital-to-analog converter means, (f) a DC comparator, (g) analog-to-digital converter means, (h) memory means, and (i) circuit means for deriving calibrating output signals over multiple ranges of resistance;

the method comprising;

(A) from time to time activating the switch means to connect simultaneously (a) the DC reference voltage source in circuit with the first resistor and at least one of the second resistors, (b) the voltage across the least one second resistor and an output of the digital-to-analog converter means to said comparator, (c) the memory to the digital-to-analog converter means so the digital-to-analog converter means is supplied with signals indicative of the calibration errors of the reference voltage source of the first resistance and the comparator derives an output signal having a magnitude representing the calibration correction error of the at least one of the second resistors, (d) the output of the comparator to the analog-to-digital converter means, and (e) an output of the analog-to-digital converter means representing the calibration correction error of the at least one of the second resistors to the memory;

(B) from time to time, after step A has been performed, connecting said at least one of the second resistors to resistance calibrating input terminals of the instrument being calibrated and connecting a memory location in the calibrator where the calibrating correcting error for said at least one of the second resistors is stored to an output of the calibrator;

said instrument being calibrated responding to the resistance connected to the resistance calibrating input terminals to derive a signal having a magnitude dependent on the resistance value at the resistance calibrating input terminals, and offsetting the magnitude of the signal derived by the instrument by the calibrating correcting error for said at least one of the second resistances at the output of the calibrator.

28. The method of claim 27 wherein several of said second resistors are connected as a chain having taps between adjacent values, and during step A connecting different ones of said taps to the comparator while different values of the stored signals representing different resistance values are supplied to the digital-to-analog converter while the digital-to-analog converter output is modified by the stored signals representing the calibrating errors for the reference voltage reference resistance.

29. Apparatus for deriving a signal indicative of a calibration correction for a first resistor having a nominal value to be used in conjunction with calibrating a resistance measuring instrument comprising a reference voltage source having a known calibration error, a second resistor having a known calibration error, an analog comparator, a memory for storing first, second, third and fourth digital signals respectively representing (a) a nominal value for the voltage to be developed across the first resistor, (b) the calibration error for the reference voltage, (c) the calibration error for the second resistor, and (d) the calibration error for the first resistor an analog-to-digital converter, said reference voltage source being connected in circuit with the first and second resistors so that a voltage approximately equal to the nominal voltage is developed across the first resistor, said first resistor and an analog output of the digital-to-analog converter being coupled to the comparator while the digital-to-analog converter is responsive to said first, second and third signals so that the comparator derives an analog signal having a magnitude representing the calibration error of the first resistor, the comparator being connected to an input of the analog-to-digital converter and the analog-to-digital converter being connected to the memory to supply the memory with the fourth signal.

30. In combination, a chain of several first resistors having taps between adjacent pairs of said first resistors, apparatus for deriving signals indicative of calibration corrections for said several first resistors, said apparatus comprising:

a reference voltage source having a known calibration error, a second resistor having a known calibration error, an analog comparator, a memory for storing first, second third and fourth digital signals respectively representing (a) a nominal value for the voltage to be developed at each of the taps, (b) the calibration error for the reference voltage, (c) the calibration error for the second resistor, and (d) the calibration error for the first resistor, an analog-to-digital converter, means for connecting said reference voltage and said second resistor in series with each other and selectively to one of said taps and for connecting said one tap to said comparator so that a voltage approximately equal to the nominal voltage at said one tap is developed across said one tap and an analog output of the digital-to-analog converter is coupled to the comparator while the digital-to-analog converter is responsive to said first, second and third signals so that the comparator derives an analog signal having a magnitude representing the calibration error of the resistance at said one tap, the comparator being connected to an input of the analog-to-digital converter and the analog-to-digital converter being connected to the memory to supply the memory with the fourth signal.

31. Method of deriving calibrating current parameters over multiple ranges for calibrating current responses of instruments over multiple ranges, the calibrating parameters being derived with a calibrator having (a) a constant current source adjusted over several values, (b) switch means, (c) a DC reference voltage source having a known calibration error, (d) digital-to-analog converter means, (e) a DC comparator, (f) an analog-to-digital converter, (g) memory means, (h) circuit means for deriving calibrating output signals over multiple current ranges, and (i) impedance means having a known calibration error;

the method comprising;

(A) (a) from time to time activating the switch means to connect simultaneously the DC reference voltage source as an input to said constant current source, (b) an output current of the constant current source across the impedance means to that a voltage having a nominal value is developed across the impedance means, (c) the voltage across the impedance means and an output of the digital-to-analog converter means to said comparator, (d) the memory to the digital-to-analog converter means so the digital-to-analog converter means is supplied with signals indicative of the calibration errors of the reference voltage source of the calibrator, (e) impedance means and the comparator derives an output signal having a magnitude representing the calibration correction error of the constant current source, (f) the output of the comparator to the analog-to-digital converter means, and (g) an output of the analog-to-digital converter means representing the calibration correction error of the constant current source to the memory;

(B) from time to time, after step A has been performed, connecting said constant current source to current calibrating input terminals of the instrument being calibrated and (b) connecting a memory location in the calibrator where the calibrating correcting error for said constant current source is stored to an output of the calibrator;

said instrument being calibrated responding to the current supplied to the current calibrating input terminals to derive a signal having a magnitude dependent on the magnitude of the current at the current calibrating input terminals and offsetting the magnitude of the signal derived by the instrument by the calibrating correcting error for said current output of the calibrator.

32. The method of claim 31 wherein said impedance means is variable, said memory storing a calibrating error for each of the variable values of the impedance means, and adjusting the value of the impedance means as the amplitude of the current derived from the constant current source is adjusted so that the nominal voltage supplied to the comparator across the impedance means remains approximately constant, the digital-to-analog converter means being responsive to signals from the memory means indicative of the calibrating error for each of the variable values of the impedance means as different values of the impedance means are connected to be responsive to current from the constant current source.

33. Apparatus for deriving a signal indicative of a calibration correction for a constant current source having a nominal value to be used in conjunction with calibrating a current measuring instrument comprising a reference voltage source having a known calibration error, a resistor having a known calibration error, an analog comparator, a memory for storing first, second, third and fourth digital signals respectively representing (a) a nominal value for the voltage to be developed across the resistor, (b) the calibration error for the reference voltage, (c) the calibration error for the second resistor, and (d) the calibration error for the constant current source, an analog-to-digital converter, said reference voltage source being connected in circuit with the constant current source resistor so that a voltage approximately equal to the nominal voltage is developed across the resistor;

said resistor and an analog output of the digital-to-analog converter being coupled to the comparator while the digital-to-analog converter is responsive to said first, second and third signals so that the comparator derives an analog signal having a magnitude representing the calibration error of the constant current source, the comparator being connected to an input of the analog-to-digital converter and the analog-to-digital converter being connected to the memory to supply the memory with the fourth signal.

34. In combination, a constant current source including means for deriving currents having several different magnitudes, said source being responsive to a reference voltage source having a known calibration correction error, apparatus for deriving calibration errors for said several different magnitudes, said apparatus comprising:

variable resistance means having a known calibration error at each of said several predetermined values, said resistance means being connected to be responsive to the output current of the constant current source so that a nominal voltage is developed across said variable resistance means for each current magnitude derived by said source, an analog comparator, a memory for storing first, second, third and fourth digital signals respectively representing (a) nominal values for the voltages to be developed across the resistance means, (b) the calibration error for the reference voltage, (c) the calibration error for the resistance means, and (d) calibration errors for different magnitudes of the current source, an analog-to-digital converter, said resistance means and an analog output of the digital-to-analog converter being coupled to the comparator while the digital-to-analog converter is responsive to said first, second and third signals so that the comparator derives an analog signal having a magnitude representing the calibration error of the current source for the value of the current derived by the source and the value of the variable resistance means responsive to the current derived by the source, the comparator being connected to an input of the analog-to-digital converter and the analog-to-digital converter being connected to the memory to supply the memory with the fourth signal.

35. In a method of deriving calibrating voltage and/or current and/or resistance parameters over multiple ranges for calibrating voltage and/or current and/or resistance responses of instruments over multiple ranges, the calibrating parameters being derived with a calibrator having:

(a) input terminals responsive to an external standard source of at least one of said parameters having a precise value, (b) switch means, (c) a DC reference voltage source, (d) digital-to-analog converter means, (e) a DC comparator, (f) an analog-to-digital converter, (g) memory means, and (h) circuit means for deriving calibrating output signals over multiple ranges for at least one of said parameters;

the method comprising;

(A) from time to time connecting an external DC voltage standard having a precise magnitude to said input terminals while activating said switch means to connect simultaneously (a) one of said input terminals to an input of said comparator, (b) said DC reference source and a signal from said memory means as inputs to said digital-to-analog converter means, a digital output indicative of said magnitude of the DC reference source and the signal from the memory means causing the digital-to-analog converter means to derive a DC signal nominally having a predetermined magnitude relative to the magnitude of the voltage standard, (c) said digital-to-analog converter means to an input of said comparator, (d) an output of said comparator indicative of the magnitude of the deviation of said inputs thereof to said analog-to-digital converter means, and (e) an output of said analog-to-digital converter means to said memory means;

the output of the analog-to-digital converter means supplied to said memory means being indicative of a calibrating error of the digital-to-analog converter means while supplied with the voltage of the DC reference voltage source;

(B) from time to time, after step A has been performed at least once, activating said switch means to connect simultaneously (a) said DC reference source to an input of said comparator, (b) said DC reference source and a signal from said memory means as inputs to said digital-to-analog converter means, a digital output indicative of said magnitude of the DC reference source and the signal from the memory means causing the digital-to-analog converter means to derive a DC signal nominally having a predetermined magnitude relative to the magnitude of the DC reference source, the memory also supplying to the digital-to-analog converter means with a signal indicative of the calibrating error derived during step A, (c) said digital-to-analog converter means to an input of said comparator, (d) an output of said comparator indicative of the magnitude of the deviation of said input thereof to said analog-to-digital converter means, and (e) an output of said analog-to-digital converter means to said memory means;

the output of the analog-to-digital converter means supplied to said memory means being indicative of a calibrating error of the DC reference source;

36. In a method of deriving calibration voltage and/or current and/or resistance parameters over multiple ranges for calibrating voltage and/or current and/or resistance responses of instruments over multiple ranges, the calibrating parameters being derived with a calibrator having:

(a) switch means, (b) a DC reference voltage source having a known calibration error, (c) digital-to-analog converter means, (d) a DC comparator, (e) an analog-to-digital converter, (f) memory means and (g) circuit means for deriving calibrating output signals over multiple ranges for at least one of said parameters, the memory means storing signals indicative of the known calibration errors;

the method comprising;

(A) from time to time, activating said switch means to connect simultaneously (a) said DC reference source to an input of said comparator, (b) said DC reference source and a signal from said memory means as inputs to said digital-to-analog converter means, a digital output indicative of said magnitude of the DC reference source and the signal from the memory means causing the digital-to-analog converter means to derive a DC signal nominally having a predetermined magnitude relative to the magnitude of the DC reference source, the memory also supplying to the digital-to-analog converter a signal indicative of the known calibrating errors of the DC reference source and the digital-to-analog converter, (c) digital-to-analog converter means to an input of said comparator, (d) an output of said comparator indicative of the magnitude of the deviation of said input thereof to said analog-to-digital converter means, and (e) an output of said analog-to-digital converter means to said memory means;

the output of the analog-to-digital converter means supplied to said memory means being indicative of a calibrating errors of the DC reference source and the digital-to-analog converter means;

(B) from time to time, after step A has been performed at least once, activating the switch means to connect simultaneously (a) said DC reference source and said memory means as inputs to said digital-to-analog converter means, and (b) said DC reference source, said comparator, said digital-to-analog converter means output and said circuit means to each other while digital-to-analog converter means responds to the known calibrating error signals and the calibrating signal error derived during step A so that the comparator derives a signal indicative of a calibrating error for the circuit means.

37. The method of claim 36 further comprising:

from time to time, after step B has been performed at least once, deriving a sequence of at least one of said calibrating parameters by;

(i) activating the switch means to connect simultaneously (a) said DC reference source and said memory means to inputs of said digital-to-analog converter means, and (b) the output of said digital-to-analog converter means to said circuit; and

(ii) with the connection of (i) established and the digital-to-analog converter means being responsive to the calibrating errors derived during step B, activating the memory means to supply signals representing a gamut of values of the at least one parameter to the digital-to-analog converter means to that the circuit means derives a gamut of analog values, one for each range, of the at least one parameter, each of the analog values in the gamut being calibrated for errors derived during step B and suitable for calibrating the instrument.