JPH08189845A - Digital measuring instrument - Google Patents

Abstract

PURPOSE: To check the operation automatically by performing the acceptability judgment from the output of an A/D converter at the time of self-diagnosis, comparing the result from judgment with the output of a reference voltage generating circuit during measurement of the input to be measured, and calculating the measuring value while the influence of variation in the circuit constituent element(s) is canceled. CONSTITUTION: At the time of self-diagnosis, switches 3, 92, m93 are turned to the NO, NC, and NC sides, and the one-volt reference voltage 2 is applied to a range changeover circuit 9 while an amplifier 91 is set to ×1 in the magnification range, and the output is passed through an A/D converter 6 followed by judgment of acceptability by a CPU 7. Then, the switches 93, 92 are turned to the NO side one by one, and the output of the converter 6 is subjected to acceptability judgment by the CPU 7 while the amplifier 91 is set to the ×0.1 and ×10 in the magnification, and meantime, the situation of the circuit operation is put under the self-diagnosis. At the time of normal measurement, the voltage 2 is measured by the converter 6 while the amplifier 91 is set to ×1 range, and the input to be measured is inputted to the circuit 9 while the switch 3 is turned to the NC side, and measurement is made by the converter 6 while the amplifier 91 is set to the ×10, ×1, ×0.1 range as applicable, and thereby, the fed voltage value is calculated. Thereby, the influence of a secular change and the temp. variation can be canceled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital measuring instrument having a self-diagnosis function.

[0002]

2. Description of the Related Art Various measuring instruments have been provided for measuring voltage, current, resistance, etc. and digitally displaying the values.
In such a measuring instrument, it is customary to provide a plurality of input ranges so that measurement can be performed in an optimum state.
On the other hand, there is also provided a digital measuring instrument having a self-diagnosis function for automatically checking whether or not the recent range is normally operating.

FIG. 4 is a block diagram of an example of a conventional measuring instrument having a self-diagnosis function. In FIG. 4, 1 is a terminal to which the input Vin to be measured is applied, 2 is a reference voltage source for generating a reference voltage Vref, and 3 is a changeover switch. Reference numeral 4 is a range switching circuit provided with three-stage switching, and an amplifier 41
Element 42 connected to the input terminal of the resistor, resistors 43 to 45 connected to the feedback circuit of the amplifier 41, and switches 46 to 4
It consists of 8. 5 is a programmable gain amplifier,
6 is an A / D converter, 7 is a CPU (microprocessor), and 8 is a display unit.

In a measuring instrument consisting of such parts,
By connecting the changeover switch 3 to the NC side when measuring the input, the measured input Vi applied to the terminal 1 is measured.
A current corresponding to n flows into the feedback resistors 43 to 45 via the input resistor 42 of the amplifier 41 forming the range switching circuit 4, and is converted into a voltage. By switching the switches 46 to 48 respectively connected to the feedback resistors 43 to 45, the input range is switched over the three stages of the magnification, for example, x1, x10, x100.

As described above, the current flowing through the input resistor 42 is converted into a voltage at the designated magnification of the range switching circuit 4, and is taken into the CPU 7 via the programmable gain amplifier 5 and the A / D converter 6. The gain of the programmable gain amplifier 5 is set to 1 at the time of measuring the input. The CPU 7 switches the input changeover switch 3 and each switch of the range changeover circuit 4, reads the output of the A / D converter 6, calculates the measured value, and displays the result on the display means 8.

Next, the operation at the time of self-diagnosis for automatically checking whether or not the range is operating normally will be described. In this case, by connecting the input changeover switch 3 to the NO side, the reference voltage Vref is supplied to the range changeover circuit 4, whereby the range is changed over. In this way, the current flowing through the input resistor 42 is converted into a voltage by the designated magnification of the range switching circuit 4, and then amplified by the programmable gain amplifier 5 with a preset gain, and the amplified output is amplified by the A / D converter 6. Are A / D converted by and are taken in by the CPU 7.

Here, for example, when the input resistor 42 constituting the range switching circuit is 1 KΩ and the feedback resistors 43 to 45 are 10 Ω, 1 KΩ and 100 KΩ, respectively, the reference voltage source 2
The output of the range switching circuit 4 is "0.01", "1", and "100" times the output Vref of the above. Therefore, the gain of the programmable gain amplifier 5 is set to "1", respectively.
00 "," 1 "," 0.01 "times, set the values in almost the same range even if the range is different, then measure those values with the CPU 7, and the range operates normally with this CPU. Whether or not it is automatically checked.

Although a measuring instrument having such a self-diagnosis function has been conventionally known, a programmable gain amplifier 5 having an accuracy higher than the diagnosis accuracy is required for self-diagnosis. The programmable gain amplifier 5 is not used for measuring the measured input Vin, which is the original function of the measuring instrument (gain is 1
It is redundant). There is a problem.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and its purpose is to use a circuit used during normal measurement and to operate a range normally with an inexpensive general-purpose product. It is to realize a digital measuring instrument with a built-in self-diagnosis function that can automatically check whether or not it is present.

[0010]

The present invention provides an input terminal to which an input to be measured is applied, a reference input for performing the self-diagnosis, and a reference input for comparing and measuring the input to be measured during normal measurement. A reference voltage generating circuit that generates a voltage, a selector switch that switches between the measured input and the output of the reference voltage generating circuit, a range switching circuit that amplifies the measured input and the output of the reference voltage generating circuit to the set magnification, range An A / D converter for converting the output of the switching circuit into a digital signal,
During self-diagnosis, the result of the A / D converter is used to determine pass / fail, and during measurement of the input to be measured, the measurement result is compared with the output of the reference voltage generating circuit to influence the fluctuation of circuit components. The microprocessor is provided with a microprocessor for calculating the measured value while canceling the above, and a display for displaying the judgment result of pass / fail during the self-diagnosis and displaying the measured value of the measured input during the normal measurement.

[0011]

According to the present invention as described above, the pass / fail judgment is made from the result of the A / D converter during the self-diagnosis, and during the measurement of the measured input, the measured value is canceled while the influence of the fluctuation of the circuit constituent elements is canceled. It is calculated.

[0012]

The present invention will be described below with reference to the drawings. FIG. 1 is a circuit configuration diagram showing an embodiment of the present invention. In each of the following embodiments, the same parts as those in FIG. 4 are designated by the same reference numerals as those in FIG. 4 and their re-explanation is omitted. In FIG.
Reference numeral 9 denotes a range switching circuit, which includes an amplifier 91, changeover switches 92 and 93, and range resistors 94 and 95. The (-) input end of the amplifier 91 is connected to the input changeover switch 3, and the output end is connected to the common potential point via the series circuit of the resistors 94 and 95. The changeover switch 92 is connected to the (+) input end of the amplifier 91, and the changeover switch 93 is connected to the input end of the A / D converter 6. The changeover switches 92 and 93 have contacts NC and NO, respectively.
The contact NC of both switches is connected to the connection point of the amplifier 91 and the resistor 94, and the contact NO is connected to the connection point of the resistors 94 and 95.

In such a configuration, (1) operation during self-diagnosis and (2) operation of a normal measuring instrument will be described. In this embodiment, the measurement range is three steps (full scale 0.1V, 1V, 10V), and the output Vre of the reference voltage 2 is
f is 1V, the value of the range resistor 94 is 18KΩ, the value of the resistor 95 is 2KΩ, and the full scale of the A / D converter 6 is 1.2V.
The case will be described. (1) Operation during self-diagnosis First, the switches 92 and 9 in the range switching circuit 9
3 is connected to the NC side to form an amplifier 91 having a gain of 1, and the switch 3 is connected to the NO side to connect the reference voltage Vr of 1V.
ef is added to the range switching circuit 9. The output of the range switching circuit 9 is converted into a digital signal by the A / D converter 6 and taken into the CPU 7, and the result is judged based on the result. Next, the switch 93 is switched to the NO side to configure the 0.1-fold range switching circuit 9, and the acceptance / rejection is judged by comparing with the A / D conversion value at 1-fold obtained in the CPU 7.
At that time, although the resolution of the A / D-converted value is one digit lower than that when it is 1, the reference voltage 2 and the range resistance 94,
If a highly accurate 95 is used, there is no problem in the accuracy of diagnosis. Further, the switch 92 is also switched to the NO side, and compared with the A / D conversion value at the time of 0.1 times obtained, and the CPU 7 determines whether the result is acceptable or not. In these processes, the switches 92 of the range switching circuit 9,
93 is operating independently and correctly, or range resistor 9
It is checked whether the resistance value of 4,95 is normal.

(2) Operation as a normal measuring instrument When measuring in the 0.1 V range, first switch N
The output of the reference voltage source 2 is added to the range switching circuit 9 on the O side, and the switch 9 in the range switching circuit 9 is added.
2 and 93 are connected to the NC side, the gain of the amplifier 91 is multiplied by 1, and the reference voltage Vref is measured by the A / D converter 6.
The result is that the output of the reference voltage source 2 is Vref and A /
When the D conversion value is Vr, it is expressed by the following equation. Vref = Vr (1) Next, the switch 3 is connected to the NC side, the input Vin to be measured is added to the range switching circuit 9 from the terminal 1, the switch 92 is set to the NO side, and the gain of the input amplifier 91 is set to 10 times. Then, perform the same measurement. The result is that the gain is G1
0 and the A / D converted value is Vm, it is expressed by the following equation. Vin · G10 = Vm (2) From the equations (1) and (2), the value of the measured input Vin is calculated by the following equation. Vin = (Vm / Vr) · (Vref / G10) (3) The calculation of the formula (3) is performed by the CPU 7, and the calculation result is displayed on the display unit 8. When measuring in the 1V range, the reference voltage Vref is measured in the same manner as described above, and then the switches 3, 92, 93 are connected to the NC side and the input Vin to be measured is measured by the input amplifier 91 of 1 time. From the result, Vin is calculated by the following equation. Vin = (Vm / Vr) · Vref (4) When measuring in the 10V range, Vref is similarly set.
Then, the switches 3 and 92 are set to the NC side and the switch 93 is set to the NO side, and the measured input Vin is measured by the input amplifier 91 of 0.1 times. From the result, the value of Vin can be calculated by the following equation. Vin = (Vm / Vr) · (Vref / G0.1) (5) The calculation of the expressions (3) to (5) is also performed by the CPU 7, and the calculation result is displayed on the display 8. The first term of these equations (3) to (5) is the ratio of the A / D converted values. Therefore, the A / D converter 6 need only be stable in the short term only between the measurement of the input Vin to be measured and the measurement of the reference voltage Vref, and the influence due to aging and the temperature coefficient can be canceled. As a component forming the A / D converter 6, an inexpensive general-purpose product can be used.

In FIG. 2, a range switching circuit 9a having the same structure as this switching circuit is provided in front of the range switching circuit 9. In the apparatus of FIG. 2, the full-scale 0.1V, 1V and 10V ranges are operated by the range switching circuit 9
The switch 92a of a is connected to the NC side to connect the input amplifier 91
By configuring the gain of a to be 1 time, it can be realized in the same manner as the device of FIG. 1, but in FIG. 2, a measurement range of 0.01 V can be further obtained.

In this case, first, the switch 3 is connected to the NO side to add the reference voltage Vref to the range switching circuit 9a,
The gain of the input amplifier 91a is set to 10 times by setting the switch 92a to the NO side. Further, the switch 92 of the range switching circuit 9 is set to the NC side and the switch 93 is set to the NO side to configure the gain of the amplifier 91 to 0.1 times, thereby configuring the A / D converter 6
To measure. When G10 'is the gain of the amplifier 91a, G10' is expressed by the following equation. Vref · G10 ′ · G0.1 = Vr (6)

Next, the gain of the range switching circuit 9a is set to 1
Switch 0 to NC and switch 3 to the NC side to measure input Vi
n and connect the switch 92 of the range switching circuit 9 to NO.
Side, switch 93 to the NC side, range switching circuit 9
The gain is set to 10 times and the same measurement is performed. The result is expressed by the following equation. Vin · G10 ′ · G10 = Vm (7) From the equations (6) and (7), the value of the measured input Vin can be calculated by the following equation. Vin = (Vm / Vr) * (G0.1 / G10) * Vref (8) If G10 'is stable in the short term only during the measurement of the measured input Vin and the reference voltage Vref, (6), (7). ) Is canceled, and G10 'does not exist in expression (8). Therefore, inexpensive general-purpose products can be used as the components of the range switching circuit 9a. In the conventional device of this type, the range expansion requires the addition of a comparatively expensive reference resistor, but the present invention can do this relatively easily.

FIG. 3 shows an embodiment in which a full scale 100 V measurement range is obtained in the same manner. In FIG. 3, 9b is a voltage dividing circuit provided in the preceding stage of the range switching circuit 9, and this voltage dividing circuit includes a changeover switch 93b and a resistor 94b.
(9MΩ), 95c (1MΩ), and has the same structure as that of FIGS. 1 and 2 except for the voltage dividing circuit 9b. In such a configuration, first, the switch 3 is set to the NO side, the reference voltage source 2 is connected, the switch 93b is set to the NO side, and the voltage dividing circuit 9b is configured to be 0.1 times. Then, the contact of the switch 92 in the range switching circuit 9 is set to the NO side and the switch 93 is set to the NC side to configure the gain of the amplifier 91 to 10 times, and the A / D converter 6 measures. As a result, the following equation holds when G0.1 'is the voltage division ratio of the voltage dividing circuit 9b. Vref · G0.1 ′ · G10 = Vr (9)

Next, with the gain of the voltage dividing circuit 9b being 0.1 times, the switch 3 is connected to the NC side and the measured input Vin
And switch 92 of the range switching circuit 9 to NC.
Side, the switch 93 is set to the NO side, the amplifier 91 is configured to be 0.1 times, and the same measurement is performed. The result is expressed by the following equation. Vin · G0.1 ′ · G0.1 = Vm (10) From the equations (9) and (10), the value of the measured input Vin is calculated by the following equation. Vin = (Vm / Vr). (G10 / G0.1) .Vref (11) The gain G0.1 'of the voltage dividing circuit 9b is also canceled like the gain G10' of the range switching circuit 9a described in FIG. Therefore, the components forming the voltage dividing circuit 9b may be general-purpose products.

According to the present invention, a digital measuring instrument having a self-diagnosis function is realized by utilizing a circuit used for the measurement of a normal measured input such as an amplifier constituted by a reference voltage or a reference resistance. be able to. In addition, the measured value is calculated by comparing the DUT with a built-in reference voltage using an input amplifier composed of a reference resistor.
There is an effect that the influence of the change over time or the temperature coefficient of the elements constituting the / D converter can be canceled and they can be replaced with inexpensive general-purpose products.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing an embodiment of a digital measuring device according to the present invention.

FIG. 2 is a circuit configuration diagram showing another embodiment of the digital measuring device according to the present invention.

FIG. 3 is a circuit configuration diagram showing another embodiment of the digital measuring device according to the present invention.

FIG. 4 is a circuit configuration diagram of an example of a conventional digital measuring instrument of this type.

[Explanation of symbols]

 1 Input terminal under test 2 Reference voltage source 3 Changeover switch 6 A / D converter 7 CPU 8 Display 9 Range change circuit 91 Input amplifier 92, 93 Switch 94, 95 Reference resistance

Claims (1)

[Claims] 1. An input terminal to which an input to be measured is applied, a reference voltage for generating a voltage which serves as a reference input for performing the self-diagnosis and serves as a reference input for comparative measurement of the input under measurement during normal measurement. Generator circuit, changeover switch for switching between the input to be measured and the output of the reference voltage generation circuit, range switching circuit to amplify the output of the input to be measured and the output of the reference voltage generation circuit to the set magnification respectively, and the output of the range switching circuit to a digital signal A / D converter to be converted, pass / fail judgment based on the result of the A / D converter during self-diagnosis, and compare the measurement result with the output of the reference voltage generation circuit during measurement of the input to be measured. A microprocessor that calculates measured values while canceling the influence of fluctuations in circuit components
And a digital measuring instrument equipped with a display that displays the pass / fail judgment result during self-diagnosis and displays the measured value of the measured input during normal measurement.