JP2001174489A - Digital voltmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the measurement accuracy by compensating for a measurement error without taking much time and labor. SOLUTION: The voltmeter is comprised of a switch 1 for sequentially switching an input voltage 101 and incorporated reference voltages 1 and 2 at a predetermined timing, an amplifier 3 set for every range, a selector 4 for selecting the amplifier of a proper range, an A/D converter 5 for converting an output voltage of the amplifier 3 into a digital data 108, and a control circuit 6 to which are inputted the digital data 108 and a switching signal 102 and which outputs a selecting signal 103 thereby selecting the proper range, detects an error of each range from the reference voltages 1 and 2, generates correction data, compensates for a measured value of the input voltage by the correction data and outputs display data to a display device 8.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital voltmeter, and more particularly to a digital voltmeter provided with means for automatically switching a switching range and automatically correcting an error in each range.

[0002]

2. Description of the Related Art Conventionally, this type of digital voltmeter generally has a predetermined gain in which the input voltage in each range is standardized using a variable gain amplifier or variable attenuation attenuator for switching the amplification degree for each range. After amplifying the voltage to a certain voltage, A / D converting the data into digital data, the data is converted into display data, and the measured voltage value is digitally displayed. However, in the measurement process, an error occurs in the displayed value due to an error in the amplifier gain, attenuator attenuation, or an error in the A / D converter. In order to correct this, for example, correction data is measured or calculated in advance and stored in a ROM, and display data is corrected using this correction data during A / D conversion.

For example, an example of a digital voltmeter described in Japanese Patent Application No. 3-73222 will be described below. FIG. 6 is a block diagram showing this conventional example.

Reference numeral 30 denotes an input terminal, and reference numeral 40 denotes a range switching circuit, which switches a voltage dividing resistor 41 for dividing a measured voltage applied to the input terminal 30 according to a range and a divided voltage generated in the voltage dividing resistor 41. It comprises a switch 42, a preamplifier 43, a resistor 24 for changing its gain, and a switch 45. The range switching circuit 40 includes switches 42 and 45
Are switched in accordance with the range, the input measurement voltage can be switched to 1/1000 to 10 times. The output terminal of this range switching circuit has a standardized voltage Ex of 2 V at the maximum for each range. Is obtained. No adjustment means such as a variable resistor is used in this range switching circuit. As a result, a slight error between the ranges occurs in the voltage Ex due to a gain error in the voltage dividing resistor 41 and the preamplifier 43.

[0005] Reference numeral 50 denotes an analog-to-digital converter (hereinafter referred to as an AD converter). This A / D converter includes a voltage Ex passing through the range switching circuit 40 and a reference voltage E.
The voltage Ex is converted into a digital signal in accordance with the ratio Ex / Es to s. 60 is an A / D converter 4
This is a display circuit that receives an output signal of 0 and displays its value. 70 is a range signal generation circuit. Reference numeral 80 denotes a read-only memory (hereinafter, referred to as a ROM) in which data for compensating an error for each range in the range switching circuit 40 is written in advance. 100 is an analog multiplier, 90 is an addition circuit, and 110 is a resistance voltage dividing circuit. The analog multiplier 110 multiplies the divided reference voltage Es' by a coefficient k 'and outputs a voltage represented by k'Es'.
Is controlled by the data.

The reference voltage Es' is applied to one input terminal of the adder circuit 90, and the output of the analog multiplier 100 is applied to the other input terminal. The addition circuit 90 adds the voltages applied to both input terminals. As a result, the voltage dividing coefficient of the voltage dividing circuit 110 and the analog multiplier 100
Assuming that a coefficient including the coefficient k ′ is k, the adding circuit 90 outputs a voltage Es represented by Es ′ (1 + k). This voltage Es is applied to the AD converter 50 as a reference voltage. The AD converter 50 uses the voltage Es as a reference, and calculates Ex / E
The measured voltage Ex is converted into a digital signal at the ratio of s.

As described above, in this digital voltmeter, the coefficient k 'of the analog multiplier 100 is controlled by the output of the ROM 80 in which data for correcting an error for each range is written in advance. Since the reference voltage Es of the D converter 50 is configured to change, an error between the ranges generated by the range switching circuit 40 is automatically corrected for each range.

The compensation data to be written into the ROM 80 is read by using a ROM writing device 801 indicated by a dotted line, applying an accurate measurement voltage to the input terminal 30 for each range, reading the A / D converted digital output at that time, and reading the error. With compensation data as R
Write to OM80.

[0009]

As described above, in the conventional example, a ROM writing device 801 is prepared for compensation data for compensating for a measurement error, and the ROM writing device 801 is connected to the device by connecting it with a dotted line in FIG. After setting the ROM 80, an accurate measurement voltage for each range is applied to the input terminal, compensation data is obtained from the digital output value at this time, the compensation data is written into the ROM 80, and the written ROM 80 is incorporated in the main body. ing. As described above, there is a problem that writing the compensation data is extremely troublesome. In addition, updating the compensation data is troublesome because the above-described writing operation is required each time. Therefore, if the same compensation data is used for a long period of time, errors such as changes in the diameter of each component element may occur. On the other hand, there is a problem that sufficient compensation is not provided. In addition, compensation is A
Since the conversion is performed at the stage of the analog voltage before the / D conversion, there is a problem that it is unstable with respect to a temperature change or the like.

[0010]

A digital voltmeter according to the present invention is a digital voltmeter which has a plurality of switching ranges and performs digital display.
1 set to the center voltage value of the input voltage where the ranges overlap
Alternatively, a plurality of reference voltage sources for correcting measurement values, a first switch that sequentially selects and outputs the input terminal and the reference voltage source at a predetermined timing by a first switching signal, A sample-and-hold circuit for holding the output voltage of the switch, and a voltage held by the sample-and-hold circuit, and when the input voltage is a voltage within the corresponding range, up to a predetermined output voltage set commonly for each range. A plurality of amplifiers provided for each of the ranges for amplifying and outputting the signals, a plurality of amplifiers commonly connected on the input side, and a second switch for sequentially selecting and outputting the output voltage of each of the amplifiers according to a second switching signal; An A / D converter that converts an output voltage of the second switch into digital data, a first switching signal generation circuit that generates the first switching signal, and an A / D converter that outputs the A / D converter. Digital data and the first switching signal are input, and when the first switching signal selects the reference voltage source, the second switching signal is output and each range is selected to select the reference voltage source. The digital data is read out, the measurement error in each range is calculated from the digital data and the built-in reference data, and this is held as correction data. Then, when the first switching signal selects the input terminal, the second A control circuit that outputs a switching signal, selects an appropriate range of the input voltage at this time, reads the digital data at that time, calculates a measured value from the digital data and the correction data, and outputs the calculated value as display data; A display for inputting the display data and displaying the measured value.

For example, the control circuit inputs the first switching signal and the digital data, outputs the second switching signal in accordance with a predetermined measurement algorithm, selects an appropriate range, and converts the digital data at that time. I / O for performing interface processing for outputting to a read internal circuit and outputting the display data calculated by the internal circuit to an external circuit
A control circuit, the first switching signal input via the I / O control circuit, the digital data, and the built-in reference data, the correction data according to the measurement algorithm and a first calculation program; CPU that calculates and outputs the display data in accordance with the measurement algorithm and the second calculation program from the switching signal, the digital data, and the correction data.
A first internal ROM for storing the measurement algorithm and the first and second calculation programs, and a second internal ROM for storing the correction data, the display data, and the reference data.
And a data bus connecting the I / O control circuit, the CPU, and the first and second built-in ROMs.

Further, the control circuit may include an external communication circuit for generating measurement result data in a predetermined format from the display data and transmitting the measurement result data to a terminal device provided outside via a communication line.

Further, the first switching signal generation circuit may monitor the on / off of a power supply and output the first switching signal for selecting the reference voltage source when the power is turned on.

Further, the first switching signal generating circuit may monitor the ambient temperature and output the first switching signal for selecting the reference voltage source when detecting a temperature change exceeding a predetermined value. good.

[0015]

Embodiments of the present invention will now be described with reference to the drawings.

FIG. 1 is a block diagram showing a first embodiment of the present invention, FIG. 2 is a block diagram showing a second embodiment of the present invention, and FIG. 3 is a third embodiment of the present invention. FIG. 4 is a block diagram showing an embodiment, FIG. 4 is a characteristic diagram showing input voltage range switching in FIG. 1, and FIG. 5 is a flowchart showing (a) correction data measurement processing and (b) input voltage measurement processing in FIG. is there. FIG. 6 is a block diagram of a conventional example.

A digital voltmeter according to a first embodiment of the present invention will be described with reference to FIG. This digital voltmeter has three switching ranges and all measurement ranges are 3.
It is an example of a digital voltmeter that digitally displays an input voltage in a range of 47 mv to 5 v (63.2 dB). First, the configuration will be described.

An input terminal 9 to which an object to be measured is connected, two reference voltage sources 10 and 11 for correcting measured values set to the center voltage value of an input voltage 101 having overlapping ranges, an input terminal 9 and a reference A switch 1 using an analog multiplexer for sequentially selecting and outputting the voltage sources 10 and 11 at a predetermined timing according to a switch signal 102; a sample and hold circuit 2 for holding the output voltage of the switch 1; When the input voltage is within the corresponding range and the input voltage is within the corresponding range, the output voltage is amplified to a predetermined output voltage that is set in common for each range. Three amplifiers 3a, 3
b, 3c, a switch 4 using an analog multiplexer for sequentially selecting and outputting the output voltage of each amplifier 3 by a switch signal 103, and an A / D converter 5 for converting the output voltage of the switch 4 into digital data 108. And the switching signal 10
2, digital data 108 output from the A / D converter 5, and a switching signal 102.
When the switching signal 102 selects the reference voltage sources 10 and 11, the switching signal 103 is output and the digital data 108 is read out by selecting each range, and the measurement in each range is performed based on the digital data and the built-in reference data. An error is calculated and held as correction data. Next, when the switching signal 102 selects the input terminal 9, the switching signal 103 is output, an appropriate range at this time is selected, and the digital data 108 of the input voltage 101 is read. Digital data 10
A control circuit 6 for calculating and outputting display data 104 for displaying a decibel value of a measured value and measurement result data 105 to be transmitted to an external terminal device 20 from the correction data 8 and the correction data; And a display 8 for displaying the result. The reference voltage sources 10 and 11 use highly stable Zener diodes with temperature compensation, and generate extremely stable reference voltages 1 and 2 driven by a stabilized power supply.

The internal structure of the control circuit 6 includes a switching signal 10
2, an I / O control circuit 51 for inputting digital data 108 and outputting a switching signal 103, display data 104, and timing control signals 106 and 107 in accordance with a predetermined measurement algorithm to perform interface processing with an external circuit;
Measurement from the switching signal 102, the digital data 108 and the correction data, and the switching signal 102, the digital data 108 and the correction data according to the measurement algorithm and the first calculation program from the switching signal 102, the digital data 108 and the built-in reference data input via the I / O control circuit 51. A CPU 54 for calculating and outputting the display data 104 in accordance with the algorithm and the second calculation program, and a built-in ROM 52 for storing the measurement algorithm and the first and second calculation programs
A built-in ROM 53 for storing correction data, display data 104, reference data, and measurement result data 105;
An O control circuit 51, a CPU 54, and a data bus 56 connecting the built-in ROMs 52 and 53 are provided.

The switching signal generating circuit 7 generates and outputs a selection signal 102 which selects the input terminal 9 at a predetermined timing, for example, always, and continuously selects the reference voltage sources 10 and 11 at intervals of 100 days. .

The switching signal generating circuit 7 always selects an input terminal, monitors on / off of the power supply, and outputs a switching signal 102 for sequentially selecting the reference voltage sources 11 and 12 when the power is turned on. This further improves the accuracy of the measured values.

The switching signal generating circuit 7 constantly selects an input terminal, outputs a switching signal 102 for monitoring the ambient temperature, and sequentially selecting the reference voltage sources 11 and 12 when detecting a temperature fluctuation exceeding a predetermined value. By doing so, measurement errors due to temperature fluctuations can be eliminated.

Next, the operation will be described with reference to FIGS. FIG. 4 illustrates the switching range of the input voltage 101 to be measured applied to the input terminal 7, and the entire range is 3.7.
5mv to 5v (63.2dB) is set to range 1 as shown in the figure.
-3 are divided into three ranges, and the upper and lower ends of each range are set so as to overlap with each other.
2, reference voltages 1 and 2 are set. The input voltage 101 of each range is amplified by the corresponding range of the amplifier 3 and becomes a normalized output voltage 420 mv to 5 v. That is, the amplifier 3a has a gain of × 1, the amplifier 3b has a gain of × 11, and the amplifier 3c has a gain of × 121. The amplifier 3
Since a has a gain of 1, it can be directly connected. However, since an analog multiplexer circuit is used for the switch 4, it is necessary to make the output characteristics of each amplifier uniform, and amplifiers having the same characteristics are used.

In FIG. 1, one of the input voltage 101 and the reference voltages 1 and 2 applied to the input terminal 7 is selected by the switch 1 in the order described above and enters the sample and hold circuit 2, where the measurement at that time is performed. The input voltage is temporarily held until the operation is completed. The held input voltage is applied to the amplifier 3a,
3b and 3c, the output voltage of the amplifier 3 in an appropriate range is selected by the next switch 4 by a method described later,
The standardized output voltage is input to the A / D converter 5. The A / D converter 5 is a 12-bit A / D converter with a voltage ratio of 0.
It has a resolution capable of reading 05 dB and converts an analog voltage into digital data 108. Digital data 1
08 is input to the control circuit 6 and processed as follows.

FIG. 5 (a) shows that the switch 1 has the reference voltages 1 and 2.
This shows the processing flow in the control circuit 6 when is selected. At the start time, the switch 4 selects the amplifier 3a by the switching signal 103. In this state, first, the digital data 108 of the reference voltage 1 is I / O It is read by the O control circuit 51. The value at this time is set to a0 (S51). Next, the digital data 10 of the reference voltages 1 and 2 is switched to the amplifier 3b.
Read 8. The values at this time are a1 and b1 (S5
2). Next, the digital data 108 of the reference voltage 2 is read by switching to the amplifier 3b. The value at this time is set to b2 (S
53). Next, the correction values S1 and S2 between the ranges are calculated from the read values a0, a1, b1 and b2 using the calculation formulas shown in the drawing, and the error in each range is calculated using the S1 and S2 and the reference data stored in the built-in ROM 53. The value is calculated by the CPU 54 and stored in the built-in ROM 53 as correction data (S54).

FIG. 5B shows a processing flow in the control circuit 6 when the switch 1 selects the input voltage 101. At the start time (S55), the switch 4 operates the amplifier 3a.
Is selected. When the input voltage 101 is applied in this state, the digital data 108 at that time is transferred to the I / O control circuit 5.
Read by 1 (S56). It is determined whether the read value is outside the upper limit range (S57). If Yes, measurement is disabled. If No, it is further determined whether the value is outside the lower limit range (S58). If it is Yes, the voltage is in the appropriate range. If Yes, the voltage is switched to the next amplifier 3b and read (S59). Similarly, S60, S61,
In step S62, an appropriate range is determined, and the digital data is read. The digital data read in the appropriate range is subjected to arithmetic processing by the CPU 54 (S63).
Stored in OM53. Then, it is sent to the display 8 or the external terminal device 20 via the I / O control circuit 51 (S
64).

Next, a second embodiment of the present invention will be described with reference to FIG. The configuration of this digital voltmeter
Compared with the digital voltmeter shown in FIG. 1, A / D converters 5a, 5b, 5c are provided for each output of
The difference is that the digital data 108 output from the / D converter 5 is selected by the control circuit 12 and the switch 4 and the sample hold circuit 2 in FIG. The feature of this digital voltmeter is that although three AD converters are used as compared with the digital voltmeter in FIG. 1, the selection of an appropriate range and the AD conversion are processed in parallel, so that the measurement time can be shortened.

In the following description, since blocks having the same reference numerals as those in FIG. 1 have the same functions, the description will be omitted, and the description will be made around different blocks. The output voltage amplified by each amplifier 3 is converted into digital data 108 by the corresponding A / D converter 4 and input to the control circuit 12.
The I / O control circuit 121 of the control circuit 12 inputs the digital data 108a to 108b,
Is selected, the digital data 108 in the proper range is selected using the reference indicating the range of the digital data in the proper range, and its value is read.
Output to internal circuit. The CPU 124 of the control circuit 12, the built-in ROM 122, the external communication circuit 125, and the bus 126 are the CPU 54, the built-in ROM 52, the external communication circuit 55 of FIG.
It has the same function as the bus 56. Further, the sample and hold circuit is unnecessary because the digital voltage 21 does not perform range switching. The calculation processing of the correction data and the display data is the same as that in the example of FIG.

Next, a third embodiment will be described with reference to FIG. The configuration of this digital voltmeter is different from the digital voltmeter shown in FIG. 1 in that the amplifier 13 is a gain switching type amplifier, and a switch for switching the amplifier output is omitted. It has the same configuration. The feature of this digital voltmeter is that the cost can be reduced because the number of amplifiers is reduced and the switch is omitted. However, there is a disadvantage that the amplifier 13 takes a long time to stabilize after the gain switching and the measurement time becomes long. The gain of the amplifier 13 can be switched between × 1, × 11 and × 121 by switching the feedback resistance. Other configurations and operations are the same as those of the above-described example of FIG.

[0030]

As described above, the digital voltmeter of the present invention has a built-in reference voltage source for correction, and generates correction data as needed at a predetermined timing using this reference voltage source.
Since the error of the measured voltage is automatically compensated for by the correction data, there is no need to input the correction data in advance, so that there is an effect that this trouble can be saved. In addition, since the correction data is updated as needed, and the correction processing is performed at the stage of digital data, higher accuracy can be compensated and the measurement result has higher accuracy.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the first invention.

FIG. 2 is a block diagram showing an embodiment of the second invention.

FIG. 3 is a block diagram showing an embodiment of the third invention.

FIG. 4 is a characteristic diagram illustrating a switching range of an input voltage in FIG. 1;

FIG. 5A shows a process of measuring correction data in FIG.
(B) It is a flowchart which shows the measuring process of an input voltage, respectively.

FIG. 6 is a block diagram showing a conventional example.

[Explanation of symbols]

 1, 4 switcher 2 sample and hold circuit 3a, 3b, 3c, 13 amplifier 5, 5a, 5b, 5c AD converter 6, 12 control circuit 7 switching signal generation circuit 8 display 9 input terminal 20 external terminal device 51, 121 I / O control circuit 52,53,122,123 Internal ROM 54,124 CPU 55,125 External communication circuit 56,126 Bus

Claims (8)

[Claims] In a digital voltmeter having a plurality of switching ranges and performing digital display, an input terminal to which a device under test is connected and one or a plurality of terminals set to a central voltage value of an input voltage whose range overlaps. A reference voltage source for measuring value correction, a first switch for sequentially selecting and outputting the input terminal and the reference voltage source at a predetermined timing by a first switching signal, and a first switch for the first switch. A sample-and-hold circuit for holding an output voltage, and inputting the voltage held by the sample-and-hold circuit, and amplifying the input voltage to a predetermined output voltage set commonly for each range when the input voltage is within a corresponding range. A plurality of amplifiers which are provided for each of the ranges for output and output, and whose input sides are connected in common; a second switch for sequentially selecting and outputting the output voltage of each of the amplifiers by a second switching signal; An A / D converter for converting the output voltage of the second switch into digital data, a first switch signal generating circuit for generating the first switch signal,
The digital data output from the / D converter and the first switching signal are input, and when the first switching signal selects the reference voltage source, the second switching signal is output to select each range. The digital data of each of the reference voltage sources is read out, a measurement error in each range is calculated from the digital data and the built-in reference data, and the calculated error is held as correction data. When the selection is made, the second switching signal is output, an appropriate range of the input voltage at this time is selected, the digital data at that time is read out, a measured value is calculated from the digital data and the correction data, and this is used as display data. A digital voltmeter, comprising: a control circuit that outputs the data; and a display that inputs the display data and displays a measured value. 2. A digital voltmeter having a plurality of switching ranges and performing digital display, wherein an input terminal to which a device under test is connected and one or a plurality of terminals set to a central voltage value of an input voltage having an overlapping range. A reference voltage source for correcting a measured value, a first switch for sequentially selecting and outputting the input terminal and the reference voltage source at a predetermined timing according to a first switch signal, and an output of the first switch A voltage is input, and when this input voltage is a voltage in the corresponding range, the voltage is amplified to a predetermined output voltage that is set commonly to each range, and output is provided for each range, and the input side is connected in common. A plurality of amplifiers, a plurality of A / D converters for converting output voltages of the respective amplifiers into digital data, and a first switching signal generating circuit for generating the first switching signal;
The digital data output from each of the A / D converters and the first switching signal are input, and when the first switching signal selects the reference voltage source, each digital data read at this time and a built-in reference A measurement error in each range is calculated from the data and held as correction data. Then, when the first switching signal selects the input terminal, a predetermined selection is made from the digital data of the input voltage at that time. A control circuit for selecting an appropriate range based on a reference, reading out the digital data thereof, calculating a measurement value from the correction data as the digital data, and outputting this as display data, and a display for inputting the display data and displaying the measurement value A digital voltmeter characterized by comprising: 3. A digital voltmeter having a plurality of switching ranges and performing a digital display, wherein an input terminal to which a device under test is connected is connected to one or a plurality of terminals set to a central voltage value of an input voltage whose range overlaps. A reference voltage source for measuring value correction, a first switch for sequentially selecting and outputting the input terminal and the reference voltage source at a predetermined timing by a first switching signal, and a first switch for the first switch. A sample-and-hold circuit for holding an output voltage, and a plurality of amplifiers for inputting an output voltage of the sample-and-hold circuit and amplifying the input voltage to a predetermined output voltage commonly set for each range when the input voltage is within a corresponding range. A gain-variable amplifier for switching the amplification degree by a second switching signal, an A / D converter for converting an output voltage of the amplifier into digital data, and the first switch. A first switching signal generation circuit for generating a switching signal, the digital data output from the A / D converter and the first switching signal, and the first switching signal selects the reference voltage source. Sometimes, the second switching signal is output to select each range, read out the digital data of each reference voltage source, calculate each range measurement error from the digital data and the built-in reference data, and hold this as correction data. Then, when the first switching signal selects the input terminal, the second switching signal is output to select an appropriate range at this time, read out the digital data at that time, read the digital data and the correction data. A control circuit for calculating a measured value from the data and outputting the calculated value as display data, and a display for inputting the display data and displaying the measured value. Tal voltmeter. 4. The control circuit receives the first switching signal and the digital data, outputs the second switching signal according to a predetermined measurement algorithm, selects an appropriate range, and selects the digital data at that time. And an I / O control circuit for performing an interface process for outputting the display data calculated by the internal circuit to an external circuit and outputting the first display data calculated by the internal circuit to the external circuit.
The correction data according to the measurement algorithm and the first calculation program from the switching signal and the digital data and the built-in reference data, and the measurement algorithm and the second from the first switching signal, the digital data and the correction data. A CPU for calculating and outputting each of the display data according to the calculation program of 2,
A first internal ROM for storing the measurement algorithm, the first and second calculation programs, a second internal ROM for storing the correction data, the display data, and the reference data, and the I / O control 4. The digital voltmeter according to claim 1, further comprising a circuit, a data bus connecting the CPU and the first and second internal ROMs. 5. The control circuit inputs the first switching signal and each of the digital data output from each of the A / D converters, and selects a proper range of digital data from each of the digital data. An I / O control circuit that performs an interface process of selecting and reading the data according to data, outputting the read data to an internal circuit, and outputting the display data calculated by the internal circuit to an external circuit; 1 according to the measurement algorithm and the first calculation program from the digital signal and the built-in reference data, the correction data, the first switching signal, the digital data and the correction data from the measurement algorithm and A CPU for calculating and outputting the display data in accordance with a second calculation program; A first built-in ROM for storing a constant algorithm and the first and second calculation programs;
A second internal ROM for storing the correction data, the display data, the reference data, and the selection reference data;
3. The digital voltmeter according to claim 2, further comprising an / O control circuit, a data bus connecting the CPU and the first and second internal ROMs. 6. The control circuit includes an external communication circuit for generating measurement result data in a predetermined format from the display data and transmitting the measurement result data to a terminal device provided outside via a communication line. Item 6. A digital voltmeter according to item 4 or 5. 7. The circuit according to claim 1, wherein the first switching signal generation circuit monitors on / off of a power supply and outputs the first switching signal for selecting the reference voltage source when the power supply is turned on. Item 7. A digital voltmeter according to any one of Items 1 to 5 or 6. 8. The first switching signal generating circuit outputs the first switching signal for selecting the reference voltage source when monitoring an ambient temperature and detecting a temperature change equal to or more than a predetermined value. Digital voltmeter according to any one of 1 to 5 or 6.