JPH08189940A - Digital measuring instrument - Google Patents

Abstract

PURPOSE: To obtain a digital measuring instrument the circuit constituting element of which can be replaced with a widely used inexpensive element by correcting the fluctuation of the zero point and gain during continuous measurement by using the measured value obtained through a switch in the preceding cycle in the turning-off cycle of the switch. CONSTITUTION: Inputs 1 to be measured, zero-point potential, and reference potential from a reference voltage source 8 are supplied to a preamplifier 6 through switches 4, 5, and 11. The output of the preamplifier 6 is fetched to a CPU 9 after A/D conversion and measured values are displayed on a display 1. The CPU 9 corrects the fluctuation of the zero point and gain during continuous measurement of the inputs 1 by turning on the switches 4 and 5 and turning off the switch 11 in one measurement cycle and turning on the switches 4 and 11 and turning off the switch 5 in the succeeding cycle, and then, using the measured value obtained through one of the switches in the preceding cycle in the turning-off cycle of the switch.

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 capable of automatically correcting the fluctuation of zero point and gain during continuous measurement.

[0002]

2. Description of the Related Art FIG. 6 shows an example of a conventional digital measuring instrument in which the zero point is automatically corrected. In the figure, reference numeral 1 denotes a voltage source to be measured, which is connected between the Hi-side terminal 2 and the Lo-side terminal 3. Reference numerals 4 and 5 are complementary switches, the switch 4 is connected between the terminal 2 and the preamplifier 6, and the switch 5 is connected between the input end of the preamplifier 6 and the common. Reference numeral 7 is an A / D converter that includes a reference voltage source 8 and converts the output of the preamplifier 6 into a digital signal.
Is a microprocessor (hereinafter referred to as CPU), and 10 is a display unit. On / off of the switches 4 and 5 is controlled by the CPU 9.

In such a configuration, as shown in the timing chart of FIG. 7, the switch 4 and the switch 5 are alternately turned on and off, whereby the measured voltage 1 and the common potential are A / A via the preamplifier 6 respectively. It is applied to the D converter and converted into a digital signal with reference to the reference voltage 8. Both converted signals are taken into the CPU 9, respectively, and the difference between the two signals is obtained, and the value is displayed on the display 10. This display value is obtained by canceling the fluctuation of the zero point of the preamplifier 6 and the A / D converter 7, and such an operation is generally known as an auto-zero function.

However, such an auto-zero function can (1) correct the fluctuation of the zero point, but cannot correct the fluctuation of the gain of the preamplifier 6. (2) As a result, it is necessary to suppress the gain variation in the device of FIG. 6, but for this reason, the key parts such as the preamplifier 6 and the A / D converter 7 are required to have a temporal change or a small temperature coefficient. .

On the other hand, as shown in FIG. 8, there is also known an apparatus capable of correcting both zero point fluctuation and gain fluctuation. In the following figures, the same components as those in FIG. 6 are allotted with the same reference numerals as those in FIG. 6 and their re-explanation is omitted. FIG.
In, 11 is a switch. The reference voltage source 8 of the A / D converter 7 is adapted to be applied to the preamplifier 6 via the switch 11. In the measuring instrument having such a configuration, the switches 4 and 5 are switched as described with reference to FIG. 6 to perform measurement while canceling the fluctuation of the zero point. When performing the self-calibration of the gain fluctuation, the switches 4 and 5 are turned off, and the switch 11 is turned on so that the reference voltage 8 is supplied to the A / D converter 7 via the preamplifier 6.
In addition, the value of the reference voltage is measured. As a result, the gain fluctuations of the preamplifier 6, the A / D converter 7, etc. are corrected. Therefore, the gain variation is only the variation of the reference voltage 8, and the measurement certainty after self-calibration is high.

Although such a digital measuring device can perform self-calibration of gain fluctuation, (1) Since the calibration is performed independently of normal measurement, self-calibration cannot be performed during continuous measurement by auto sampling. (2) Therefore, it is necessary that the key parts have a temporal change or a small temperature coefficient.

[0007]

SUMMARY OF THE INVENTION The object of the present invention is to eliminate the above-mentioned drawbacks of conventional measuring instruments and to correct the zero point and gain fluctuations during continuous measurement, thus reducing the cost of circuit components. It is to realize a device that can be replaced with another general-purpose product.

[0008]

According to the present invention, first, second and third switches for supplying a measured input, a zero point potential and a reference voltage to a preamplifier, respectively, and converting the output of the preamplifier into a digital signal. An A / D converter, a microprocessor in which the output of the A / D converter is taken in, and an indicator for displaying a measurement value obtained through the microprocessor are provided, and in one cycle of the measurement cycle, the first and The second switch is turned on and the third switch is turned off, and in the next cycle, the first and third switches are turned on and the second switch is turned off. Configured to compensate for zero point and gain variations during continuous measurement of the input being measured by using the measurement value obtained via that switch of It is characterized in that the.

[0009]

According to the present invention, in the cycle in which the switch is off, by using the measurement value obtained through the switch in the previous cycle, the zero point and the gain are continuously measured during the continuous measurement of the input to be measured. Correct the fluctuation.

[0010]

1 is a block diagram showing an embodiment of the device of the present invention. In FIG. 1, 11 is a switch, one end of which is connected to the input end of the preamplifier 6 and the other end of which is connected to the reference voltage source 8. The ON / OFF of the switches 4, 5 and 11 is controlled by the CPU 9. When the switch 4 is ON and the switches 5 and 11 are OFF, the input 1 to be measured is measured as X, the switch 5 is ON and the switch 4 is ON. The operation of the apparatus shown in FIG. 1 will be described below, where Y is a case where zero is measured when the switches 11 and 11 are off, Z is a case where the reference voltage 8 is measured when the switches 11 are on and the switches 4, 5 are off. It becomes like.

To calculate the value displayed on the display 10, the process of X (Vin + Vof) .G = Vx (1) The process of Y Vof.G = Vy (2) The process of Z (Vre + Vof) .G = Vz (3) In the equations (1) to (3), Vin: measured voltage Vof: offset voltage Vre: reference voltage Vx, Vy, Vz: A / D conversion value G: gain of preamplifier 6 . From the expressions (1) to (3), the following expression (4) is established. {(Vin + Vof-Vof) * G} / {(Vre + Vof-Vof) * G} = (Vx-Vy) / (Vz-Vy) (4) Here, while measuring the processes of X, Y, and Z, the offset voltage Vof and the gain G of the preamplifier 6 are measured.
Is stable in the short term, the measured voltage Vin is expressed by the equation (4) as follows: Vin = {(Vx-Vy) / (Vz-Vy)}. Vre (5). Equation (5) is the ratio of A / D converted values (first term)
And the reference voltage Vre. Therefore, the fluctuation of the gain G of the preamplifier 6 and the fluctuation of the A / D conversion value are canceled.

The operation sequence of the apparatus shown in FIG. 1 will be described below with reference to the timing chart shown in FIG. First, in the first cycle of measurement value display, Vx is measured by the A / D converter 7 based on the equation (1), and then Vy is measured by the equation (2). In this case, the value of Vz based on the equation (3) is required to obtain the measured voltage Vin, but the value of Vz used previously is used. The CPU 9 performs the calculation of the equation (5) based on the values of Vx and Vy measured this time and the value of Vz measured the last time, and also controls to display the value on the display 10, and the value of Vin obtained as a result is displayed. Displayed by the instrument 10.

In the next measurement cycle, Vx based on the equation (1) and Vz based on the equation (3) are measured, the value of the measured voltage Vin is determined by the microprocessor 9, and the value is displayed. To do. In this case, the value obtained in the previous measurement cycle is used as Vy based on the equation (2). Therefore, it is necessary that the values of Vy and Vz are sufficiently stable with respect to the measurement cycle, but this is not a particular problem in a device with a short measurement cycle such as a digital multimeter.

As described above, in the apparatus of FIG. 1, it is possible to obtain an apparatus capable of continuously measuring the measured voltage 1 while correcting the fluctuation of the zero point and the gain. Since the influence of such fluctuations of the preamplifier 6 or the A / D converter 7 is canceled, inexpensive general-purpose products can be used as the components of them, and the cost can be reduced.

In the apparatus of FIG. 1, the preamplifier 6 has a plurality of gains, and the gain used when measuring the value of the voltage under test 1 may differ from the gain used when measuring the reference voltage 8. The operation of the apparatus of FIG. 1 in such a case will be described as follows. When measuring the voltage under test 1 with the switch 4 on and the switches 5 and 6 off, X is measured with the same gain of the preamplifier 6 as the switch 5 is on and the switches 4 and 6 are off. Yx, switch 6 is on, switches 4, 5 are off, and reference voltage 8 is measured. Z, switch 5 is on with gain of preamplifier 6 same as Z, switches 4, 6 are off. Let Yz be the case of measuring zero with. The measurement sequence is → X → Yx → X → Z → Display → X → Yz → Display →.

In this case, in order to calculate the display value, the process of X (Vin + Vof) .G1 = Vx (6) The process of Yx Vof.G1 = Vyx (7) The process of Z (Vre + Vof) .G2 = Vz (8) Yz process Vof · G2 = Vyz (9) Note that in (6) to (9), Vin: measured voltage Vof: Offset Voltage Vre: reference voltage _{Vx, Vy X, Vy Z,} Vz: A / D conversion values G1, G2: shows the gain of the preamplifier. Therefore, from the expressions (6) to (9), {(Vin + Vof−Vof) · G1} / {(Vre + Vof−Vof) · G2} = (Vx−Vyx) / (Vz−Vyz) (10) ∴Vin = { (Vx-Vyx) / (Vz-Vyz)}. (G2 / G1) .Vre ... (11), and if G2 is multiplied by 1, the following formula is obtained. Vin = {(Vx-Vyx) / (Vz-Vyz)}. Vre / G1 ... (12)

Here, for example, the preamplifier 6 having a gain G1
As shown in FIG. 3, reference resistors 61 and 62 and switches 63 and 6 are
4 and, for example, the reference resistance 61 is 18 kΩ and 62 is 2
If it is kΩ, this preamplifier can have a gain of 10,1,0.1 times. Therefore, for example, the reference voltage Vre
1V, and the full scale of the A / D converter 7 is 1.2V, this measuring device can have three-step measurement ranges of 0.1V, 1V, and 10V. As described above, in the device of the present invention, the measurement range can be expanded if the reference voltage source 8 and the reference preamplifier 6 are provided. Also in this case, A /
The influence of the fluctuation of the D converter 7 can be canceled.

FIG. 4 is a block diagram of another embodiment of the device of the present invention, which is a case where a measuring range is further added to the voltage measuring device. In FIG. 4, 12 is an amplifier having a gain of 1 times and G ′ times. The amplifier 12 is connected between the switch 4 and the preamplifier 6. The preamplifier 6 has the configuration shown in FIG.
By the switching operation of 4, the gain has a gain of 10, 1, 0.1 times, for example. The measurement sequence is the same as that in FIG. 2, and the calculation of the measurement value is also {(Vin + Vof−Vof) · G ′ · G1} / {(Vre + Vof−Vof) · G ′ · G2} = (Vx−Vyx) / (Vz− Vyz) (13) ∴Vin = {(Vx-Vyx) / (Vz-Vyz)}. (G2 / G1) .Vre (14).

In this device, for example, when the reference voltage Vre is 1V and the full scale of the A / D converter 7 is 1.2V,
If the gains G ', G1, and G2 are multiplied by 10, 10, and 0.1, the Vin measurement range becomes 0.01V full scale. Further, if the amplifier 12 is formed of, for example, a voltage divider circuit and its gains G ′, G1, and G2 are multiplied by 0.1, 0.1, and 10 times,
The measurement range of n is 100V full scale. G'is A
It is necessary to make the input of the / D converter 7 approximately 1 V full scale, but it is not included in the final Vin formula and is canceled. Therefore, the resistor forming the amplifier 12 may be an inexpensive general-purpose product. As described above, in the present invention, if there is one reference preamplifier 6, the extension of the measurement range can be easily realized by using general-purpose components.

FIG. 5 is a block diagram of an embodiment in which the device of the present invention is applied to resistance measurement. In FIG. 5, 1 is H
It shows the measured resistance connected between the i terminal 2 and the Lo terminal 3. Reference numeral 13 is a constant current source, 14 and 15 are switches, and 16 is a resistance element. The output terminal of the constant current source 13 is connected to the Hi terminal 2 via the switch 14 and is also connected to one end b of the switch 15. The other end a of the switch 15 is connected to the common via the resistance element 16 and is also connected to the input end of the preamplifier 6 via the switch 16.

In the apparatus having such a configuration, the case where the voltage generated in the resistance to be measured 1 is measured with the switches 4 and 14 on and the switches 5 and 15 and 11 off is X,
With switches 4 and 11 off and switch 5 on,
When zero is measured, the reference resistor 16 is set to Y, the switches 4, 5 and 14 are off, and the switches 15 and 11 are on.
The operation will be described assuming that Z is the case of measuring the voltage generated at. The sequence in this case is the same as that in FIG. To calculate the display value X process (Rin · Is + Vof) · G = Vx (15) Yx process Vof · G = Vy (16) Z process (Rre · Is + Vof) · G = Vz (17) ) In the equations (15) to (17), Rin: value of measured resistance 1 Vof: offset voltage Rre: value of reference resistance 16 Vx, Vy, Vz: A / D conversion value G: preamp 6 gain Is : Indicates the output current of the constant current source 13. From equations (15) to (17), {(Rin · Is + Vof−Vof) · G} / {(Rre · Is + Vof−Vof) · G} = (Vx−Vy) / (Vz−Vy) (18) ∴Rin = {(Vx-Vy) / (Vz-Vy)}. Rre (19).

The above equation (19) is the ratio of A / D values (first
And the reference resistance value 16, the influence of fluctuations in the constant current source 13 for resistance measurement, the preamplifier 6, the A / D converter 7, etc. can be corrected. The device of the present invention can also measure the current in the same manner as the resistance measurement.

[0023]

As described above, in the present invention, by continuously measuring and displaying the measured point and a plurality of reference points in the apparatus, continuous measurement can be performed at the same measurement cycle as when the auto-zero function is used. It is possible to obtain a digital measuring instrument in which the zero point and the gain can be corrected. Further, by obtaining the measured object by comparing it with the reference in the device, it is possible to cancel the zero point and the gain variation due to the other circuit components, and replace those circuit components with inexpensive general-purpose products. There are features.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a digital measuring device of the present invention.

FIG. 2 is a timing chart for explaining the operation of the measuring instrument in FIG.

FIG. 3 is a configuration diagram of an example of the preamplifier shown in FIG.

FIG. 4 is a block diagram showing another embodiment of the measuring instrument of the present invention.

FIG. 5 is a block diagram showing another embodiment of the measuring instrument of the present invention.

FIG. 6 is a block diagram of an example of a conventional digital measuring device.

7 is a timing chart for explaining the operation of the measuring instrument of FIG.

FIG. 8 is a block diagram of an example of a conventional digital measuring device.

[Explanation of symbols]

 1 input to be measured 2 Hi terminal 3 Lo terminal 4, 5, 11, 14, 15, 16 switch 6 preamplifier 7 A / D converter 8 reference voltage source 9 CPU 10 indicator 12 amplifier 13 constant current source

Claims (1)

[Claims] 1. A first, a second and a third switch for supplying an input to be measured, a zero-point potential and a reference voltage to a preamplifier respectively, an A / D converter for converting the output of the preamplifier into a digital signal, and this A A microprocessor for receiving the output of the A / D converter, and a display for displaying a measurement value obtained via the microprocessor are provided, and the first and second switches are turned on and off in one cycle of the measurement cycle. 3 is turned off and in the next cycle the first and third switches are turned on, the second switch is turned off and in the cycle when the switch is off it is obtained via that switch in the previous cycle. The measurement device is characterized in that the fluctuations of the zero point and the gain are corrected during continuous measurement of the input to be measured by using the measured values. .