JP3244212B2 - Digital measuring instrument - Google Patents

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 compensating for zero point and gain fluctuations during continuous measurement.

[0002]

2. Description of the Related Art FIG. 6 shows an example of a conventional digital measuring instrument in which a 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 denote complementary switches. The switch 4 is connected between the terminal 2 and the preamplifier 6, and the switch 5 is connected between the input terminal of the preamplifier 6 and the common. Reference numeral 7 denotes an A / D converter that includes a reference voltage source 8 and converts the output of the preamplifier 6 into a digital signal.
Denotes a microprocessor (hereinafter referred to as CPU) and 10 denotes a display unit. On / off of the switches 4 and 5 is controlled by the CPU 9.

In such a configuration, the switch 4 and the switch 5 are alternately turned on and off as shown in a timing chart of FIG. 7, whereby the voltage 1 to be measured and the common potential are A / A via the preamplifier 6 respectively. It is applied to a D converter and converted into a digital signal with reference to a reference voltage 8. Both converted signals are taken into the CPU 9 to determine the difference between the two signals, 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, with such an auto-zero function, (1) fluctuation of the zero point can be corrected, but fluctuation of the gain of the preamplifier 6 cannot be corrected. (2) As a result, in the apparatus shown in FIG. 6, it is necessary to suppress the fluctuation of the gain. For this reason, key parts such as the preamplifier 6 and the A / D converter 7 need 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 drawings, the same components as those in FIG. 6 are denoted by the same reference numerals as those in FIG. FIG.
, 11 is a switch. The reference voltage source 8 of the A / D converter 7 is applied to the preamplifier 6 via the switch 11. In the measuring instrument having such a configuration, the measurement is performed while switching the zero point fluctuation by switching the switches 4 and 5 as described with reference to FIG. When performing the self-calibration of the gain variation, the switches 4 and 5 are turned off, and the switch 11 is turned on to supply the reference voltage 8 to the A / D converter 7 via the preamplifier 6.
And the value of the reference voltage is measured. As a result, gain fluctuations of the preamplifier 6 and the A / D converter 7 are corrected. Therefore, the gain variation is only the variation of the reference voltage 8, and the measurement confidence after self-calibration is increased.

Although such a digital measuring instrument can perform self-calibration for 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 change with time or have a small temperature coefficient.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks of the conventional measuring instrument, correct the zero point and gain fluctuations during continuous measurement, and thereby reduce the cost of circuit components. Another object of the present invention is to realize a device that can be replaced with a general-purpose product.

[0008]

SUMMARY OF THE INVENTION The present invention provides a method for measuring a resistance to be measured.
Each of the reference resistors provided in parallel with this measured resistor
First and second switches for supplying the current of the constant current source to the
The voltage generated at the resistance to be measured, the zero point potential, and the reference resistance
The third and the third supply the generated voltage to the preamplifier respectively.
4th and 5th switches and the output of the preamplifier
A / D converter for converting into a digital signal, and this A / D converter
The microprocessor from which the output of the
Table showing the measured values obtained through the microprocessor
Indicator in one cycle of the measurement cycle
Turns off the second and fifth switches, and
When the third switch and the fourth switch are turned on alternately
In both cases, the fourth switch is turned off in the next cycle.
And the first and third switches and the second and fifth switches.
Switch on alternately, the readings taken in the previous cycle
During continuous measurement of the measured resistance
The present invention is characterized in that it is configured to correct the fluctuation of the zero point and the gain .

[0009]

According to the present invention, in the cycle in which the switch is turned off, the measured value obtained through the switch in the previous cycle is used, so that the zero point and the gain are continuously measured during the continuous measurement of the input under test. Compensate for fluctuations.

[0010]

FIG. 1 is a block diagram showing an embodiment of the apparatus of the present invention. In FIG. 1, a switch 11 has one end connected to the input terminal of the preamplifier 6 and the other end connected to the reference voltage source 8. The ON / OFF of the switches 4, 5 and 11 is controlled by the CPU 9, and when the input 1 to be measured is measured while the switch 4 is on and the switches 5 and 11 are off, the switch X is turned on, and the switch 4 is turned on. The operation of the apparatus shown in FIG. 1 is described as follows: Y when measuring zero with switches 11 and 11 off, and Z when measuring reference voltage 8 with switches 11 on and switches 4 and 5 off. It looks 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 expressions (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 equations (1) to (3), the following equation (4) holds. {(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 a short term, the measured voltage Vin is expressed as Vin = {(Vx−Vy) / (Vz−Vy)} · Vre (5) from the equation (4). Equation (5) is the ratio of A / D conversion values (first term)
And the reference voltage Vre. Therefore, the fluctuation of the gain G and the fluctuation of the A / D conversion value of the preamplifier 6 are canceled.

Here, the sequence of the operation of the apparatus shown in FIG. 1 will be described with reference to the timing chart shown in FIG. First, in the first measurement value display cycle, 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, and the value of Vz used is the value measured last time. 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 last time, and performs control for displaying the value on the display 10. The value of Vin obtained as a result is displayed. Displayed by the container 10.

In the next measurement cycle, the measurement of Vx based on the equation (1) and the measurement of Vz based on the equation (3) are performed, the value of the voltage Vin to be measured is obtained by the microprocessor 9, and the value is displayed. I 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 does not pose a particular problem in an apparatus having a short measurement cycle such as a digital multimeter.

As described above, the apparatus shown in FIG. 1 is capable of continuously measuring the voltage under measurement 1 while correcting the fluctuations of the zero point and the gain. Since the influence of the fluctuation of the preamplifier 6 or the A / D converter 7 is canceled, an inexpensive general-purpose product can be used as a component of the component and the cost can be reduced.

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

In this case, 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) Process of Yz 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 equations (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) When G2 is multiplied by 1, the following equation is obtained. Vin = {(Vx−Vyx) / (Vz−Vyz)} · Vre / G1 (12)

Here, for example, a preamplifier 6 having a gain G1
Are connected to the reference resistors 61 and 62 and the switches 63 and 6 as shown in FIG.
For example, the reference resistance 61 is 18 kΩ and the reference resistance 62 is 2
Assuming kΩ, the preamplifier can have a gain of 10, 1, 0.1 times. Therefore, for example, the reference voltage Vre
Is 1 V, and the full scale of the A / D converter 7 is 1.2 V, the measuring device can have three measurement ranges of 0.1 V, 1 V, and 10 V. As described above, in the device of the present invention, the measurement range can be extended if the reference voltage source 8 and the reference preamplifier 6 are provided. 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 apparatus according to the present invention, in which a measuring range is added to the voltage measuring apparatus. In FIG. 4, reference numeral 12 denotes an amplifier having gains of 1 and G '. 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 No. 4, for example, a gain of 10, 1, 0.1 times is obtained. The measurement sequence is the same as in FIG. 2, and the calculation of the measured value is {(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 1 V and the full scale of the A / D converter 7 is 1.2 V,
If the gains G ', G1 and G2 are multiplied by 10, 10, and 0.1, the measurement range of Vin becomes 0.01 V full scale. If the gain G ', G1, and G2 are set to 0.1, 0.1, and 10 times, for example, by dividing the amplifier 12 into a voltage dividing circuit, Vi
The measurement range of n is 100V full scale. G 'is A
It is necessary to set the input of the / D converter 7 to approximately 1 V full scale, but it is not included in the final expression of Vin and is canceled. Therefore, the resistors constituting the amplifier 12 may be inexpensive general-purpose products. As described above, in the present invention, if there is one reference preamplifier 6, the measurement range can be easily extended 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
3 shows a resistance to be measured connected between the i terminal 2 and the Lo terminal 3. 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 to one end b of the switch 15. The other end a of the switch 15 is connected to the common via a resistance element 16 and to the input terminal of the preamplifier 6 via the switch 16.

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

The above equation (19) gives the ratio of A / D values (first
) And the value of 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, and the like can be corrected. Note that the device of the present invention can also measure a current in the same manner as the resistance measurement.

[0023]

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

[Brief description of the drawings]

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

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

FIG. 3 is a configuration diagram of an example of a preamplifier of FIG. 1;

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 instrument.

FIG. 7 is a timing chart for explaining the operation of the measuring device of FIG. 6;

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

[Explanation of Signs] 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 Display 12 Amplifier 13 Constant current source

Claims (1)

(57) [Claims] 1. A resistance to be measured is provided in parallel with the resistance to be measured.
Of the constant current source to each of the reference resistors
1. a second switch, and a voltage generated at the resistance to be measured
And the zero-point potential and the voltage generated at the reference resistor, respectively.
Third, fourth and fifth switches for supplying the amplifier;
A / D for converting the output of the preamplifier into a digital signal
A converter, and a my to receive the output of the A / D converter.
Microprocessor and through this microprocessor
A display for displaying the measured value to be measured , wherein in one cycle of the measurement cycle, the second,
A fifth switch is turned off, and the first and third switches are turned off.
And the fourth switch are turned on alternately, and the next
In the vehicle, the fourth switch is turned off, and the
1st, 3rd switch and 2nd, 5th switch alternately
Turn on and use the measurements obtained in the previous cycle
The zero point and gain during continuous measurement of the measured resistance.
It is configured to compensate for fluctuations in
Measuring device.