JPH0715493B2 - Amplifier offset voltage measuring device - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a measuring device for measuring a DC offset voltage in an operational amplifier such as a differential amplifier, and more particularly to measuring an input offset voltage of an integrated operational amplifier. The present invention relates to an amplifier offset voltage measuring device suitable for performing the above.

[Technical Background of the Invention and Problems Thereof] In an operational amplifier such as a differential amplifier, the output voltage often does not become 0 even when the input voltage is not applied. This is because the input offset voltage exists in the operational amplifier, and this input offset voltage is represented by Voffset in FIG. That is, with the non-inverting input terminal of the operational amplifier connected to the ground potential, the input voltage Vin
Corresponding to the input voltage Vin at which the value of the voltage Vout at the output terminal becomes 0 when the value of Vin is sequentially increased.

The relationship between the input and output voltages of the operational amplifier is given by the following equation 1.

Vout = Gain × ΔVin (1) where Vout is the output voltage, Gain is the voltage gain of the operational amplifier, and ΔVin is the differential input voltage.

That is, the output voltage is the gain times the applied differential input voltage. However, when ΔVin is 0, an error occurs such that Vout does not become 0. This error is due to the input offset voltage described above.
The relationship between the input and output voltages of the operational amplifier in consideration of ffset is given by the following two equations.

Vout = Gain × (ΔVin−Voffset) 2 Since the input offset voltage appears as an output error when using the operational amplifier as described above, it is necessary to cancel the input offset voltage in advance before using it. Further, in order to perform this cancellation, it is necessary to measure the input offset voltage value.

Conventionally, such an input offset voltage value is measured by, for example, sequentially increasing the input voltage of the operational amplifier in the range of ± 15 mV at intervals of 0.01 mV, which is the final measurement accuracy, and changing the output voltage value of the operational amplifier each time. There is known a method of measuring, plotting with a plotter or the like to create a graph, extracting an input voltage value in the vicinity of an output voltage passing 0V, and calculating an input offset voltage.

However, in the method of measuring the input offset voltage by sequentially changing the input voltage of the operational amplifier at regular intervals, which is the final measurement accuracy, as described above, the input voltage of the operational amplifier is within the range of ± 15 mV as described above. When changing at 0.01 mV intervals, data must be processed 3000 times. Therefore, the measurement time becomes very long,
It has the drawback of being inefficient.

[Object of the Invention] The present invention has been made in view of the above circumstances, and an object thereof is to be able to measure an offset voltage of an amplifier in a necessary measurement range with sufficient accuracy and to measure a measurement time. It is an object of the present invention to provide an offset voltage measuring device for an amplifier that can significantly reduce

[Summary of the Invention] In order to achieve the above object, the present invention includes an amplifier to be measured, a variable voltage source, a voltage measuring circuit, and a procedure for measuring an offset voltage of the amplifier. And a control unit for controlling the voltage measuring circuit and performing data processing, and sequentially changing the set voltage of the voltage source at a constant value interval according to the measurement procedure and inputting it to the input terminal of the amplifier. The output voltage of the amplifier is measured by the voltage measurement circuit, and the control unit performs data processing based on the measured value to detect a pair of input voltage values before and after the output voltage of the amplifier reaches a predetermined value, and then perform the next measurement. The input voltage change range of the amplifier is set to a small value, and these operations are repeated for a plurality of cycles with a small change value of the set voltage. The input voltage value corresponding to the predetermined value is calculated by the control unit by, for example, the proportional distribution method, and the offset voltage of the amplifier is measured.

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

FIG. 1 is a block diagram showing a schematic configuration of an amplifier offset voltage measuring device according to the present invention. In the figure
11 is a measuring unit including an operational amplifier for measurement, 12 is a variable voltage source that supplies an input voltage to the measuring unit 11, 13 is a voltmeter, and 14 is a voltmeter.
Is a controller having a CPU, and 15 is a plotter. Each of these devices is connected to each other via a signal cable, for example, a GPIB cable.

The controller 14 stores, as a program, a measuring procedure for measuring the offset voltage of the operational amplifier provided in the measuring section 11, and the controller 14 stores the variable voltage source 12 and the voltmeter 13 in the program. Also, the operation of the plotter 15 is controlled. That is, the variable voltage source 12 is controlled by the controller 14 to set the output voltage value, and the voltage set here is supplied to the measuring unit 11 as an input voltage. The voltmeter 13 measures the output voltage from the measuring unit 11 and supplies the measured data to the controller 14. The controller 14 processes this measurement data, outputs it as graphic data to the plotter 15, and finally calculates the value of the input offset voltage.
The plotter 15 draws based on the graphic data from the controller 14 and records the input offset voltage value calculated by the controller 14 on the plot image.

FIG. 2 is a circuit diagram showing a detailed structure of the measuring unit 11 in the apparatus having the above structure. In the measuring section 11, an inverting input terminal 21 and a non-inverting input terminal 2 are provided as operational amplifiers for measurement.
2. A differential input type operational amplifier 26 having an output terminal 23 and positive and negative polarity power supply terminals 24 and 25 is provided. For example, a voltage Vcc of + 15V is applied to one power supply terminal 24 of the operational amplifier 26, and a voltage Vcc of -15V is applied to the other power supply terminal 25.
_EE is supplied respectively. A pair of resistors 28 and 29 are connected in series between the terminal 27 to which the output voltage Vin from the variable voltage source 12 is supplied and the ground potential, and the inverting input terminal 21 of the operational amplifier 26 is connected to the pair of resistors 28 and 29 in series. Connected to the serial connection point 30 of. The non-inverting input terminal 22 of the operational amplifier 26 is connected to the ground potential. An output resistor 31 is connected between the output terminal 23 of the operational amplifier 26 and the ground potential.

In general, the phenomenon that the polarity of the output voltage of the operational amplifier is reversed occurs in an extremely minute range of the input voltage, for example, in the range of several mV. Therefore, when measuring the input offset voltage, it is necessary to set the minimum unit of the input voltage to about 0.01 mV. However, since the output accuracy of the variable voltage source 12 is limited, the output voltage Vin from the variable voltage source 12 is divided by the pair of resistors 28 and 29 and supplied to the inverting input terminal 21, For example, the value of resistor 28 is set to 100 KΩ, the value of resistor 29 is set to 100 Ω, and the voltage Vin is 1/1000.
And is supplied to the inverting input terminal 21 as Vin '.

Next, a schematic operation will be described. First, the value of the input voltage Vin 'to the inverting input terminal 21 of the operational amplifier 26 is sequentially increased at relatively large intervals, and the output terminal 23 is increased each time.
The voltage Vout at is measured with a voltmeter 13. Then, the controller 14 detects the polarity reversal characteristic of the voltage Vout. next,
Within the range showing the polarity reversal characteristic, the value of the input voltage Vin ′ is sequentially increased at intervals smaller than the previous time, and the output terminal 23
The voltage Vout at is measured with a voltmeter 13. Then, the controller 14 again detects the polarity inversion characteristic of the voltage Vout.

In this way, the controller 14 detects in which range the inversion characteristic required to detect the offset voltage value is generated, and the measurement accuracy is increased step by step by one digit until the desired cycle is reached. Is repeatedly executed to detect the final input offset voltage in the measurement cycle with the desired measurement accuracy.

Next, detailed operation will be described. The measurement conditions were such that the change range of the input voltage Vin ′ was −15 mV to +15 mV, and the minimum input voltage interval (final measurement accuracy) was 0.01 mV.

First, the output voltage Vin of the variable voltage source 12 is set under the control of the controller 14, and the value is sequentially increased by 1V in the range of -15V to + 15V. At this time, since this voltage is attenuated to 1/1000 by the pair of resistors 28 and 29 in the measuring unit 11, the value of the voltage Vin ′ supplied to the inverting input terminal 21 of the operational amplifier 26 is from −15 mV to −15 mV. It increases by 1 mV step by step. Each time the voltage Vin 'changes, the voltmeter 13 measures the voltage Vout at the output terminal 23 of the operational amplifier 26, converts it into a digital signal, and sends it to the controller 14. The controller 14 performs data processing using this digital signal to generate graphic data and outputs it to the plotter 15. After that, the plotter 15 draws a graph as shown in FIG. 3, for example, based on the graphic data from the controller 14. In the case of the characteristics shown in FIG. 3, the output voltage Vout has the polarity reversed between the point A and the point B. Therefore, the controller 14 detects the input voltage at one point A exhibiting this polarity reversal characteristic, that is, 2 mV which is a value smaller by 1 mV than 3 mV, as the low voltage side of the measurement range in the next cycle, and at the other point B. 5 mV which is 1 mV greater than the input voltage of 4 mV
Is detected as the high voltage side of the measurement range in the next cycle.

Next, the value of the output voltage Vin of the variable voltage source 12 is set again, and the value of the voltage Vin ′ is changed from the low voltage side 2 mV of the measurement range detected in the previous cycle to the high voltage side 5 mV. Then, the values are sequentially increased at intervals of 0.1 mV, which is one digit smaller than the previous cycle, and the plotter 15 draws a graph as shown in FIG. 4 based on the voltage measured by the voltmeter 13 at this time. In the case of the characteristics shown in FIG. 4, the output voltage Vout is inverted in polarity between points C and D. Therefore, the controller 14 has an input voltage of 3.5 mV at one point C showing this polarity inversion characteristic.
3.4mV which is smaller than 0.1mV by 0.1mV is detected as the low voltage side of the measurement range in the next cycle, and 3.7mV which is larger by 0.1mV than the input voltage 3.6mV of the other point D.
Is detected as the high voltage side of the measurement range in the next cycle.

Further, the value of the output voltage Vin of the variable voltage source 12 is set again, and the value of the voltage Vin ′ is measured from the low voltage side of 3.4 mV detected in the previous cycle to the high voltage side of 3.7 mV. Then, it is sequentially increased at intervals of 0.01 mV which is one digit smaller than the previous cycle, and based on the voltage measured by the voltmeter 13 at this time,
The plotter 15 draws a graph as shown in FIG. In the case of the characteristics shown in FIG. 5, the output voltage Vout is inverted in polarity between the points E and F. So controller 14
Is the input voltage at one point E that shows this polarity inversion characteristic
3.55 mV and the input voltage 3.56 mV of the other point F are detected.

By the way, in the characteristic diagram shown in FIG. 5, the output voltage Vo
The input voltage Vin ′ at the point where ut crosses 0 V is the actual offset voltage, but it is almost impossible to obtain this offset voltage by actual measurement. Therefore, the controller 14 approximately obtains the offset voltage value from the pair of input voltages 3.55 mV and 3.56 mV obtained in the final measurement cycle as follows. That is, the offset voltage value becomes the input voltage Vin 'when Vout = 0 on the linear function line having a negative slope connecting the points E and F as shown in the characteristic diagram of FIG. Here, the triangle EFG and the triangle EHI are geometrically similar to each other. The distance Xcomp of the line segment HI, which is the base of the triangle EHI, is X on the coordinate axis in the input voltage Vin ′ direction.
When the axis and the coordinate axis in the output voltage Vout direction are the Y axis, the coordinates of the point E are (XE, YE), and the coordinates of the point F are (XF, YF), they are given by the following equations.

Xcomp = {XE | XF-XE |} / | YE-YF | ... 3 Xcomp given by the above three equations is a correction value necessary for obtaining the offset voltage value, and this value was obtained in the final measurement cycle. An accurate offset voltage value can be obtained by adding the pair of input voltages of 3.55 mV. The offset voltage value calculated in this manner is recorded on the graph image as described above by the plotter 15.

According to such a device, when the input offset voltage value is measured with a measurement accuracy of 0.01 mV in the range of ± 15 mV, for example, the first measurement cycle performs 30 measurements in 1 mV steps, and the next measurement cycle 30 measurements are performed in 0.1 mV steps, and 30 measurements are performed in 0.01 mV steps in the final cycle. That is, it is sufficient to measure the output voltage Vout 90 times in total. Therefore, the controller 14
The number of data processing performed in is significantly reduced compared to the conventional 3000 times. Therefore, the measurement time becomes much shorter than that of the conventional one, and the measurement efficiency can be improved.

Next, FIGS. 7 to 9 show characteristic diagrams (graphs) of the input voltage Vin 'and the output voltage Vout plotted by the plotter 15 when the input offset voltage of the operational amplifier is actually measured. As the operational amplifier, "TA7558P" manufactured by Toshiba Corporation was used. The characteristic diagram in Fig. 7 was obtained when the input voltage Vin 'was increased in 1mV steps in the range of -15mV to + 15mV.
The polarity of the output voltage Vout is inverted between mV and 1 mV.
Therefore, in this measurement cycle, the next measurement range is set within the range of -1 mV to +2 mV.

The characteristic diagram in Fig. 8 is obtained when the input voltage Vin 'is increased in the range of -1 mV to +2 mV in 0.1 mV step, and the input voltage Vin' is between +0.7 mV and +0.8 mV. The polarity of the output voltage Vout is inverted at. Therefore, in this measurement cycle, the next measurement range is set within +0.6 mV and +0.9 mV.

The characteristic diagram in Fig. 9 was obtained when the input voltage Vin 'was increased in the range of + 0.6mV and + 0.9mV in 0.01mV steps. The input voltage Vin' was + 0.71mV and + 0.72mV. The polarity of the output voltage Vout is inverted between and. Therefore, after this measurement cycle, the final offset voltage value (VIO) of 0.7144 mV is obtained by the arithmetic processing based on the proportional distribution method.
Recorded with the graph above FIG.

In order to automatically perform the above series of measurements, a program having the following procedure may be prepared in the controller 14. That is, the start point of applying the input voltage is Vs, the end point of applying the input voltage is Vp, and the change value of the set voltage of the input voltage is Vs.
If the start point (high voltage side) of the output voltage reversal is Xa and the end point (low voltage side) of the output voltage reversal is xb, Vs = -15mV, Vp = + 15mV, Vd =
Xa and Xb are measured at 1 mV. In the second measurement cycle, Vs and Vp are calculated based on the data of Xa and Xb measured in the first cycle, and Vd of the previous cycle is calculated.
Set to 1/10 and measure Xa and Xb again. This calculation formula is shown below.

Vs (second time) = Xa (first time) -Vd (first time) ... 4 Vp (second time) = Xb (first time) -Vd (first time) ... 5 Vd (second time) = Vd (first time) × 1/10 ... 6 In the third measurement cycle, Vs and Vp are calculated based on the data of Xa and Xb obtained in the second measurement cycle, and Vd is set to 1/10 of the previous cycle, Xa and again
Measure Xb. The arithmetic expression in this case is similar to the previous cycle as shown below.

Vs (third time) = Xa (second time) -Vd (second time) ... 7 Vp (third time) = Xb (second time) -Vd (second time) ... 8 Vd (third time) = Vd (second time) x 1/10 ... 9 Thus, the measurement conditions after the second cycle can be sequentially obtained by the calculation based on the program prepared in the controller 14. Here, FIGS. 7 to 9
The conditions in the case of the figure are shown collectively in FIG. It can be seen that the inversion characteristic is captured with high accuracy in this way.

FIG. 11 is a flow chart of a measurement program stored in the controller 14 in advance and for executing the above-mentioned calculation. First, in step A, a program necessary for executing the main program is initialized. Next, in step B, a character string variable common in the execution of the program,
Define constants, etc. Next, in step C, the measurement conditions that do not change when performing the measurement are set. That is, in this step C, the variable voltage source 12 in FIG. 1 is used in the control mode of the controller 14, and the power source voltages Vcc, V
Set _EE and earth potential. Then step D
To run the subprogram. Then, in the next step E, it is determined whether or not this subprogram has been executed three times (control of the number of times the subprogram is executed). If the subprogram has not been executed three times, the subprogram is repeatedly executed in step F until the condition is satisfied.

On the other hand, the subprogram first corrects the measurement conditions in step G. The correction of the measurement condition is, for example, setting of a constant and unit operation of a value required at the time of measurement or drawing. Next, the input voltage is changed in step H based on the corrected conditions, and the output voltage at each input voltage is measured 30 times to obtain data. Next, in step I, two pieces of data necessary for calculating the offset voltage are detected from the read data, and the detected data are substituted into an arithmetic expression for obtaining the offset voltage to calculate the offset voltage value.
In the next step J, the graph size is set, the vertical axis and horizontal axis are plotted, and the graph scale is added.
A characteristic diagram is drawn based on the 30 data obtained by the above measurement. Also, the offset voltage value obtained by the above calculation is recorded on the plot diagram. Then, in the next step, the execution of the sub program is ended and the process returns to the main program.

As described above, in the apparatus of the above-described embodiment, the range showing the inversion characteristic required for the detection of the offset voltage value is detected in the first measurement cycle, and the accuracy of the measurement is increased stepwise in the two and three measurement cycles to obtain the inversion characteristic. Since only the portion indicated by is expanded sequentially, the number of times of measuring the output voltage can be significantly reduced as compared with the conventional method. As a result, the time required for measurement is shortened.

It is needless to say that the present invention is not limited to the above-mentioned embodiment and various modifications can be made. For example, in the above embodiment, the case where the measurement cycle is repeated three times and the final measurement accuracy is set to 0.01 mV has been described. However, this is to increase the measurement cycle as necessary so as to perform the measurement with higher accuracy. You may Further, although the case where the proportional distribution method is used as the method of finally calculating the offset voltage has been described, this is the simplest of the proportional distribution method and can be sufficiently used as the measurement data. However, it goes without saying that a correction method of a higher order than this correction method may be used.

[Effects of the Invention] As described above, according to the present invention, the amplifier offset voltage can be measured with sufficient accuracy in the necessary measurement range, and the measurement time can be greatly shortened. An offset voltage measuring device can be provided.

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of an apparatus according to an embodiment of the present invention, FIG. 2 is a circuit diagram showing a detailed configuration of a part of the apparatus having the above configuration, and FIGS. 7 is a characteristic diagram for explaining the embodiment, FIGS. 7 to 9 are characteristic diagrams when the offset voltage is actually measured by the apparatus of the above embodiment, and FIG. 10 is a characteristic diagram when the offset voltage is shown in FIGS. FIG. 11 is a diagram collectively showing the measurement conditions, FIG. 11 is a flow chart of a program used in the apparatus of the above embodiment, and FIG. 12 is a characteristic diagram for explaining the input offset voltage. 11 …… Measuring section, 12 …… Variable voltage source, 13 …… Voltmeter, 14…
… Controller, 15… Plotter, 26… Operational amplifier.

Claims (1)

[Claims] 1. An amplifier to be measured, a variable voltage source, a voltage measuring circuit, and a procedure for measuring an offset voltage of the amplifier, and control and data of the voltage source and the voltage measuring circuit based on the measuring procedure. A control section for performing processing is provided, and the set voltage of the voltage source is sequentially changed at constant intervals according to the measurement procedure and input to the input terminal of the amplifier, and the output voltage of the amplifier at this time is measured by the voltage measurement circuit. The control unit performs data processing based on the measured value, detects a pair of input voltage values before and after the output voltage of the amplifier reaches a predetermined value, and sets them as the input voltage change range of the amplifier in the next measurement. The above operation is repeated to calculate the input voltage value corresponding to the above-mentioned predetermined value from the pair of input voltage values detected in the final cycle using the proportional distribution method in the control unit. An offset voltage measuring device for an amplifier, characterized in that