JPH05196654A - measuring device - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a measuring device in which the bias current and the input impedance of the input amplifier do not cause a measurement error even when an input amplifier having a common performance is used. [Structure] An input amplifier having _a bias current I _a and an input impedance R _a at an input terminal, an AD converter for converting an output signal of the input amplifier into a digital signal, and one end having a different known resistance value Two resistors (R, r) connected to a common potential, a resistor (R _P ), one end of which is connected to the measurement terminal, and the other end of the three resistors are selected to input the input terminals of the input amplifier. Switch means for leading to
(SW _a ), a series circuit of a constant current source (I _S ) and a second switch means (SW _S ) connected between the measurement terminal (17) and the common potential, and A
A μP for introducing the output signal of the D converter and controlling the switching of the two switch means to calculate the value of the internal voltage V _O of the signal generating source.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a measuring device for obtaining a signal according to the magnitude of an introduced analog signal. Generally, the measuring apparatus is provided with an input buffer amplifier, but the input impedance of this input buffer amplifier and the bias current give an error to the measured value. This application is
The present invention mainly relates to a measuring device capable of reducing the influence of the error factors.

[0002]

2. Description of the Related Art There is a measuring apparatus which introduces an analog signal such as voltage or current from a signal source to be measured and converts it into a digital signal for measurement. In such a measuring device, a buffer amplifier configured by using an operational amplifier or the like is usually used for the input section. An ideal buffer amplifier, the input impedance R _a is infinite, the bias current I _a of the input is zero. However, since each actual amplifier has a certain finite value, an error occurs in the measured value.

For example, when the buffer amplifier has a bias current I _a of a certain value, this bias current I _a flows to the signal generation source of the measurement object connected to the measurement terminal. Since the signal generation source has the output impedance R _s , the voltage of V _bias = I _a · R _s (1) is generated at the output terminal of the signal generation source when this bias current flows. Since the voltage V _bias of the equation (1) thus generated is superimposed on the original output voltage V _i of the signal generation source and added to the measuring device,
It causes a measurement error.

An input protection circuit is usually provided before the buffer amplifier in order to prevent the input circuit section of the measuring device from being damaged by an excessive input due to an erroneous connection or the like. An ideal input protection circuit has a parallel impedance of infinity and a series impedance of 0 when looking into the measuring device from between the measuring terminals. However, actual input protection circuits each have a finite value. As a result, a measurement error occurs.

FIG. 2 is a block diagram showing an input section of a conventional measuring apparatus. In the figure, reference numeral 7 denotes an input buffer amplifier (hereinafter, simply referred to as an input amplifier), which includes an ideal amplifier 9, bias current sources I ₊ and I ₋ connected to the outside of the ideal amplifier 9, and an input impedance R _+. , R ₋ are equivalently represented. The ideal amplifier 9 has infinite input impedance and zero bias current.

Reference numeral 5 is an input protection circuit, and its equivalent series impedance is represented by R _P. In such a configuration, the current I _i separated from the bias current I ₊ flows through the impedance R _P of the input protection circuit 5. Therefore, the voltage V _A represented by the equation (2) is applied to the (+) input terminal of the ideal amplifier 9 in FIG. R _S is the output impedance of the measurement target 1. V _A = (R _P + R _S ) · I _i (2) This voltage V _A is generated due to the bias current I ₊ , and is superimposed on the voltage V _O of the measuring object 1 to be measured by the measuring device. Since it is added, it causes a measurement error.

Such an error can be reduced by connecting a resistor R _P 'having a value as shown in equation (3) to the (-) input terminal. R _P '= R _P + R _S (3) That is, a current I _i separated from the bias current I ₋ also flows through the resistance R _P ' of the (-) input terminal (I ₊ = I in a normal operational amplifier). _- a). Therefore, at this time, if the voltage generated in the resistor R _P ′ is V _B , then V _B = R _P ′ · I _i (4) Since V _A = V _B from the relationship of the equation (3), the voltage Vd between the (+) and (−) input terminals of the ideal amplifier 9 is V _d = V _A.
−V _B = 0, and the influence of the bias currents I ₊ and I ₋ is
To be removed.

However, since the output impedance R _S of the measuring object 1 is different for each measuring object, the canceling resistance value R _P 'in the relation of the equation (3) is set at the design stage of the measuring device. It is impossible to determine. Therefore, in the conventional measuring device, a resistor R _P 'where R _P ' = R _P is provided for convenience. That is, in the conventional device,
The error caused by the output impedance R _S of the measurement target 1 was not canceled.

On the other hand, in the field of process control and the like, when the temperature of the field is measured by a thermocouple,
Generally, the thermocouple and the measuring device are connected with a long cable,
In addition, a lightning arrester is provided. In such a case, depending on the length of the cable, the presence or absence of a lightning arrester, etc., the output impedance R as seen from the measurement device 1 side (thermocouple side)
_S (see FIG. 2) has a value of about 0 to 500Ω, and the value fluctuates depending on factors such as temperature.

Since the electromotive force of the thermocouple is about several tens of mV, the current I _i caused by the bias current I ₊ (see FIG. 2).
Depending on the value of, the voltage drop across the resistor R _S becomes a problem as a measurement error. Therefore, in the case of performing temperature measurement using a thermocouple (or introducing a minute detection signal, not limited to the thermocouple), in the field of process control and the like, the bias current is small and the input impedance is high and expensive. By using an amplifier as the input amplifier 7, the absolute value of the measurement error was suppressed.

[0011]

On the other hand, in recent years, in the field of measuring instruments and process control, the electronic circuit portion of the device has been integrated into an IC. In other words, a certain electronic circuit part that was conventionally assembled discretely is integrated into one IC.
It has been attempted to improve the performance of the device, downsize it, and reduce the price. Thus, there is a big problem when the measuring device of FIG. 2 is viewed from the viewpoint of IC.

That is, as the input amplifier 7, an amplifier having a small bias current and a high input impedance is used as an IC.
In order to realize it, a special IC process or a trimming process is required for its manufacture. That is, the conventional circuit of FIG. 2 (that is, the circuit that requires an amplifier having a small bias current and a high input impedance as the input amplifier 7) has a configuration unsuitable for an IC.

An object of the present invention is to provide a measuring device in which the bias current and the input impedance of the input amplifier do not cause a measurement error even if an input amplifier having a common performance is used.

[0014]

SUMMARY OF THE INVENTION The present invention is a device for measuring an internal voltage V _O of a signal source having an output impedance R _S connected to a measurement terminal, and a bias at an input terminal for introducing a signal to be measured. An input amplifier having _a current of I _a and an input impedance of R _a , an AD converter for converting an output signal of the input amplifier into a digital signal,
Of the two resistors (R, r) having different known resistance values, one end of which is connected to a common potential, the one end of which is connected to the measurement terminal (R _P ), and the three resistors of which are The other end is selected and the current is passed through the first switch means (SW _a ) and the second switch means (SW _S ) which lead to the input terminal of the input amplifier.
(I _S ) is superposed on the signal to be measured and a constant current source (12) for changing the current at the measurement terminal (17) and an output signal of the AD converter are introduced and switching between the two switch means is performed. Is controlled by the first switch means to select the two resistors (R, r) respectively from the output of the AD converter to the bias current I _{a of the} input amplifier and the input impedance R _a.
And the respective output values of the AD converter when the resistance (R _P ) is selected by the first switch means and the second switch means is turned on and off, and the calculated values of I _a and R _a . And a microprocessor for calculating the value of the internal voltage V _O of the signal generation source from the microprocessor.

[0015]

When the two resistors (R, r) are selected by the first switch means, the respective outputs (AD _R and A) of the AD converter are selected.
D _r ) has a relationship of AD _R = I _a · (R // R _a ) AD _r = I _a · (r // R _a ). Therefore, the bias current I _a and the input impedance R _a of the input amplifier can be calculated from these two equations.

Output value of the AD converter when the resistance (R _P ) is selected by the first switch means and the second switch means is turned off
AD _O has a relationship of V _io = AD _O · (R _a + R _P ) / R _a −I _a · (R _P // R _a ) I _io = (V _iO −AD _O ) / R _P. V _io is the voltage at the measurement terminal in this case, and I _io is the current flowing at the measurement terminal. Therefore, using the calculated values of I _a and R _a and the measured value AD _O , V _{i o} ,
I _io can be calculated.

[0017] Select resistance (R _P) in the first switching means and the output value AD _S of the AD converter when to turn on the second switching _{means, V iS = AD S · (} R a + R P ) / R _a −I _a · (R _P // R _a ) I _iS = (V _is −AD _S ) / R _P −I _S. V _iS is the voltage at the measurement terminal in this case, and I _iS is the current flowing through the measurement terminal. Therefore, using the calculated values of I _a and R _a and the measured value AD _O , V _{i S} and
I _iS can be calculated.

On the other hand, the internal voltage V _O of the signal generation source has a relationship of V _O = (V _iS · I _io −V _io · I _iS ) / (I _io −I _iS ). The value of the internal voltage V _O of the signal generation source can be calculated from the calculated values of V _io , I _io , V _iS , and I _iS .

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing an example of the essential structure of a measuring apparatus according to the present invention. In the figure, reference numeral 20 denotes a measuring device of the present invention, which includes measuring terminals 17 and 19. This measuring device 20
As described below, the internal voltage V _O of the signal generation source 21 having the output impedance R _S connected to the measurement terminals 17 and 19 is influenced by the bias current I _a of the input amplifier 11 and the input impedance R _a. Can be measured without Reference numeral 21 denotes a signal generation source having an output impedance R _S and an internal voltage V _O. An example of such a signal generation source is a thermocouple or the like.

Reference numeral 11 is an input amplifier. In the example of FIG.
The signal to be measured is led to the (+) input terminal. on the other hand,(-)
The input terminal is directly connected to the common potential. It should be noted that the present invention can be realized even if the (-) input terminal is connected to the common potential via a certain resistor, but in order to explain the invention easily, the (-) input terminal is directly connected to the common potential. A connection example will be described. Further, contrary to FIG. 1, the present invention is also realized by introducing a signal to be measured to the (−) input terminal and connecting the (+) input terminal to a common potential. Further, the illustration of the feedback circuit provided in the input amplifier 11 is omitted.

Assuming that the bias current at the (+) input terminal that introduces the signal to be measured is I _a and the input impedance is R _a , in the present invention, these values of I _a and R _a are input amplifiers of ordinary numerical values. Even if there is, it is not affected by the measured value. In addition, in FIG. 1, since the (−) input terminal is directly connected to the common potential, the input impedance R ₋ of the (−) input terminal described in FIG. 2 is short-circuited. Therefore, in FIG. 1, the illustration of R ₋ is omitted. Further, the bias current I _{− at} the (−) input terminal does not induce any voltage because the input impedance R ₋ is short-circuited. Therefore, in FIG. 1, the illustration of the bias current I ₋ at the (−) input terminal is also omitted. Reference numeral 13 is an ideal amplifier, which is the same as that already described in FIG.

Reference numeral 10 is an AD converter for converting the analog output signal of the input amplifier 11 into a digital signal. R and r are resistors having different values, and their values are known. One end is connected to the common potential, and the other end is connected to the switching contact of the switch SW _a . In this case, r << R is set. Resistance R
_P is, for example, the equivalent series impedance of the input protection circuit (not shown in FIG. 1) described in FIG. 2, one end of which is connected to the measurement terminal 17 and the other end of which is connected to the switching contact of the switch SW _a. ing. The fixed contact of the switch SW _a is connected to the (+) input terminal of the input amplifier 11.

Reference numeral 12 is a constant current source for outputting a constant current I _S , one end of which is connected to a common potential. The other end of the constant current source 12 is connected to the measurement terminal 17 via the switch SW _S. The measurement terminal 19 is connected to the common potential. Reference numeral 15 is a microprocessor (hereinafter, simply referred to as μP15), which introduces the output signal of the AD converter 10 and controls switching between the two switches SW _a and SW _S. Then, the calculation to be described later is performed to calculate the value of the internal voltage V _O of the signal generation source 21.

The operation of the apparatus of FIG. 1 configured as above will be described.

<A> The μP 15 controls the switch SW _a to connect the resistor R to the input terminal of the input amplifier 11. As a result, the (+) input terminal of the input amplifier 11, the parallel resistance of the resistor R and the input impedance R _a (R // R _a) will be connected. Therefore, _a bias current I _{a is} applied to this parallel resistor.
, The voltage represented by the right side of equation (5) is output from the input amplifier 11, and the output of the AD converter 10 at this time is AD _R
Then, AD _R = I _a · (R // R _a ) (5)

<B> The μP 15 controls the switch SW _a to connect the resistor r to the input terminal of the input amplifier 11. As a result, the parallel resistance (r // R _a ) of the resistance r and the input impedance R _a is connected to the (+) input terminal of the input amplifier 11. Therefore, when the voltage represented by the right side of the equation (6) is output from the input amplifier 11 and the output of the AD converter 10 at this time is AD _r , AD _r = I _a · (r // R _a ) ( It is represented by 6).

<C> As described above, since there is a relation between the equations (5) and (6), the μP15 is calculated from the equations (7) and (8) by the bias current I _a of the input amplifier and the input impedance R _a. To calculate. R _a = (AD _R −AD _r ) · R · r / (R · AD _r −r · AD _R ) (7) I _a = (Rr) · AD _R · AD _r / (AD _R −AD _r ) · R ・ r (8)

<D> μP15 controls the switch SW _a to connect the resistor R _P to the input terminal of the input amplifier 11. That is, the signal from the signal generation source 21 is guided to the input amplifier 11.
And μP15 controls the switch SW _S to be off. When the output value of the AD converter 10 at this time is AD _O , the signal generation source 21
The current flowing out of the input amplifier and flowing into the (+) input terminal of the input amplifier is represented by AD _O / R _a . Therefore, the voltage V _{io at} the measurement terminal 17 in this case is calculated from the voltage AD _O · (R _a + R _P ) / R _a generated by this current AD _O / R _a to the voltage I _a · generated by the bias current I _a. By subtracting (R _P // R _a ), it is expressed by equation (9). V _io = AD _O · (R _a + R _P ) / R _a −I _a · (R _P // R _a ) (9) The current I _io flowing through the measurement input terminal 17 is expressed by the equation (10). It I _io = (V _iO −A D _O ) / R _P (10) V _io and I _{io in the} equations (10) are the R _a and I _a calculated by the equations (7) and (8). Value and the measured value AD _O obtained here
It can be calculated from P15 (the value of R _P is known).

<E> The μP 15 controls the switch SW _a to connect the resistor R _P to the input terminal of the input amplifier 11, and guides the signal from the signal generation source 21 to the input amplifier 11. And μP15
Controls the switch SW _S to be ON, and newly adds the current I _S from the constant current source 12. That is, for the case of <D>,
The current at the measurement input terminal 17 is forcibly changed. Assuming that the output value of the AD converter 10 at this time is AD _S , the current flowing out from the signal generation source 21 and the constant current source 12 and flowing into the (+) input terminal of the input amplifier is represented by AD _S / R _a. .. Therefore,
In this case, the voltage V _{iS at} the measuring terminal 17 is the current AD _S / R _a
By subtracting the voltage I _a · (R _P // R _a ) generated by the bias current I _a from the voltage AD _S · (R _a + R _P ) / R _a generated by It V _iS = AD _S · (R _a + R _P ) / R _a −I _a · (R _P // R _a ) (11) Further, the current I _iS flowing through the measurement input terminal 17 is expressed by the equation (11). It _{I iS = (V is -AD S} ) / R P -I S (12) (11), (12) formula V _iS and I _iS is, R _a calculated in (7) and (8), Using the value of I _a and the measured value AD _S obtained here, μ
It can be calculated from P15 (the value of R _P is known).

As already described in the section <E>, the function of the constant current source 12 is to change the current value at the measuring terminal 17. Therefore, the present invention can be realized even if a constant current I _S is applied to the line between the resistor R _P and the input amplifier 11 as shown in FIG. 3 differs from FIG. 1 only in the position where the constant current source 12 is added. In FIG. 3, the input amplifier 11
Illustration of the subsequent configuration is omitted.

<F> Here, the internal voltage V _O of the signal generation source 21 and the output impedance R _S have the following relationship. V _O = R _S · I _io + V _io (13) V _O = R _S · I _iS + V _iS (14) Therefore, from the equations (13) and (14), the internal voltage V _O of the signal source 21 and the output impedance R _S is represented by the following equation. V _O = (V _iS · I _io −V _io · I _iS ) / (I _io −I _iS ) (15) _RS = (V _iS −V _io ) / (I _io −I _iS ) (16) Therefore, At μP15, the calculated V _io , I _io , V _iS ,
The value of the internal voltage V _O of the signal source and the output impedance R _S can be calculated from the value of I _iS .

The voltage V _{O to} be measured represented by the equation (15) is
It is calculated based on the actual measurement value, and
Output impedance R _S , the input protection circuit series impedance R _P , the input amplifier 11 bias current I _a , and the input amplifier 11 input impedance R _a . If necessary, the operations <A> to <F> may be repeated and the calculation may be performed based on the latest data.

Although the (-) input terminal is directly connected to the common potential in FIG. 1, the present invention can be realized even if it is connected to the common potential via the resistor RB having a certain value. The reason will be explained. In this case, the bias current I ₋ at the (−) input terminal flows through the resistor RB, and the offset voltage V _OFF = RB ·
I _- is generated. This offset voltage V _OFF is superimposed on the signal applied to the (+) input terminal, but since it is a DC value, it can be easily canceled. For example, measured in advance the offset voltage V _OFF, by the method and μP15 adding negative V _{O FF} to the input amplifier 11, may be subtracted by equivalent of the offset voltage V _OFF.

Also, the above-mentioned <D> AD _O and <E> A
Focusing on the difference in D _S in μP15, if this difference is large, it is determined that the connection between the signal source 21 and the measuring device 20 is abnormal, and μP15
You can also give an alarm from.

[0035]

As described above, according to the present invention, the voltage V _O to be measured is calculated based on the measured value, and the output impedance R _S of the signal source 21 and the input protection circuit Series impedance R _P , input amplifier 11 bias current I _a , and input amplifier 11 input impedance R _a
Not affected by. And, if necessary, the above <A>
By repeating the operation of <F>, it is possible to perform the calculation based on the latest data, and it is possible to reduce the influence of temperature change, aging change, and the like. Therefore, even if an input amplifier having a common performance is used, the bias current and the input impedance of the input amplifier do not cause a measurement error and the IC can be easily formed.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration example of a measuring device according to the present invention.

FIG. 2 is a diagram showing a configuration example of a conventional measuring device.

FIG. 3 is a diagram showing another configuration example of the measuring apparatus according to the present invention.

[Explanation of symbols] 10 AD converter 11 Input amplifier 12 Constant current source 15 Microprocessor 17, 19 Measuring terminal 20 Measuring device SW _a , SW _S switch

Claims (2)

[Claims] 1. An apparatus for measuring an internal voltage V _O of a signal source having an output impedance R _S connected to a measurement terminal, wherein a bias current I _a and an input impedance at an input terminal for introducing a signal to be measured are An input amplifier that is R _a , and an A that converts the output signal of this input amplifier into a digital signal
A D converter, two resistors (R, r) having different known resistance values, one ends of which are connected to a common potential, and a resistance (R _P ) of which one end is connected to the measurement terminal (17). And a current (I _S ) via the first switch means (SW _a ) and the second switch means (SW _S ) that select the other ends of the three resistors and lead to the input terminal of the input amplifier. Is superimposed on the signal to be measured, and a constant current source (12) for changing the current at the measurement terminal (17) and an output signal of the AD converter are introduced and switching between the two switch means is controlled, The bias current I _a and the input impedance R _a of the input amplifier are calculated from the output of the AD converter when the two resistors (R, r) are selected by the first switch means, and the resistance (R R _P )
And the respective output values of the AD converter when the second switch means is turned on and off and the calculated I _a and R _a
And a microprocessor that calculates the value of the internal voltage V _O of the signal generation source from the value of 2. The microprocessor introduces a digital signal from the AD converter and controls switching of the two switch means, and the first switch means selects two resistors (R, r) respectively. When the bias current I _a and the input impedance R _a of the input amplifier are calculated from the output of the AD converter at the time, the resistance (R _P ) is selected by the first switch means, and the second switch means is turned off. From the output of the AD converter and the calculated values of I _a and R _a , the voltage V _io and the current I _io from the signal source applied to the measurement terminal are calculated, and the resistance (R _P ) Is selected and the output of the AD converter when the second switch means is turned on and the calculated values of I _a and R _a are the voltage V _iS from the signal source applied to the measurement terminal. The current I _iS is calculated, and the calculated V _io , I _io and V _iS are calculated. 2. The measuring apparatus according to claim 1, which has a function of calculating the internal voltage V _O of the signal generation source from the values of I _iS and I _iS .