JPH0731218B2 - Resistance measuring device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a resistance measuring device using a double integrating circuit,
In particular, the present invention relates to a highly accurate resistance measuring device in which circuits can be integrated.

[Conventional technology]

Conventional resistance measuring devices measure resistance using double integration, as described in JP-A-54-137378 and JP-A-57-16367.

Japanese Unexamined Patent Publication No. 54-137378 discloses a device in which a constant current source (E, Rc) is connected in series to a resistance RX to be measured and the voltage VX across RX is measured to obtain a measured value. .

Further, Japanese Patent Laid-Open No. 16367/1982 discloses a device for performing double integration by switching the reference resistor 2 and the unknown resistor 4 with a switch 12.

[Problems to be Solved by the Invention]

However, in the conventional technique disclosed in JP-A-54-137378,
There were the following issues.

(1) Since it is difficult to build a constant current source having excellent temperature characteristics and high accuracy inside the integrated circuit, the constant current source is externally attached and the cost increases.

(2) The resistance range switching is not considered.

(3) Therefore, in the case of the low resistance measurement range, a large current must be passed through the resistance to be measured, a low current source with a large drive capacity is required, and the accuracy of the current source due to impedance is required, resulting in cost reduction. Get higher

Further, in the conventional technique disclosed in Japanese Patent Application Laid-Open No. 57-16367, (4) resistance range switching is not considered. Therefore, resistance cannot be measured over a wide range from low resistance to high resistance.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a resistance measuring device capable of measuring resistance over a wide range from low resistance to high resistance with high accuracy and integrating without overloading a power supply.

[Means for Solving the Problems]

The resistance measuring device of the present invention includes a reference resistance group in which a resistance circuit in which a reference resistance and a first range switch are connected in series are connected in parallel, and the reference resistance group and the resistance to be measured are connected to a reference power source. Of a reference resistance group corresponding to the measurement range of the resistance to be measured by turning on / off the first range switch; A resistance measuring device comprising: a double integrating circuit that positively integrates both-end voltage and inversely integrates both-end voltage of the reference resistor, and is connected to the first range switch-side terminal of the reference resistor, respectively. A second range switch that is turned on and off together with the first range switch, and is connected between the measured resistance and the double integration circuit,
A first switch that is turned on when performing positive integration and turned off when performing reverse integration, and is connected between the second range switch and the double integration circuit, is turned on when performing reverse integration, and is turned off when performing positive integration. A second switch; a first voltage follower connected between the first switch and the double integrating circuit; and a first voltage follower connected between the second switch and the double integrating circuit. It is characterized by having a voltage follower of 2.

[Action]

1 and 2 show a pulse width modulation type A / D conversion circuit (PWM conversion circuit), and the operation of the present invention will be described.

In FIG. 1, the PWM conversion circuit creates a pulse width T proportional to the voltage, measures the number of clocks between T, and performs A / D conversion. The pulse width T is expressed by T = K * VIN / EC (1). Here, K is a constant determined by the integrating resistors R1 and R2, VIN is an input voltage to the PWM conversion circuit, and EC is a reference voltage to the PWM conversion circuit.

FIG. 2 will be described.

Rin is the resistance to be measured, and RON and RON are switches 2
5 and 24 are ON current limiting resistors, and Rref1 and Rref2 are
It is a reference resistance.

Connect switch 25 and Rref1 in series to connect switch 24 and Rre
f2 is connected in series, Rref1 and switch 23 are connected, and Rref2 and switch 22 are connected to form a range changeover switch group.

Reference numerals 19, 20, 21, 14, 24, 25, 22, 23, 5 and 6 are switches.

Reference numerals 7, 12, 15, 16, 17, and 18 are operational amplifiers. The operational amplifiers 15 to 17 are voltage followers for impedance conversion, the operational amplifiers 12 and 18 are inverting amplifier circuits by external resistors, and the operational amplifier 7 is It is used as an integrating circuit. Reference numeral 8 is a comparator, reference numeral 10 is a counter, and reference numeral 9 is a display circuit.

The measured resistance Rin, the switch 25 or 24, the current limiting resistances RON and RON, and the reference resistance Rref1 or Rref2 are connected in series between the reference voltage Vref and GND. The reference resistors Rref1, 2 connected in series with the measured resistance Rin are switched by the switches 25, 24 according to the resistance of the measured resistance Rin.

The reference voltage Vref side terminal of the measured resistance Rin is input to the voltage follower 17 as the voltage V1 via the switch 19. The voltage V1 is input to the integrating operational amplifier 7 via the integrating resistor R1.

When the switch 20 is OFF and the switches 25 or 24 and 19 are ON, the potential V1 with respect to GND is represented by V1 = Vref.

The other end of the measured resistance Rin is input to the voltage follower 16 as the voltage V2 via the switch 21. The voltage V2 is input to the integrating operational amplifier 7 via the inverting amplifier operational amplifier 18 and the integrating resistor R1 ′.

Switch 14 OFF, Switch 21 ON, Switch 25 or 2
When 4 is ON, the potential V2 with respect to GND is V2 = Vref * (RON + Rref) / (Rin + RON + Rref) ....
It is expressed as (2).

When R1 = R1 'and R0 = R0', the input voltage VIN of the PWM conversion circuit is VIN = V1-V2 = Vref * Rin / (Rin + RON + Rref) ....
It can be expressed as (3) and becomes equal to the voltage across the measured resistance Rin.

One end of the reference resistor Rref1 is connected to the GND terminal and the other end is
The voltage V3 is input to the voltage follower 15 via the switch 23 while being connected to the limiting resistor RON.

When switching the range, another reference resistor Rref2
Is connected to the GND terminal at one end like the reference resistor Rref1,
The other end is connected to the limiting resistor RON and is also input as a voltage V3 to the voltage follower 15 via the switch 22.

The voltage V3 is input to the integrating operational amplifier 7 via the switch 5 and the integrating resistor R2, and is also input to the integrating operational amplifier 7 via the iterative amplifier circuit operational amplifier 12, the switch 6, and the integrating resistor R2.

The reference voltage E C of the PWM conversion circuit can be expressed as E C = Vref * Rref / (Rin + RON + Rref) (4) and is equal to the voltage across the reference resistor Rref.

In the embodiment of the present invention, the reference resistance Rref is switched according to the resistance range of the resistance to be measured so that EC becomes 1/3 of Vref.

From equations (1), (3) and (4), T = K * VIN / EC = K * Rin / Rref (5)

Therefore, even if the range is changed, the range selector switch
It does not depend on the ON resistance and current limiting resistance.

The offset voltage of the operational amplifier can be canceled by opening / closing the switches 19, 20 and the switches 21, 14 using an auto-zero circuit.

The advantage of the embodiment using PWM is that it is not affected by the temperature drift of the reference voltage Vref and the voltage value of Vref. Further, Vref may be either positive or negative, and can take any value as long as it does not exceed the dynamic range of the operational amplifier. The switches 19, 20, 21, and 14 enable integrated circuits for range switching that are not affected by the ON resistance of internal analog switches, temperature drift, or resistance to static electricity.

Furthermore, the value of the limiting resistance RON is also arbitrary, but Rref, Rin
In the low resistance range where is small, the current flowing from Vref to GND is limited.

〔Example〕

FIG. 3 shows a resistance measuring device using the double integration type A / D conversion circuit of the present invention.

Like the conventional resistance measuring device using the double integration type A / D conversion circuit, the voltage VIN across the measured resistance is positively integrated for a certain period of time Tref, and then the inverse voltage EC across the reference resistance is reversed. The polarity voltage -EC is integrated. (EC is inversely integrated). By counting the number of clocks of the inverse integration time T, A / D
Can be converted.

The equation is expressed by T = VIN / EC * Tref.

By turning on the auto-zero analog switches 32 and 34, the offset voltage of each operational amplifier is canceled.

The voltage Vin across the resistance Rin to be measured is input to the integrating operational amplifier 48 as described in the section of action, and integrated as VIN = Vref * Rin / (Rin + RON + Rref) during the first integration period (positive integration period).

The voltage EC across the reference resistor Rref is the switch 37 or 38,
A switch 39, a voltage follower 45, an operational amplifier 46 for an inverting amplifier circuit, and an operational amplifier 48 for integration through an integrating resistor R1 '.
And the reverse voltage is integrated as -EC = -Vref * Rref / (Rin + RON + Rref) during the second integration period (inverse integration period).

Therefore, T = Rin / Rref * Tref, and since the reference resistance Rref and the reference time Tref are known, the measured resistance Rin can be measured as a function of the inverse integration time T.

Here, when the positive integration is performed, the switches 31 and 33 are turned on, and the switch 39
Is OFF, switch 39 is ON at the time of inverse integration, switches 31, 33
Is turned off, the voltage across the resistor Rin to be measured is input to the double integrator circuit during the positive integration, and the reverse polarity voltage across the reference resistor is input during the reverse integration. Also, voltage followers 44,4
5 has a high input impedance and a low output impedance, and outputs the input voltage value to the double integrator circuit. Therefore, almost no current flows through the switches 31 and 33 that are turned on during the positive integration, the current is negligible, and the voltage itself across the resistance to be measured can be input to the double integration circuit. Also, switch 3 that turns on during reverse integration
9, almost no current flows to switch 37 or 38,
The current is negligible and the voltage across the reference resistor itself can be input to the double integrator circuit.

Therefore, the voltage drop between the range changeover switch 35 and the current limiting resistor RON due to the current flowing through the circuit in which the reference resistor Rref1-range change switch 35-current limiting resistor RON-measured resistor Rin is connected in series between the reference power source and GND Is not output to the double integrator circuit by the voltage followers 44, 45 and the switches 31, 33, 39.

〔The invention's effect〕

The resistance measuring device of the present invention includes a reference resistance group formed by connecting in series a resistance circuit in which a reference resistance and a first range switch are connected in series, a reference resistance group and a resistance to be measured, and a reference power source. And a ground connected in series, and a measurement circuit in which one of the reference resistance groups corresponding to the measurement range of the resistance to be measured is selected by turning on and off the first range switch. Between the second range switch connected to the first range switch side terminal of the reference resistor and turned on and off together with the first range switch, and between the measured resistance and the double integration circuit. Connected to the
It is connected between a first switch that is turned on when performing positive integration and turned off when performing reverse integration, and is connected between the second range switch and the double integration circuit, turned on when performing reverse integration, and turned on when performing positive integration. A second switch; a first voltage follower connected between the first switch and the double integration circuit; and a first voltage follower connected between the second switch and the double integration circuit. Since the voltage follower of 2 is provided, it is possible to measure only the voltage across the resistance to be measured and the reference resistance without being affected by the on resistance of the range switch of the resistance circuit. This has an effect that the resistance can be measured with high accuracy over a wide range.

[Brief description of drawings]

FIG. 1 is a block diagram of a PWM conversion type resistance measuring device of the present invention. FIG. 2 is a circuit diagram of the PWM conversion type resistance measuring device of the present invention. FIG. 3 is a circuit diagram of the double integral resistance measuring device of the present invention. 15,16,17,44,45 …… Voltage follower 46 …… Inverting amplification operational amplifier 7,48 …… Integration operational amplifier 8,50 …… Comparator 10,51 …… Counter 9,52 …… Display circuit Rin …… Measured resistance RON …… Current limiting resistance Rref1,2 …… Reference resistance R1 …… Integration resistance C1 …… Integration capacitor R0 …… Inverting amplification resistance 31, 32, 33, 34, 35, 36, 37, 38 , 39 …… switch

Claims (1)

[Claims] 1. A reference resistance group in which a resistance circuit in which a reference resistance and a first range switch are connected in series are connected in parallel, the reference resistance group and a resistance to be measured, a reference power source and a ground. A measurement circuit which is connected in series between, and which selects one of the reference resistance groups corresponding to the measurement range of the resistance to be measured by turning the first range switch on and off, and a voltage across the resistance to be measured. A double integrator circuit that positively integrates and inversely integrates the voltage across the reference resistor, wherein the resistance measuring device is connected to the first range switch side terminal of the reference resistor, respectively. A second range switch that turns on and off together with a range switch, and is connected between the measured resistance and the double integration circuit,
It is connected between a first switch that is turned on when performing positive integration and turned off when performing reverse integration, and is connected between the second range switch and the double integration circuit, turned on when performing reverse integration, and turned on when performing positive integration. A second switch; a first voltage follower connected between the first switch and the double integration circuit; and a first voltage follower connected between the second switch and the double integration circuit. 2. A resistance measuring device comprising: a voltage follower of 2.