JP3158862B2 - RTD circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resistance temperature detector circuit, and more particularly to a resistance temperature detector circuit suitable for signal processing of different types of resistance temperature detectors.

[0002]

2. Description of the Related Art FIG. 2 is a circuit diagram showing a main part of a conventional resistance temperature detector circuit. In FIG. 2, R _t has three terminals A, B,
b, a terminal A is connected to a constant current source CS for applying a reference current I, and a terminal b is connected to a reference resistor R _C.
To the common potential point. OP _{1 to} OP ₃ are operational amplifiers. The non-inverting input terminal of the operational amplifier OP ₁ is connected to terminals A RTD R _t is, to the inverting input terminal resistance with the terminal b of the RTD R _t through the resistor R ₁ is connected output terminal of the operational amplifier OP ₁ through R ₃ are connected. The non-inverting input terminal of the operational amplifier OP ₂ is connected the terminal b of the RTD R _t is, to the inverting input terminal resistance with the terminal b of the RTD R _t through the resistor R ₂ is connected output terminal of the operational amplifier OP ₂ through R ₄ are connected. The non-inverting input terminal of the operational amplifier OP ₃ is connected the terminal B of the RTD R _t, the resistance together with the output terminal of the operational amplifier OP ₁ is connected via a resistor R ₅ to the inverting input terminal R ₆ Via the operational amplifier O
Output terminals of the P ₂ are connected, and further through the resistor R ₇ connected to the output terminal of the operational amplifier OP _3. The output terminal E _out is connected to the output terminal of the operational amplifier OP _3.

In such a configuration, R ₁ = R ₂ = R _a
By obtaining a condition that can eliminate the influence of the resistance value r as R ₃ = R ₄ = R _b , R _b = R _a + 2R _C. For example, if R _C = 100Ω, then R ₁ = R ₁
= R ₂ = 9.8 kΩ and R ₃ = R ₄ = R ₅ = R ₆ = 10 kΩ. Note that R ₇ = 5 kΩ. In this case, the output voltage E _out becomes E _out = I (R _t −R _c ), and the influence of the resistance value r of the lead wire of the resistance bulb R _t can be eliminated.

[0004] Platinum (Pt) based or copper (Cu) system is used in accordance with the temperature measuring range as RTD R _t. That is, for example, when measuring the temperature at -200 ° C. to 550 ° C., a platinum-based material is used, and a current I of, for example, 1 mA is applied. In this case, the resistance value of the resistance temperature detector R _t is the change in 17Omu～300omu, the output voltage E _out -
It changes between 83 mV and 200 mV. On the other hand, for example, -2
When measuring the temperature from 00 ° C. to 300 ° C., a copper-based material is used and a current I of, for example, 2 mA is applied. Resistance value in this case, the temperature measuring resistor R _t is 1Ω~20
Ω, and the output voltage E _out is −198 mV to −16.
It changes by 0 mV. FIG. 3 is a graph of these relationships.

[0005]

Here, assuming that the span of the measurement range is, for example, ± 200 mV, the measurement output voltage at 550 ° C. becomes +200 mV in the high-temperature range in the case of platinum, so the upper limit of the measurement range is +200 mV. Is practically zero, but it is considered that the frequency of such high-temperature measurement is practically small and practically acceptable.

However, in the circuit configuration shown in FIG.
_When a copper-based material is used as _t , -200 ° C to 300 °
The measured output voltage at 0 ° C. near the center of the temperature measurement range of C is about −180 mV, and the lower limit of the measurement range is −20.
The noise margin for 0 mV is only -20 mV. As a result, a measurement error may occur when the measured output voltage is converted into a digital signal by being applied to the integrating A / D converter. That is, when sine wave noise having a magnitude exceeding the noise margin is input together with the measurement output voltage E _out as shown in FIG. 4, for example, the portion (S ₂ ) of the sine wave noise exceeding the noise margin in the negative polarity portion (S ₂ ) S ₃₎
Is a measurement error. If the sine wave noise does not exceed the noise margin, the plus polarity portion (S ₁ ) and the minus polarity portion (S ₂ ) of the sine wave noise cancel each other out and no measurement error occurs.

[0007] The present invention has been made to solve the above problems, and an object, with sufficient noise margin even in the case of using a copper-based as RTD R _t can be secured, measured It is an object of the present invention to realize a temperature measuring resistor circuit capable of performing highly accurate temperature measurement without being affected by the resistance value of the lead wire of the temperature resistor.

[0008]

Means for Solving the Problems The present invention, in order to solve this problem, three terminals A, B, and RTD R _t with b, terminal A of the RTD R _t , A reference resistor R _C having a known resistance value connected between the terminal b of the resistance thermometer R _t and the common potential point, and an output voltage of the terminal A of the resistance thermometer R _t. a first operational amplifier which calculates and outputs the difference between the output voltage of the fourth operational amplifier, calculates outputs a difference between output voltages of the fourth operational amplifier terminals b of RTD R _t a second operational amplifier, the output voltage and a third for calculating outputting a difference between the output voltage of the terminal B of the second sum and the temperature measuring resistor R _t of the output voltage of the operational amplifier of the first operational amplifier operational amplifier and a first switch connected to a connection point between the terminal b of the reference resistor R _C and the temperature measuring resistor R _t, the reference resistor R _C and the common potential A second switch connected to the connection point of these first, the fourth operational amplifier output of the second switch is operated as a voltage follower is connected to the non-inverting input terminal is provided with, RTD R when using platinum-based as _t turns off the second switch to turn on the first switch, the second switch by turning off the first switch when using copper as a measuring resistor R _t It is characterized by turning on.

[0009]

First in accordance with the type of RTD R _t using [action], on the second switch to turn off the drive, the first through the fourth operational amplifier operating the output voltage as a voltage follower, the 2 to the inverting input terminal of the operational amplifier. Since the fourth operational amplifier operating as a voltage follower is connected between the reference resistor R _C and the inverting input terminals of the first and second operational amplifiers in this manner, the first and second operational amplifiers are connected. The current flowing from the inverting input terminal side to the reference resistor R _C can be substantially neglected, and highly accurate measurement can be performed. The output voltage of the fourth operational amplifier from turning on the second switch to turn off the first switch when using copper as a measuring resistor R _t becomes the voltage of the common potential point, the 3, the noise margin of the output voltage of the operational amplifier can be increased.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.
You. FIG. 1 is a circuit diagram showing one embodiment of the present invention, and FIG.
The same reference numerals are given to the same parts. In FIG.
Reference resistance R_CAnd RTD R_tThe connection point with terminal b of
1 switch SW₁Is connected to the reference resistor R_CAnd common potential
A second switch SW is provided at the connection point with the point._TwoIs connected
You. These first and second switches SW₁, SW _TwoOutput side of
Is a fourth operational amplifier operating as a voltage follower
OP_FourIs connected to the non-inverting input terminal. And the performance
Operational amplifier OP_FourOutput terminal is a resistor R₁Via operational amplifier
OP₁And the resistor R_TwoTo
Via operational amplifier OP_TwoConnected to the inverting input terminal of
You. Other configurations are the same as those in FIG.

In such a configuration, the resistance temperature detector R _t
A first switch SW ₁ when using a platinum-based to turn the second off the switch SW ₂ and, when using copper-based as RTD R _t in the first off the switch SW ₁ as to turn on the second of the switch SW _2. Next, the operation of each of these cases will be described. <When using a platinum-based> When using a platinum-based as RTD R _t in the second off the switch SW ₂ is turned on the first switch SW _1, a constant current source CS is 1mA current I Inhale.

At this time, assuming that the potential of the terminal A is V _A , the potential of the terminal B is V _B , and the potential of the terminal b is V _b , V _A = −I (R _t + R _C + 2r) V _B = −I ( R _C + r) V _b = −IR _C

Meanwhile, the output voltage V ₄ of the operational amplifier OP ₄ will V ₄ = V _b. From these, the output voltage V of the operational amplifier OP _1
1 is V ₁ = − (R ₃ / R ₁ ) V _b + {1+ (R ₃ / R ₁ )} V _A =
IR _C + 2V _A , and the output voltage V ₂ of the operational amplifier OP ₂ is V ₂ = − (R ₄ / R ₂ ) V _b + ｛1+ (R ₄ / R ₂ )｝ V _b =
It becomes IR _C + 2V _b.

As a result, the output voltage E of the operational amplifier OP _3
out is: E _out = − (R ₇ / R ₅ ) V ₁ − (R ₇ / R ₆ ) V ₂ + [1+
{R ₇ / (R ₅ · R ₆ / (R ₅ + R ₆ ))}] V _B = − (V _{1
/ 2) - (V 2/} 2) + 2V B = -IR C -V A -V b +2
_{V B = -IR C + I (} R t + R C + 2r) + IR C -2I
(R _C + r) = I (R _t −R _C ), and highly accurate measurement can be performed without being affected by the resistance value r of the lead wire of the resistance bulb R _t .

[0015] <When using copper-based> to turn on the second switch SW ₂ in the first off the switch SW ₁ when a copper-based as RTD R _t, the constant current source CS 2
It is assumed that a current I of mA is drawn. At this time, the terminal A
Potential V _A, the potential of the terminal B V _B, although the potential V _b of the terminal b is the same as the case of platinum-based, the output voltage V ₄ of the operational amplifier OP ₄ becomes zero.

From these, the output voltage V ₁ of the operational amplifier OP ₁ is given by: V ₁ = − (R ₃ / R ₁ ) V ₄ + ｛1+ (R ₃ / R ₁ )｝ V _A =
2V _A , and the output voltage V ₂ of the operational amplifier OP ₂ becomes V ₂ = − (R ₄ / R ₂ ) V ₄ + ｛1+ (R ₄ / R ₂ )｝ V _b =
2 _Vb .

As a result, the output voltage E of the operational amplifier OP _{3
out is, E out = - (V 1} /2) - (V 2/2) + 2V B = -V A -
_{V b + 2V B = I (} R t + R C + 2r) + IR C -2I (R C
+ R) = become IR _t, enabling measurement with high accuracy without being affected by the resistance value r of the leads just as RTD R _t in the case of platinum-based.

[0018] Then, focusing on the noise margin of the output voltage E _out of this case, the output voltage E _out will be shifted to a position starting from the 100mV as indicated by a slightly thick broken line in FIG. 3, in FIG. 2 This is a great improvement over the conventional configuration. With such a configuration, the same resistance temperature detector circuit be mixed using RTD R _t of platinum-based and copper-based in never noise margin is greatly reduced as in the prior art either since perform measurement with high accuracy without being affected by the temperature measuring resistor R _t is a the resistance value r also lead, is also suitable as embedded circuits such as a recorder or data logger.

[0019]

As described above, according to the present invention,
In a relatively simple circuit configuration, the copper can be secured a sufficient noise margin in the case of using as a measuring resistor R _t, without being affected by the resistance of the RTD leads It is possible to realize a resistance thermometer circuit capable of performing highly accurate temperature measurement.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

FIG. 2 is a circuit diagram showing an example of a conventional resistance temperature detector circuit.

FIG. 3 is an operation explanatory diagram of FIGS. 1 and 2;

FIG. 4 is an explanatory diagram of a noise margin of a conventional resistance temperature detector circuit.

[Explanation of symbols]

R _t RTD R _C reference resistance OP Operational amplifier CS Constant current source SW switch

Claims (1)

(57) [Claims] 1. A resistance thermometer R _t having three terminals A, B and b.
When a current source connected to the terminal A of the RTD R _t, connected resistance between the common potential point and the terminal b of the RTD R _t is a known reference resistance R _C, temperature measurement first operational amplifier and, calculating the output voltage and the fourth terminal b of RTD R _t for calculating outputting a difference between the output voltage and the output voltage of the fourth operational amplifier terminal a of the resistor R _t a second operational amplifier which calculates and outputs the difference between the output voltage of the amplifier, the output terminal B of the first operational amplifier output voltage and a second sum of the output voltage of the operational amplifier and the temperature measuring resistor R _t third operational amplifier and a first switch connected to a connection point between the terminal b of the reference resistor R _C and the temperature measuring resistor R _t, the reference resistor R _C to a common potential for calculating outputting a difference between the voltage The second connected to the connection point with the point
And switch, these first, the fourth operational amplifier output of the second switch is operated as a non-inverting input connected to a voltage follower to the terminal provided, when using a platinum-based as RTD R _t first and the switch is turned on to turn off the second switch, when using copper-based as RTD R _t resistance thermometer, characterized in that turning on the second switch to turn off the first switch Body circuit.