CN102829888A - Method for eliminating three-wire heating resistor measurement errors - Google Patents

Abstract

The invention discloses a method for eliminating three-wire heating resistor measurement errors. A three-wire heating resistor measuring circuit comprises a multi-way selecting switch (MUX), a programmable gain amplifier (PGA), an analog-digital converter (ADC), double constant current sources (IDAC1 and IDAC2), a reference voltage sampling resistor (Rref) and a three-wire heating resistor (RTD); the circuit is characterized in that the connection relationships of the two constant current sources (IDAC1 and IDAC2) and the heating resistor are exchanged through the multi-way selecting switch (MUX) during the measurement; the heating resistor is measured twice; and the average value of the two heating resistor measuring values is used as a reference calculation temperature value. The method has the advantages that with the processing method, the influence of a double constant current matching error on a heating resistor sampling value is eliminated greatly, and the high precision temperature acquisition is achieved.

Description

Subdue the method for three-wire system thermal resistance measuring error

Technical field

The present invention relates to a kind of measuring method of temperature, a kind of especially method of subduing two constant current source three-wire system thermal resistance metering circuit measuring error.

Background technology

The temperature amount is that industrial occasions is used one of maximum physical quantity, and thermal resistance is a kind of temperature sensor commonly used, is characterized in: accuracy is high, have that the linearity is good, stable performance.Divide by the mode of connection, thermal resistance has following three kinds: two-wire system thermal resistance, three-wire system thermal resistance, four-wire system thermal resistance.Because it is too big that two-wire system thermal resistance accuracy of measurement is influenced by line resistance, so often adopt three-wire system or four-wire system thermal resistance in the practical application.Three-wire system thermal resistance cost is lower, and wiring is more convenient, is present modal a kind of mode.

Three-wire system thermal resistance acquisition mode has bridge method and two constant current source methods.Along with the development of chip technology, two constant current source methods are widely used.Some chip manufacturers have released the high integration thermal resistance acquisition chip based on this method.Two constant current source methods are measured three-wire system thermal resistance temperature-measurement principle and are described below: the method at first measures U based on Ohm law (R=U/I), calculates R then, draws corresponding temperature value at last through tabling look-up.As shown in Figure 1, metering circuit comprises 6 parts, and multidiameter option switch (MUX), programmable operational amplifier (PGA), analog-digital converter (ADC), two constant current source (IDAC1, IDAC2), reference voltage sampling resistor (Rref), three-wire system thermal resistance (RTD).Though three-wire system thermal resistance conductor length is identical, the solid conductor impedance differs very little, in order to make demonstration thorough, supposes that two connection resistances in three-wire system thermal resistance two ends are Rw1 and Rw2.Rref is the reference voltage sampling resistor, for analog-digital converter provides reference source.Two constant current sources are connected on two connecting lines at thermal resistance two ends among Fig. 1, and constant current source produces exciting current.When the exciting current that provides when two constant-current sources equates, calculate, survey between thermoelectric resistance and the actual thermoelectric resistance and concern as shown in Equation (1) through deriving.

$\mathrm{Rrtd}=\frac{\mathrm{RREF}}{\mathrm{Rref}}\×\mathrm{RRTD}+\frac{\mathrm{RREF}}{\mathrm{Rref}}\×(\mathrm{Rw}1-\mathrm{Rw}2)$ Formula (1)

In the formula:

Rrtd is the thermoelectric resistance of actual measurement;

RRTD is actual thermoelectric resistance;

Rref is actual reference electrode resistance;

RREF is the theoretical reference resistance value;

Rw1 is the resistance of a lead of thermal resistance a end;

Rw2 is the resistance of a lead of thermal resistance b end;

It is thus clear that, when adopting two constant current source methods that the three-wire system thermal resistance is sampled, wanting to realize high precision, two constant current source matching degrees must reach highly consistent.Yet with existing technology, thermal resistance chip manufacturer all can't guarantee very high matching precision.Two constant current source matching precisions directly influence the temperature acquisition precision.

Application number is 200810036435.6 patent of invention, has introduced a kind of thermal resistance metering circuit.Two constant current source matching precisions are not considered in this invention fully, and do not have the measuring error that this factor causes is done any measure, therefore can not reach very high measuring accuracy.Generate the sampling precision that to use the three-wire system thermal resistance to realize the four-wire system thermal resistance in the literary composition and be worth discussion.

Summary of the invention

The objective of the invention is provides a kind of method of subduing three-wire system thermal resistance measuring error to the defective that exists in the prior art.Three-wire system thermal resistance metering circuit comprises: multidiameter option switch (MUX), programmable operational amplifier (PGA), analog-digital converter (ADC), two constant current source (IDAC1; IDAC2), reference voltage sampling resistor (Rref), three-wire system thermal resistance (RTD); When it is characterized in that measuring; Exchange two constant current source (IDAC1 through multidiameter option switch (MUX); IDAC2) with the annexation of thermal resistance, thermal resistance is measured twice in front and back, and is according to the accounting temperature value with the mean value of this twice thermal resistance measured value.

Measure in the circuit at the two constant current sources of three-wire system thermal resistance; Through exchanging two constant current sources and thermal resistance annexation front and back; Thermal resistance is carried out the one-shot measurement value respectively; And, significantly subdue of the influence of two constant current source matching errors to the thermal resistance sampled value the method that two measured values are averaged, realize high precision temperature acquisition.

For achieving the above object, this practical technique scheme is: at first, multidiameter option switch is set, referring to Fig. 2, as Fig. 2 circuit arrangement as shown in, once sample, obtain thermal resistance measured value Rrtd1 shown in formula (3).

$\mathrm{Rrtd}1=\frac{2i_1}{i_1+i_2}\×\frac{\mathrm{RREF}}{\mathrm{Rref}}\×\mathrm{RRTD}+\frac{{2i}_1\×\mathrm{Rw}1}{i_1+i_2}\×\frac{\mathrm{RREF}}{\mathrm{Rref}}-\frac{2i_2\×\mathrm{Rw}2}{i_1+i_2}\×\frac{\mathrm{RREF}}{\mathrm{Rref}}$ Formula (3)

In the formula:

i ₁Exciting current for constant current source IDAC1 generation;

i ₂Exciting current for constant current source IDAC1 generation;

Then, the conversion multidiameter option switch, referring to Fig. 3, as Fig. 3 circuit arrangement as shown in, sample once more, obtain thermal resistance measured value Rrtd2 shown in formula (4).

$\mathrm{Rrtd}2=\frac{2i_2}{i_1+i_2}\×\frac{\mathrm{RREF}}{\mathrm{Rref}}\×\mathrm{RRTD}+\frac{{2i}_2\×\mathrm{Rw}1}{i_1+i_2}\×\frac{\mathrm{RREF}}{\mathrm{Rref}}-\frac{2i_1\×\mathrm{Rw}2}{i_1+i_2}\×\frac{\mathrm{RREF}}{\mathrm{Rref}}$ Formula (4)

At last, through asking the mean value of Rrtd1 and Rrtd2, draw the thermal resistance sampled value shown in formula (5).

$\mathrm{Rrtd}=\frac{1}{2}\×(\mathrm{Rrtd}1+\mathrm{Rrtd}2)=\frac{\mathrm{RREF}}{\mathrm{Rref}}\×\mathrm{RRTD}+\frac{\mathrm{RREF}}{\mathrm{Rref}}\×(\mathrm{Rw}1-\mathrm{Rw}2)$ Formula (5)

Can know through contrast formula (1) and formula (5),, can the measuring error that two constant current source matching errors bring thermal resistance be eliminated fully through the method.

After drawing thermoelectric resistance, can draw corresponding temperature value through look-up table.In the two constant current sources measurement of three-wire system thermal resistance circuit, adopt the method can significantly improve the thermal resistance measuring accuracy.

Advantage of the present invention is significantly to subdue the influence of two constant current source matching errors to the thermal resistance sampled value, realizes high precision temperature acquisition.

Description of drawings

The two constant current source methods of Fig. 1 are surveyed three-wire system thermal resistance principle schematic;

The two constant current sources of the first sampling of Fig. 2 are connected synoptic diagram with thermal resistance;

The two constant current sources of Fig. 3 double sampling are connected synoptic diagram with thermal resistance.

Among the figure: MUX multidiameter option switch, PGA programmable operational amplifier, ADC analog-digital converter, the two constant current sources of IDAC1, the two constant current sources of IDAC2, Rref reference voltage sampling resistor, RTD three-wire system thermal resistance, Rw1 conductor resistance, Rw2 conductor resistance, the end of a thermal resistance RTD, an end, the i of b thermal resistance RTD ₁Exciting current, i for constant current source IDAC1 generation ₂Exciting current for constant current source IDAC2 generation.

Embodiment

Further specify embodiments of the invention below in conjunction with accompanying drawing:

Three-wire system thermal resistance metering circuit is by multidiameter option switch MUX, programmable operational amplifier PGA, analog-digital converter ADC, two constant current source IDAC1, and two constant current source IDAC2, reference voltage sampling resistor Rref, three-wire system thermal resistance RTD form.The annexation that three-wire system thermal resistance metering circuit is exchanged constant current source IDAC1, constant current source IDAC2 and thermal resistance RTD through multidiameter option switch MUX, resistance value that twice measurement of warp thermal resistance RTD obtains and warp calculate obtains mean value.

In the resistance value process of two groups of thermal resistances of twice measurement, when measuring for the first time, through multidiameter option switch MUX a that constant current source IDAC1 is connected to thermal resistance RTD is held, constant current source IDAC2 is connected to the b end of thermal resistance RTD, obtains thermal resistance measured value Rrtd1; When measuring for the second time, through multidiameter option switch MUX the b that constant current source IDAC1 is connected to thermal resistance RTD is held, constant current source IDAC2 is connected to a end of thermal resistance RTD, obtains thermal resistance measured value Rrtd2.Referring to Fig. 2, Fig. 3.

Then, obtain the mean value of Rrtd1 and Rrtd2 through the aforementioned calculation formula, and with this value as final thermal resistance sampled value, can draw the temperature value of correspondence then through look-up table.

Claims (3)

1. method of subduing three-wire system thermal resistance measuring error; Three-wire system thermal resistance metering circuit comprises: multidiameter option switch (MUX), programmable operational amplifier (PGA), analog-digital converter (ADC), two constant current source (IDAC1; IDAC2), reference voltage sampling resistor (Rref), three-wire system thermal resistance (RTD); When it is characterized in that measuring, (IDAC1 is IDAC2) with the annexation of thermal resistance to exchange two constant current sources through multidiameter option switch (MUX); Before and after measure thermal resistance twice, and be according to computing formula accounting temperature value with the mean value of these two thermal resistance measured values.

2. method of subduing three-wire system thermal resistance measuring error according to claim 1 is characterized in that said computing formula is:

$\mathrm{Rrtd}=\frac{1}{2}\×(\mathrm{Rrtd}1+\mathrm{Rrtd}2)=\frac{\mathrm{RREF}}{\mathrm{Rref}}\×\mathrm{RRTD}+\frac{\mathrm{RREF}}{\mathrm{Rref}}\×(\mathrm{Rw}1-\mathrm{Rw}2)$

In the following formula: Rrtd is the thermoelectric resistance of actual measurement; RRTD is actual thermoelectric resistance; Rref is actual reference electrode resistance; RREF is the theoretical reference resistance value; Rw1 is the resistance of a lead of thermal resistance a end; Rw2 is the resistance of a lead of thermal resistance b end.

3. method of subduing three-wire system thermal resistance measuring error according to claim 1; It is characterized in that in the resistance value process of two groups of thermal resistances of twice measurement in said front and back; When measuring for the first time; The annexation of two constant current sources and thermal resistance is a end that constant current source (IDAC1) is connected to thermal resistance (RTD), and constant current source (IDAC2) is connected to the b end of thermal resistance (RTD); When measuring for the second time, two constant current sources and thermal resistance annexation are the b end that constant current source (IDAC1) is connected to thermal resistance (RTD), and constant current source (IDAC2) is connected to a end of thermal resistance (RTD).

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