JPH0511584B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field to which the Invention Pertains] The present invention relates to a bridge output detection circuit, and particularly to a bridge output detection circuit that compensates for the zero point of the bridge output and compensates for sensitivity temperature characteristics.

[Prior art and its problems]

This type of bridge output detection circuit is used, for example, in a pressure transducer equipped with a silicon pressure-sensitive diaphragm. The variations cause variations in the zero point of the pressure transducer output. In addition, the degree of change in resistance value with respect to pressure change of the strain gauge is -0.1
Since it has a negative temperature coefficient of ~-0.3/°C, it produces a negative sensitivity temperature characteristic. In particular, when using a silicon pressure-sensitive diaphragm in a high-precision pressure transducer, compensation for zero point springing and compensation for sensitivity temperature characteristics are essential.

As shown in Figure 7, a commonly used circuit for zero point compensation and sensitivity temperature characteristic compensation is a circuit that connects a variable resistor or trimming resistor to a part of the strain gauge bridge to perform zero point compensation. In addition, a circuit that compensates for the sensitivity temperature characteristics by connecting a resistor with an appropriate temperature coefficient different from that of the strain gauge between the strain gauge bridge and the drive power supply, and an output stage as shown in Figure 8. Zero-point compensation is performed by applying a voltage divided by a resistor to the non-inverting input terminal of the operational amplifier, and the sensitivity is adjusted by using a resistor with a temperature dependence opposite to the sensitivity temperature characteristic of the bridge as the feedback resistor of this operational amplifier. Circuits that compensate for temperature characteristics are known.

In FIG. 7, SG1 to SG4 are strain gauges, RX and RY are variable resistors or trimming resistors, and by changing the values of RX and RY, the balance of the bridge is changed and the zero point is compensated for. In addition, RZ is a resistor that has an appropriate temperature dependence that is more negative than that of the strain gauge, and this resistance causes the drive voltage of the strain gauge bridge to have a positive temperature dependence, which increases the output sensitivity of the strain gauge bridge. There is a tendency for the temperature characteristics to be as follows. This positive temperature characteristic compensates for the aforementioned negative sensitivity temperature characteristic. This compensation can be done by simply connecting a resistor to the bridge, so it has the advantage of a relatively simple circuit, but on the other hand,
It has the following drawbacks.

(1) Due to the sensitivity temperature characteristic compensation resistor RZ, the zero point component generated by the zero point compensation resistors RX and RY also becomes temperature dependent. Particularly in a high-precision pressure transducer, if the amount of zero point compensation is large, it is necessary to compensate for the temperature dependence of this zero point.

(2) Since a resistor is connected between the strain gauge bridge and the driving power source, the driving voltage of the bridge is reduced, and as a result, the sensitivity of the bridge is reduced.

Further, in FIG. 8, R1 and R2 are fixed resistors, RV and RW are resistors having an appropriate positive temperature coefficient, R3 and R4 are variable resistors or trimming resistors, and OP1 is an operational amplifier. These resistors R1 to R4, RV, RW and operational amplifier OP1 constitute a differential amplifier. A strain gauge bridge output voltage component VG is applied to differential input terminals T1 and T2 of this circuit. This voltage VG
may be the strain gauge bridge output voltage itself, or may be an amplified strain gauge bridge output voltage. Then, this voltage VG is amplified by this differential amplifier and outputted from the output terminal of the amplifier OP1 as a pressure transducer output. Usually R1-R2, RV=RW,
Further, for the sake of simplicity, assuming that R3 and R4 have sufficiently smaller resistance values than RV and RW, the output voltage Vout of the amplifier OP1 is expressed by the following equation (1).

Vout＝R4／R3＋R4VC＋RV／R1VG ……(1) Here, VC is the voltage applied to resistor R3.

By appropriately changing the resistances R3 and R4, the first term on the right side of equation (1) changes, and the zero point is compensated for. Furthermore, by providing the resistors RV and RW with appropriate positive temperature coefficients, the temperature dependence of the second term on the right side can be eliminated. In other words, the sensitivity temperature characteristics are compensated. According to this method, zero point compensation and sensitivity temperature characteristic compensation do not affect each other, and bridge sensitivity does not decrease, but the zero point change range due to resistors R3 and R4 is 0.
Above, the value is less than or equal to VC, and it is not possible to have a zero point that is negative or greater than or equal to V. Therefore, for example, in a pressure transducer that detects only near atmospheric pressure when the reference pressure side of the diaphragm is vacuum, a positive differential pressure is always applied to the diaphragm, so a negative reference voltage is generated to compensate for the output caused by that differential pressure. The disadvantage is that it is not applicable.

[Purpose of the invention]

Therefore, the present invention eliminates the above-mentioned drawbacks, and provides a bridge output detection method that does not reduce the bridge output sensitivity, has a very small influence between zero point compensation and sensitivity temperature characteristic compensation, and has a wide zero point compensation range. The purpose is to provide circuits.

[Key points of the invention]

The present invention includes a first operational amplifier having one of its bridge output lines connected to a non-inverting input, and a second operational amplifier having one of its bridge output lines connected to a non-inverting input.
an operational amplifier, a resistor connected between the inverting inputs of the first and second operational amplifiers, and a first resistor connected between the output and the inverting input of each of the first and second operational amplifiers. and a second feedback resistor, and a zero point compensation resistor connected between the inverting input of either one of the first and second operational amplifiers and a predetermined voltage power supply, By using the feedback resistor of the operational amplifier that is not connected to the zero point compensation resistor as the sensitivity temperature characteristic compensation resistor, try to reduce the influence between the zero point compensation and the sensitivity temperature characteristic compensation. That is.

[Embodiments of the invention]

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

FIG. 1 shows an embodiment of a pressure transducer to which the present invention is applied. In the figure, a strain gauge bridge output voltage Vi is applied to terminals T3 and T4. Terminal T
3 is connected to the non-inverting input terminal of the operational amplifier OP2, and the terminal T4 is connected to the non-inverting input terminal of the operational amplifier OP3. Also, power supply Vcc and earth GND
Resistors R5 and R6 are connected in series between them, and these resistors R5 and R6 are variable resistors or trimming resistors. And the connection point of resistors R5 and R6 is OP3
A resistor R7 is connected between the inverting input terminal of the amplifier OP2 and the inverting input terminal of the amplifier OP3. Output terminal T5 of amplifier OP2
A resistor with a positive temperature coefficient is connected between the input terminal and the inverting input terminal.
Ra is connected, and a resistor R8 is connected between the output terminal T6 and the inverting input terminal of the amplifier OP3. By operational amplifiers OP2, OP3 and resistors R5 to R8, Ra,
A differential input and differential output amplifier is configured. Here, the differential input terminals are T3 and T4, and the differential output terminals are T5 and T6.

Next, the operation of the embodiment of the present invention having the above configuration will be explained. Here, for simplicity, the offset current and voltage of the operational amplifier are assumed to be zero. It is also assumed that the average of the potential V3 of the input terminal T3 relative to the ground and the potential V4 of the input terminal T4 relative to the ground is half of the power supply.
This can be expressed as a formula as follows.

V3=Vcc/2-Vi/2...(2) V4=Vcc/2+Vi/2...(3) Here, Vi is defined as the potential of terminal T4 viewed from terminal T3.

The differential output voltage Vo of this amplifier is expressed by the following equation (4).

Vo={R8+Ra/R7+1+R8(R5+R6)/2R5R6}Vi
+R8 (R5-R6)/2R5R6・Vcc...(4) R5 and R6 are zero point compensation resistors, and this resistor R
The zero point component V _D generated by 5 and R6 is the second term on the right side of equation (4), and is expressed by the following equation (5).

V _D = R8 (R5 − R6)/2R5R6·Vcc (5) This V _D equation does not include the temperature-dependent resistance Ra. Therefore, V _D does not have temperature dependence due to the influence of sensitivity temperature characteristic compensation. R5=
If it is R6, VD=0. Here R5 is R5
The change ΔVD1 in the zero point component V _D when changing from the state of =R6=RS to infinity (open) is expressed by the following equation (6).

△VD1=R8/R _S・Vcc/2 ...(6) Similarly, when R6 is changed from the state of R6=R5= _RS to infinity (open), the change in VD △VD2 is calculated by the following formula. It becomes like (7).

△VD2=-R8/R _S・Vcc/2 ...(7) Therefore, from the state of R5=R _S and R6=∞, R5=
∞, the change in the zero point component V _D when it is changed to the state of R6=R _S is expressed by _the following equation (8).

△V _D = △V _D 1-△V _D 2 = R8/R _S・Vcc ...(8) As is clear from this formula, △V _D is the resistance R5,
The values of R6 and R8 can be selected from zero to infinity.

The first term on the right side of equation (4) is a term related to the strain gauge bridge output voltage Vi. Here, the temperature coefficient of resistance Ra is assumed to be α (α>0). Further, the strain gauge bridge output voltage Vi is divided into a component generated according to the differential pressure, that is, a span component V _S and a zero point component V _Z. Here, the temperature coefficient of the span component V _S is β (β <
0), and for the sake of simplicity, it is assumed that the zero point component V _Z has no temperature dependence. From the above, Ra and Vi can be expressed as the following equation (9),
Expressed as (10).

Ra=Rao(1+αT)...(9) Vi=V _Sp (1+βT)+V _Z ...(10) Here, T is temperature, Rao is Ra at T=0,
V _S0 is V _S at T=0. Expressions (9), (1) and (10) are expressed as
The first term on the right side of the differential output voltage Vo in (4) (set as Voi)
By substituting into , Voi is expressed as the following equation (11).

Voi={R7+R8+Rao/R7+R8(R5+R6)/2R5R6} {1+Rao αT/R7+R8+Rao+R7R8(R5+R6)/2R
5R6}×{V _Sp (1+βT)+V _Z } In equation (11), the temperature coefficient γ of the term related to the span component V _Sp of the strain gauge bridge is expressed by the following equation (12). Here, the second-order temperature term is omitted for simplicity.

γ=Rao α/R7+R8+Rao+R7R8(R5+R6)/2R5R6+
β
...(12) The first term on the right side of equation (12) is the temperature coefficient of circuit amplification. The condition for sensitivity temperature compensation is γ=0. Normally, R7 is set smaller than Ra and R8, so by changing the value of Rao or R8, the temperature coefficient of the amplification degree can be changed in the range from around zero to around α, allowing a wide range of compensation. It is possible.

In order to sufficiently reduce the influence of zero point compensation on compensation of sensitivity temperature characteristics, the width of change (denoted as △γ) in the first term on the right side of equation (12) with respect to changes in zero point compensation resistors R5 and R6 must be It needs to be sufficiently small.

As an example, R7=1.5KΩ, R8=30KΩ, Ra
= 50KΩ, α = 0.4%/℃, the values of R5 and R6 are R5 = R _S and R6 = ∞, so R5 = ∞, R6
Figure 2 shows the relationship between R _S and △V _D and the relationship between R _S and △γ when the value is changed to =R _S . From Figure 2,
As an example of making △V _D sufficiently large and △γ sufficiently small, △V _D should be 2Vcc or more, and △γ should be 5×10 ^-2 %/
If you want to keep it below ℃, you can set R _S in the range of 2.5KΩ to 15KΩ.

As mentioned above, by selecting an appropriate value for Rao or R8, the sensitivity temperature characteristics can be compensated, and by further selecting the values for R5 and R6, the zero point compensation range △V _D can be sufficiently wide and It is possible to sufficiently reduce changes in sensitivity temperature characteristics due to zero point compensation.

FIG. 3 is a circuit diagram showing another embodiment, in which the connection point of the zero point compensation resistors R5 and R6 is connected to the operational amplifier OP2.
A sensitivity temperature characteristic compensation resistor Rb with a positive temperature coefficient is connected between the output terminal of OP3 and the inverting input terminal, and a resistor R9 is connected between the output terminal of OP2 and the inverting input terminal. In the case that
It exhibits the same effect as the embodiment shown in FIG.

FIG. 4 is a circuit diagram showing still another embodiment,
This corresponds to the case where R6 in the embodiment shown in FIG. 1 is left open. It is possible to output only the negative zero point by the zero point compensation resistor R5. The first embodiment except that the range of zero point compensation is narrower compared to the embodiment shown in FIG.
It exhibits the same effect as the embodiment shown in the figure. Similarly, by opening R5 in the embodiment of FIG. Only positive zeros can be output.

FIG. 5 is a circuit diagram showing still another embodiment.
The resistance R5 in Figure 1 is a temperature-dependent resistance.
By replacing it with Rc, the temperature characteristics at the zero point can be compensated by selecting an appropriate value for the resistance temperature coefficient of the resistor Rc.

If the resistor R6 in FIG. 1 is replaced by a temperature-dependent resistor, it is possible to compensate for a temperature characteristic having a slope opposite to that of the embodiment shown in FIG.

FIG. 6 is a circuit diagram showing still another embodiment.
A diode D1, which is a nonlinear resistor, is added to the resistor 5 in FIG. ₁ , and the dependence of the characteristics on the power supply voltage can be compensated for. The diode portion can be replaced with other non-linear resistors. Adding a non-linear resistance to resistor R6 allows compensation in the opposite direction to that of the embodiment of FIG.

〔Effect of the invention〕

According to the present invention as described above, a differential amplifier is constructed using two operational amplifiers, and a zero point compensation resistor is connected to the inverting input of one, and the feedback resistor of the operational amplifier to which this resistor is not connected is Since the resistor is used as a resistance for compensating sensitivity-temperature characteristics, the influence of zero-point compensation on sensitivity-temperature characteristics is extremely reduced, and the zero-point compensation width can be made sufficiently large.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a diagram showing an example of the relationship between R _S , △V _D and △γ,
FIGS. 3 to 6 are circuit diagrams showing other embodiments of the present invention, and FIGS. 7 and 8 are circuit diagrams showing conventional examples. SG1~SG4...Strain gauge, R1, R2,
R7, R8, R9...Resistance, R3, R4, R5, R
6... Variable resistor, R _Z ... Resistance with a more negative temperature coefficient than the strain gauge, R _V , R _W , Ra, Rb, Rc... Resistance with temperature dependence, D ₁ ... Diode, OP1~
OP3...Operation amplifier, P, Vc, Vcc...Power supply, GND
...Earth potential, Vi _N , V _G , Vi... Strain gauge bridge output component, Vout, Vo... Amplifier circuit output, T ₁ ~
_T6 ...Terminal.

Claims (1)

[Claims] 1 a first operational amplifier having one of the bridge output lines connected to the non-inverting input; a second operational amplifier having the other bridge output line connected to the non-inverting input; and the first and second operational amplifiers. a resistor connected between the inverting inputs of the first and second
first and second feedback resistors connected between the output and the inverting input of each of the operational amplifiers, and between the inverting input of either one of the first and second operational amplifiers and a predetermined voltage power supply. a zero point compensation resistor connected to the first and second
A bridge output detection circuit characterized in that a feedback resistor of an operational amplifier to which the zero-point compensation resistor is not connected among the feedback resistors is used as a sensitivity-temperature characteristic compensation resistor.