Measuring and testing – Fluid pressure gauge – Electrical
Reexamination Certificate
2001-09-28
2003-07-29
Williams, Hezron (Department: 2855)
Measuring and testing
Fluid pressure gauge
Electrical
Reexamination Certificate
active
06598484
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a physical quantity detection device for detecting a physical quantity through resistance variation.
2. Description of the Prior Art
A detection circuit for a semiconductor pressure sensor using the piezoresistance effect is known.
FIGS. 6A and 6B
show examples of such a detection circuit. Japanese patent publication No. 2976487 discloses this detection circuit for a strain gage with temperature characteristic compensation.
The piezoresistance effect in diffusion resistors decreases with increase in the temperature, which decreases the sensitivity. On the other hand, the resistance increases. Particularly, the increase in the resistance depends on a density of impurity in the diffused resistors forming the strain gages. When the strain gages are driven with a constant current, the voltage applied to the strain gages increases with increase in the temperature. This provides compensation of decrease in the sensitivity in the sensor in accordance with the density of the impurity in the diffused resistors. This is the reason for using the circuits shown in
FIGS. 6A and 6B
.
In
FIGS. 6A and 6B
, when the diffused resistors Ra to Rd sense no strain (stress), that is, if no physical quantity is applied to this sensor, it is desirable that Ra=Rb=Rc=Rd. In this condition the output of the sensor &Dgr;Vout=0.
However, dispersion in manufacturing results in Ra≠Rb≠Rc≠Rd, so that &Dgr;Vout≠0. This is referred to as an offset voltage Voff.
In the condition that the offset voltage is developed, that is, &Dgr;Vout=Voff≠0, if a constant current is applied to the strain gage, the larger offset voltage, the more the temperature characteristic in the offset voltage (hereinafter referred to as offset temperature characteristic) increases. Therefore, to compensate the offset temperature characteristic, it is necessary to add a separate compensation circuit.
Moreover, it is necessary to obtain data of the offset temperature characteristic with large variation of temperature during the manufacturing process and to adjust the offset temperature characteristic on the basis of the data.
Japanese patent publication No. 2976487 discloses an example of the prior art offset temperature compensation circuit as shown in FIG.
7
.
In this circuit, the resistors R
9
, R
10
, R
11
correspond to the offset temperature compensation circuit.
Prior to explanation of the offset temperature compensation circuit, operation of the whole circuit will be described.
The operational amplifier OP
1
operates to equalize the voltage drop in the resistor R
3
to that in the resistor R
5
. Therefore, if a resistor having a temperature coefficient of resistance (TCR) of almost zero is used as the resistor R
5
, a current Io flowing through the bridge circuit including gage resistors Ra to Rd is substantially constant, though the temperature varies.
Here, using diffused resistors including boron as the gage resistors Ra to Rd and making the density of the p type impurity in the gage resistors Ra to Rd about 10
20
cm
−3
provides temperature compensation to sensitivity. This corresponds to the sensitivity-temperature compensation circuit.
The operational amplifier OP
2
and the operational amplifier OP
3
connected to a resistor R
6
are used as voltage follower circuits to convert the bridge output voltage into a current with the resistor R
6
. The current is supplied to an operational amplifier OP
4
through transistors Tr
1
and Tr
2
having Darlington connection. The operational amplifier OP
4
amplifies the current. The resistor R
8
connected to the input of the operational amplifier OP
4
is used for zero point adjustment.
The offset voltage of the bridge circuit can be made zero by laser-trimming the resistor R
1
or R
2
which is formed with CrSi thin film resistor having an almost zero TCR. However, because its TCR is largely different from those of the gage resistors Ra to Rd (about 1600 ppm/° C.), the offset voltage varies with the temperature.
In this circuit, operation of the resistors R
9
, R
10
, R
11
for offset temperature compensation will be described. Here, these resistors have a relation R
9
=R
10
<<R
11
. This condition makes the current flowing through the resistor R
11
constant though the temperature varies.
At first, because the bridge circuit is driven with a constant current, the applied voltage thereto varies at a TCR which is the same as that of the gage resistors Ra to Rd. Accordingly, the voltage potential V
6
decreases with increase in temperature. On the contrary, the voltage difference between the voltage potentials V
6
and Vd increases.
Therefore, laser-trimming the resistor R
9
to have the voltage Vf relatively approached the ground potential decreases the current flowing through the resistor R
11
with increase in temperature. On the other hand, laser-trimming the resistor R
10
to have the potential Vf approached the potential Vd increases the current flowing through the resistor R
11
with increases in temperature. That is, trimming the resistor R
9
or R
10
provides a temperature characteristic to the current flowing through the resistor R
11
. This temperature characteristic compensates the temperature characteristic in the bridge output. The offset temperature characteristic can be compensated in this way.
However, this operation requires measurement at the room temperature and at a high temperature for every circuit and laser trimming to obtain a target resistance which is calculated for the desired potential of Vf.
SUMMARY OF THE INVENTION
The aim of the present invention is to provide a superior physical quantity detection circuit.
According to the present invention, a first aspect of the present invention provides a physical quantity detection device comprising: a first sensing resistor having a first resistance varying in accordance with a first physical quantity relating a detection physical quantity; a second sensing resistor having a second resistance varying in accordance with a second physical quantity relating said detection physical quantity, first ends of said first and second sensing resistors being connected to a first voltage potential; a first current source for flowing a first constant current through said first sensing resistor, a first end of said first current source being connected to a second end of said first sensing resistor; a second current source for flowing a second constant current through said second sensing resistor, a first end of said second current source being connected to a second end of said second sensing resistor, second ends of said first and second current sources being connected to a second potential which is different from said first voltage potential; and outputting means for outputting a voltage difference signal indicative of said detection physical quantity between the second ends of said first and second sensing resistors.
According to the present invention, a fourth aspect of the present invention provides a physical quantity detection device based on the third aspect, wherein second ends of said first and second resistors are connected to each other at a junction point, said physical quantity detection device further comprising a third resistor connected to said junction point, wherein said first and second resistors are connected to said second potential through said third resistor, and a voltage potential of said junction point of said first and second resistors is supplied to an input of said operational amplifier.
According to the present invention, a fifth aspect of the present invention provides a physical quantity detection device based on the third aspect, wherein second ends of said first and second resistors are connected to each other at a junction point, and a voltage potential of said junction point is supplied to an input of said operational amplifier.
According to the present invention, a fifth aspect of the present invention provides a physical quantity detection d
Denso Corporation
Ellington Alandra
Posz & Bethards, PLC
Williams Hezron
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