Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Using a particular bridge circuit
Reexamination Certificate
2001-11-08
2004-04-20
Deb, Anjan K. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Using a particular bridge circuit
C324S071100, C324S721000, C073S726000, C338S003000
Reexamination Certificate
active
06724202
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 with temperature compensation.
2. Description of the Prior Art
A physical quantity detection device for detecting a physical quantity through resistance variation with temperature compensation is disclosed in Japanese patent No. 2976487.
FIG. 5
is a schematic circuit diagram of this prior art physical quantity detection device (pressure sensor). This physical quantity detection device has four operational amplifiers OP
11
to OP
14
.
The operational amplifier OP
11
compensates a temperature characteristic of sensitivity by supplying a constant current to a Wheatstone bridge including strain gages for detecting a pressure. The operational amplifier OP
12
and the operational amplifier OP
13
are used as voltage-followers to suppress an error due to directly drawing an output current from the bridge output. The operational amplifier OP
14
amplifiers the pressure detection signal and shifts the zero point. This physical quantity detection device provides a physical quantity detection signal with accuracy over a wide temperature range.
However, in consideration of reduction in cast and in the area occupied by this circuit on an IC chip, the number of the operational amplifiers should be reduced.
Japanese patent No. 3-67211 (U.S. Pat. No. 4,576,052) discloses another prior art physical quantity detection device satisfying this requirement.
FIG. 6
is a schematic circuit diagram of this prior art physical quantity detection device including two operational amplifiers. This prior art physical quantity detection device provides the same function as the physical quantity detection device shown in
FIG. 5
with only two operational amplifiers. In the case that the physical quantity detection device is used as a pressure sensor in motor vehicles, it is generally required that the physical quantity detection device is driven by a single power supply of 5 V, so that the output voltage range is from 0.5 to 4.5 V. Moreover, it is further required that the output voltage increases with an increase in pressure. Thus, in consideration of these general requirements, the prior art physical quantity detection device shown in
FIG. 6
has the following disadvantages.
First, in the physical quantity detection device (pressure sensor) shown in
FIG. 6
, the operational amplifier OP
21
is used as a differential amplifier. Thus, the operational amplifier OP
21
outputs 0 V when there is no pressure on the stain gages Ra′ to Rd′, and the output voltage increases with increase in the pressure.
In the general operational amplifier, the output voltage is limited at high and low voltages in the output voltage range. That is, near the supply voltage, the output voltage is limited by the voltage (Vcc) of the power supply, and near the ground level, the output voltage is subjected to the low voltage limit such as transistor's |Vce(sat)|≈0.2 V. Moreover, when a difference between two pressures is measured, this circuit cannot generate a negative voltage, so that the output cannot represent the negative pressure.
Second, in the above-mentioned structure, because of the single supply voltage, the output voltage range starts from 0.5 V. Thus, it is required to shift the zero point of the output signal. Accordingly, the operational amplifier OP
22
at the last stage should be used as an inverting type operational amplifier having a summing input. However, in this structure, the output voltage of the last stage of operational amplifier OP
22
decreases with increase in the output voltage at the previous stage of the operational amplifier OP
21
when the pressure increases. That is, the output characteristic of the pressure sensor shown in
FIG. 6
is as shown in FIG.
7
. This is an inverted characteristic with respect to the required output characteristic for the pressure sensor.
The pressure sensor disclosed in Japanese patent application provisional publication No. 3-51733 (U.S. Pat. No. 5,042,307) clears this requirement.
FIG. 8
is a schematic circuit diagram of this prior art pressure sensor(physical quantity detection device). This pressure sensor is developed to have operation which is the same as the prior art pressure sensor having four operational amplifiers shown in FIG.
5
. In this pressure sensor shown in
FIG. 8
, a feedback resistor Rh has a temperature dependency for compensating a temperature characteristic of sensitivity. Moreover, its offset temperature characteristic can be adjusted also. However, if only offset is simply adjusted, because the feedback resistor Rh has temperature dependency, in order to cancel the temperature dependency, it is necessary to equalize TCRs (temperature coefficients of resistance) in the combined resistance of the resistors Ri, R
27
, and R
29
and the combined resistance of the resistors Rj, R
28
, and R
30
to that of the combined resistance of the resistors Rh and R
26
.
Moreover, in the actual offset adjustment and the actual offset temperature adjustment, the resistors R
27
, R
29
, R
28
, and R
30
are adjusted adequately. However, trimming any of these resistors varies the offset temperature characteristic due to the presence of the temperature dependency in the feedback resistor Rh. That is, in this pressure sensor, accurate adjustment cannot be provided because the offset and offset temperature dependency cannot be adjusted independently.
There is a further problem in this pressure sensor. That is, the non-inverting input of the operational amplifier OP
32
at the second stage is supplied with the output of the bridge, so that this operational amplifier OP
32
operates with a reference potential including the pressure detection signal which is not amplified. Thus, adjusting the offset and offset temperature characteristic in the second stage by trimming any of the resistors R
27
, R
28
, R
30
slightly exercises an influence on the pressure detection signal component. That is, this structure exercises an influence on the adjustment in the sensitivity. Thus, if the offset and the offset temperature characteristic are adjusted after adjustment of the sensitivity, the accuracy in the sensitivity will decrease because the adjustment in the sensitivity deviates slightly from the previous state. Inversely, if the offset in the bridge, the offset (voltage), and the offset temperature characteristic are adjusted before the sensitivity adjustment, the offset and offset temperature characteristic deviates from the adjusted values.
Accordingly, the physical quantity detection device such as the pressure sensor is required to output an accurate detection signal with temperature compensation using a single power supply and a low number of amplifiers even at a voltage near the ground potential and a negative potential, and the adjustment should be easy with accuracy.
SUMMARY OF THE INVENTION
The aim of the present invention is to provide a superior physical quantity detection device.
According to the present invention, a first aspect of the present invention provides a physical quantity detection device including: an operational amplifier; a first resistor connected between an inverting input of the operational amplifier and a first reference potential; a second resistor connected between the inverting input of the operational amplifier and a second reference potential, the first and second resistors having a first temperature coefficient of resistance; a feedback resistor being connected between the inverting input of the operational amplifier and an output of the operational amplifier and having a second temperature coefficient of resistance; and a reference voltage generation circuit generating a reference voltage supplied to a non-inverting input of the operational amplifier, at least one of the first and second resistors including a sensing element of which resistance varies on the basis of a physical quantity with a temperature coefficient of sen
Deb Anjan K.
Denso Corporation
Posz & Bethards, PLC
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