Electrical resistors – Resistance value responsive to a condition – Fluid- or gas pressure-actuated
Reexamination Certificate
2000-02-14
2004-03-02
Easthom, Karl D. (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Fluid- or gas pressure-actuated
C338S007000, C338S009000, C073S862623
Reexamination Certificate
active
06700473
ABSTRACT:
FIELD OF INVENTION
The present invention relates to piezoresistive pressure sensors and more particularly temperature compensated pressure transducers.
BACKGROUND OF INVENTION
It is well known in a piezoresistive Wheatstone bridge having four equal piezoresistors of resistance R
B
of which two increase with positive strain and two decrease with an equal negative strain, that the change of voltage &Dgr;V across the bridge is given by:
Δ
V
V
B
=
Δ
R
B
R
B
and that
Δ
R
R
=
ε
GF
where &egr; is the strain, V
B
is the voltage applied across the bridge and GF is the gauge factor. It is also well known that the gauge factor decreases as a function of bridge temperature. The rate of change of gauge factor with temperature is usually referred to as TCGF or temperature coefficient of gauge factor. Thus, for a constant voltage applied across the bridge, the output will decrease as a function of temperature. It is also well known that resistance of the bridge elements increase as a function of temperature. The change of resistance with temperature is referred to as TCR or temperature coefficient of resistance. For highly doped P-type silicon, the TCGF is approximately −2%/100° F. to −3%/100° F. while the TCR is approximately +10%/100° F. Referring now to
FIG. 1
, one way to make the output voltage more independent of temperature using a constant voltage source
15
is to place a temperature independent resistor R
S
in series with the bridge
2
. Thus, as temperature increases the bridge resistance increases and more of the supply voltage appears across the bridge. For this case the bridge voltage V
B
is given by:
V
B
=
R
B
R
B
+
R
S
V
0
Thus, by appropriate choice of the ratio of R
S
to R
B
, the desired increase of bridge voltage with temperature can be obtained. This compensation scheme can be used in many applications. An example of such an apparatus and method is illustrated in U.S. Pat. No. 3,245,252, entitled “TEMPERATURE COMPENSATED SEMICONDUCTOR STRAIN GAGE UNIT” issued Apr. 12, 1966, the entire disclosure of which is hereby incorporated by reference as if being set forth herein in it entirety. However, there are some instances when such an approach is unsuitable for certain needs. An example of such an application is where certain extremely tight specifications are needed, then the passive resistor alone can not accomplish the desired effect. This is especially true when the pressure transducer must be extremely precise at either the extreme cold end or extreme hot end of the operating temperature range of the device.
It is an object of the present invention to provide an improved temperature compensated transducer suitable for use in these types of applications.
SUMMARY OF INVENTION
A dielectrically isolated temperature compensated pressure transducer including: a wafer including a deflectable diaphragm formed therein, the diaphragm being capable of deflecting in response to an applied pressure, and the diaphragm defining an active region surrounded by an inactive region of the wafer; a plurality of dielectrically isolated piezoresistive elements formed on the active region of the wafer and coupled together to form a Wheatstone bridge configuration so as to cooperatively provide an output signal in response to and indicative of an amount of deflection of the diaphragm, the plurality of piezoresistive elements being undesirably operative to introduce an undesirable error into the output according to exposure of the wafer to an environmental condition; and, a dielectrically isolated resistor formed on the inactive region of the wafer and electrically coupled in series to the plurality of piezoresistive elements so as to at least partially compensate for the undesirable error.
REFERENCES:
patent: 3245252 (1966-04-01), First et al.
patent: 4226899 (1980-10-01), Thiel et al.
patent: 4414837 (1983-11-01), Bice et al.
patent: 4462018 (1984-07-01), Yang et al.
patent: 4480478 (1984-11-01), Sato
patent: 4510813 (1985-04-01), Kanazawa
patent: 4622856 (1986-11-01), Binder et al.
patent: 4628296 (1986-12-01), Kitagawa et al.
patent: 4777826 (1988-10-01), Rud, Jr. et al.
patent: 4788521 (1988-11-01), Johnson
patent: 5197334 (1993-03-01), Guziak
patent: 5286671 (1994-02-01), Kurtz et al.
patent: 5549006 (1996-08-01), Kurtz
patent: 5877423 (1999-03-01), Mattsson
patent: 6097276 (2000-08-01), Van Den Broek et al.
Webpage at www.precisionresistor.com, printed Oct. 1, 2003, date unknown.
Bemis Andrew V.
Kurtz Anthony D.
VanDeWeert Joseph
Duane Morris LLP
Easthom Karl D.
Kulite Semiconductor Products Inc.
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