Circuit for self compensation of silicon strain gauge pressure transmitters

A circuit for compensating a silicon strain gauge pressure transmitter. The circuit includes: a current source, an embodiment of which may include an amplifying device and means for supplying the current source with an electric potential, a strain gauge bridge, a plurality of resistances that includes a feedback resistance, a series resistance, a current sampling resistance, and a load with parameters. The key to this invention is to add a series resistance and a feedback resistance to the circuit which eliminates the need for R.sub.c, the current sample resistor, to be a thermistor, and increases the operating temperature range of the sensor by compensating the sensor's parameter variations with temperature.

FIELD OF THE INVENTION 
This invention relates generally to electrical circuits and, in particular, 
to electronic circuits which are capable of self compensating silicon 
strain gauge pressure sensors. 
BACKGROUND OF THE INVENTION 
Occasions arise wherein a circuit designer of an electronic circuit that 
includes a silicon strain gauge transmitter or sensor is required to 
compensate for variations in temperature that occur. In order to maintain 
the accuracy of the measurements within specified limits, it is known to 
generally use an independent, separate element or component to compensate 
the positive temperature coefficient of sensitivity(with constant current) 
of the silicon strain gauge pressure transmitter. For example, the curves 
for typical silicon strain gauge pressure transmitters (sensors) have a 
slightly positive temperature coefficient of sensitivity with constant 
current excitation. Under conventional practice the circuit would need or 
use an additional thermally sensitive resistor, also known as a 
thermistor, with a positive temperature coefficient of sensitivity to 
temperature to compensate for the effects of temperature. Thus the circuit 
is adjusted to compensate for the effects of temperature by reducing 
current slightly as temperature increases. 
It is known in the art, shown in FIG. 1, that the output 1a of an 
operational amplifier 1 is connected to one node (input node) 2a of a 
strain gauge bridge 2 and another node (output node) 2b of the strain 
gauge bridge 2 is connected to an inverting input 1b of the operational 
amplifier 1. The output node 2b is also connected to one end of a current 
sampling resistance R.sub.c (which may be a thermistor). The other end of 
the current sampling resistance R.sub.c is connected to ground or a zero 
voltage point. A positive terminal of a reference voltage, V.sub.ref, 
represented by a voltage source, such as a battery, is connected to a 
non-inverting input 1c of the operational amplifier 1, with negative end 
of said battery connected to the zero potential point. 
It is also known in the art that, in practice, thermistors are non-linear 
devices which tend to limit or compromise the measurement accuracy of the 
strain gauge. Additionally, a thermistor's parameters may shift value upon 
exposure to high temperature, which would compromise the measurement 
range. Prior to this invention, designers were limited to using a 
non-linear element, such as a thermistor, to compensate the current 
variations due to temperature change. Furthermore, the time constants of 
the strain gauge bridge and the thermistor may be different, resulting in 
the compensation lagging or leading the temperature induced change in 
bridge resistance. As can be appreciated, this technique does not provide 
an optimum solution for accurately compensating a silicon strain gauge 
based system within a temperature range, such as a pressure sensor. 
OBJECTS AND ADVANTAGES OF THE INVENTION 
It is a first object and advantage of this invention to provide an improved 
circuit for self compensating electric current variations caused by 
temperature changes, without requiring the use of a non-linear element, 
such as a thermistor. 
It is a second object and advantage of this invention to provide an 
electronic circuit for self compensation of silicon strain gauge pressure 
transmitters wherein only the silicon stain gauge pressure transmitter's 
parameters vary with a change of temperature. 
It is a third object and advantage of this invention to provide a circuit 
that compensates for the non-linear properties of a silicon stain gauge 
pressure transmitter, without requiring the use of a separate non-linear 
component. 
It is a fourth object and advantage of this invention to provide a circuit 
which uses no temperature sensing element other than the silicon gauge 
transmitter to achieve an system accuracy within +/-0.3% in a temperature 
range of -40.degree. C. to 85.degree. C. 
It is a further object and advantage of this invention to provide a 
temperature compensated silicon stain gauge sensor that only uses commonly 
available and low priced, non-precision components. 
SUMMARY OF THE INVENTION 
The foregoing and other problems are overcome and the objects of the 
invention are realized by apparatus in accordance with embodiments of this 
invention, wherein a feedback resistor and a series resistor are 
respectively added to the current power source of the strain gauge 
pressure sensor circuit. 
The present invention resides in a new and improved circuit for self 
compensation of silicon strain gauge pressure transmitters which overcomes 
the disadvantages of the prior art. In its broader aspects, the present 
invention contemplates a circuit for self compensation of the circuitry 
with a temperature range with a load as the only element which has 
parameters that vary with temperature. The circuit comprises a current 
source, a feedback resistance, a series resistance, and the load which has 
parameters that vary with temperature. 
The present invention contemplates a circuit for self compensation of a 
non-linear resistive load that comprises a feedback resistance, a series 
resistance, the load itself such as Wheatstone bridge, an operational 
amplifier, a battery as reference voltage, and a current sampling 
resistor. Additionally, the instant invention eliminates the need for a 
thermally sensitive resistance to compensate the positive temperature 
coefficient of sensitivity of the bridge due to the compensating nature of 
the circuitry. The operational amplifier functions as a source of constant 
current for the bridge. This arrangement eliminates the need for a 
thermally sensitive resistor for compensating the positive temperature 
coefficient of the bridge, and leaves the bridge resistance R.sub.B as the 
only resistance that varies with temperature. This invention also 
eliminates a time lag between sensing temperature and correcting for 
temperature effects, since the circuit only has one element, the strain 
gauge sensor, whose parameters vary with temperature.

DETAILED DESCRIPTION OF THE INVENTION 
Reference is made to FIG. 2 for illustrating an electronic circuit for 
providing self compensation of a silicon strain gauge pressure transmitter 
in accordance with this invention. In FIG. 2 the output 11a of an 
operational amplifier 11 is connected to one node (input node) 12a of a 
strain gauge bridge 12 and an opposite node (output node) 12b of the 
strain gauge bridge 12 is connected to an inverting input 11b of the 
operational amplifier 11 through a series resistor R.sub.s 14. The output 
node 12b is additionally connected to one end of a current sampling 
resistance R.sub.c 15; where the other end of the current sample 
resistance R.sub.c 15 is connected to ground or some reference voltage 
point. A positive end of a reference voltage V.sub.ref 16 representing a 
voltage source such as a battery, is connected to the non-inverting input 
terminal 11c of the operational amplifier. A negative end of the battery 
is connected to ground or to the reference potential point. Additionally, 
a feedback resistance R.sub.f 13 is connected at one end to the inverting 
input 11b of the operational amplifier 11 and is connected at the other 
end to the output 11a of the operational amplifier 11. When constructed in 
this manner, the output voltage V.sub.o of the operational amplifier 11 is 
given by the expression: 
##EQU1## 
where R.sub.B is the equivalent resistance of the silicon strain gauge 
bridge 12. The equation shown above is for the output of the operational 
amplifier 11 applied on a power common. 
Further mathematical analysis will show that the bridge voltage, which is 
the voltage drop across R.sub.B, will follow the same curve shape as 
R.sub.B varies with temperature. The equation shows that the bridge 
excitation V.sub.o increases as the bridge resistance R.sub.B increases, 
thus compensating for the accompanying reduction in sensitivity inherent 
in silicon sensors. Additionally, the slope of the output voltage versus 
temperature can be changed by a selection process which chooses different 
values of R.sub.f, R.sub.c, and R.sub.s respectively. The result is that 
part of the output voltage V.sub.o across the strain gauge bridge 
resistance R.sub.B, attains a maximum voltage value while using a 
reasonable amount of electrical current. 
It is noted that in the instant invention only the non-linear resistance 
R.sub.B of the load or the silicon strain gauge bridge varies with the 
change in temperature. Additionally, further analysis will show that the 
voltage V.sub.B, the voltage across the load or the silicon strain gauge 
bridge, follows the same curve shape as R.sub.B varies with temperature. 
Furthermore, the instant invention compensates the reduction in 
sensitivity inherent in silicon sensors by increasing the bridge 
excitation voltage as bridge resistance increases. 
It is further noted that this invention teaches advantageously using 
commonly available and non-precision linear components to construct 
circuitry which achieves a suitable measurement accuracy of for example, 
+/-0.3% within the temperature range of -40.degree. C. to 85.degree. C. 
In general, this invention may be reduced to practice by a variety of 
means. In an embodiment of this invention, the following components may be 
used. The typical values of the components in an exemplary are listed 
below: 
The bridge 12 is a light-implant 10 K.OMEGA. Si bridge. The feedback 
resistance R.sub.f 13 is 60 K.OMEGA., the series resistance R.sub.s 14 is 
5 K.OMEGA., the current sample resistance R.sub.c is 1.3 K.OMEGA., and the 
reference voltage V.sub.ref 16 is one Volt. 
An example is now given to further illustrate the teachings of this 
invention. By example, one objective of the instant invention is to 
maintain a parameter, such as bridge sensitivity, as constant in value as 
possible. A further objective relates to span compensation, which is to 
provide the same amplified voltage span at wider temperature intervals. 
Similarly, room temperature could be used as a reference temperature for 
measuring the span. A span is defined as the output of the sensor (in mV) 
at full scale input, which is the pressure applied upon the sensor minus 
the output at zero input. As can be appreciated, the value of the span is 
relative. The output is measured between the positive and negative nodes 
of the 
Wheatstone bridge in FIG. 2. Furthermore, at any given temperature, the 
span is a function of the bridge excitation voltage, V.sub.B, measured in 
volts. The span may be expressed in the follow equation: 
EQU Span=V.sub.B .times.S(mV) 
where S is the full scale sensitivity of the strain gauge bridge measured 
in mV/V. 
The sensor is compensated as sensitivity drops over temperature by 
increasing the bridge voltage sufficiently to keep the equation balanced. 
The data listed below is for a nominal 10 K.OMEGA. bridge: 
______________________________________ 
Temperature Bridge Resistance 
Sensitivity 
-28.degree. C. 
8005.OMEGA. 11.2 mV/V 
+63.degree. C. 
11904.OMEGA. 
9.204 mV/V 
______________________________________ 
the functional relationship between bridge voltage and bridge resistance 
can be expressed in the following equation: 
##EQU2## 
where R.sub.B1 =bridge resistance cold 
S1=sensitivity cold 
R.sub.B2 =bridge resistance hot, and 
S2=sensitivity at hot. 
Exemplary values for R.sub.c and R.sub.s are as follows: 
R.sub.c =1300.OMEGA. 
R.sub.s =6000.OMEGA. 
The values of R.sub.s and R.sub.c can be arbitrarily set to optimize the 
circuit in other areas, and V.sub.ref cancels out. The equation can then 
be solved for R.sub.f. The value of R.sub.f in this example is 68.15 
k.OMEGA.. 
Regarding the above equation, it is noted that only the relative value of 
sensitivity is important, and not the actual or absolute value. It is 
further noted that the value of V.sub.ref can be used to scale the output 
since it too has no effect on the equation. Sensitivity of the sensor may 
vary by a factor of two to one in any given wafer lot, but the sensor's 
relative sensitivities, resistance values, and relative resistance values 
over temperature are tightly controlled by the production process. 
Therefore, span compensation can be achieved on a per lot basis over a 
wide temperature range, as shown above. 
It is further noted that the instant invention enables the use of lot 
compensation data for the compensation of component values. Additionally, 
the instant invention also tends to compensate for the non-linear 
characteristics of resistance and sensitivity of the sensor by introducing 
an "overbow" multiplier to the "under-bow" characteristic of the sensor. 
The instant invention teaches a circuitry comprising any current source 
that contribute to maintaining a constant current i through a load, such 
as the strain gauge bridge. The circuit includes a feedback resistance 
R.sub.f, and a series resistance R.sub.s which contribute to maintaining 
the constant current over a wide temperature range, and eliminates the 
need for the current sampling resistance R.sub.c to be a non-linear 
thermistor resistance. The circuit further includes a reference voltage 
supplying power to the current source to maintain the constant current, as 
shown in FIG. 3. 
Thus, while the invention has been particularly shown and described with 
respect to preferred embodiments thereof, it will be understood by those 
skilled in the art that changes in form and details may be made therein 
without departing from the scope and spirit of the invention.