Circuit arrangement for compensating for the temperature dependence of the sensitivity and the null point of a piezoresistive pressure sensor

A circuit arrangement for compensating for the temperature dependence of the sensitivity and of the null point of a piezoresistive pressure sensor having a temperature sensor thermally coupled thereto. The temperature sensor is coupled to an auxiliary amplifier, in the output circuit of which the feed diagonal of the pressure sensor is connected. The output diagonal of the pressure sensor is connected to an output amplifier, to which a temperature-dependent correction voltage can be applied on the input side via a resistor arrangement. To perform the compensation of the temperature dependence of the sensitivity independently of the temperature dependence of the null point, the resistor arrangement is connected directly to the output of the temperature sensor, preferably via jumpers.

BACKGROUND OF THE INVENTION 
The present invention relates to a circuit arrangement for compensating for 
the temperature dependence of the sensitivity and the null point of a 
piezoresistive pressure sensor in a bridge circuit having an auxiliary 
amplifier which is arranged thermally coupled to the pressure sensor and 
is connected to the latter, and in the output circuit of which the feed 
diagonal of the pressure sensor is connected, as well as having an output 
amplifier which is connected to the output diagonal of the pressure 
sensor, and to the input of which a temperature dependent correction 
voltage can be applied via a resistor arrangement. 
In a known circuit arrangement of this type ("Elektronik" 13/June 29, 1984, 
pages 95-98), a temperature sensor connected in series with a resistor 
forms a temperature feeler, to the output of which the input of an 
operational amplifier is connected. On the output side, the operational 
amplifier acts on a transistor which serves as a control element (chopper) 
and into the emitter-collector circuit of which the feed diagonal of the 
pressure sensor is connected in a bridge circuit; in this manner, a 
temperature-controlled voltage supply for the pressure sensor is made 
available, by which the temperature dependence of the sensitivity of the 
pressure sensor is compensated. For compensating for the temperature 
dependence of the null point of the pressure sensor in the known circuit 
arrangement, a resistor arrangement is provided which is connected on the 
one hand via resistors to one input of an output amplifier connected to 
the output diagonal of the pressure sensor and, on the other hand, to a 
fixed reference voltage; the reference potential of the output amplifier 
is made available by means of a further operational amplifier which is 
connected on the input side parallel to the feed diagonal of the pressure 
sensor, so that the compensation of the temperature behavior of the null 
point of the pressure sensor depends, in the known circuit arrangement, on 
the magnitude of the voltage at the feed diagonal of the pressure sensor. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a circuit 
arrangement for compensating for the temperature dependence on the 
sensitivity and the null point of a piezoresistive pressure sensor in a 
bridge circuit, wherein the compensation of the temperature dependence of 
the sensitivity can be performed independently of the compensation of the 
temperature dependence of the null point by decoupling the circuit parts 
required therefor from each other without reaction. 
According to the invention, the resistor arrangement, in a circuit 
arrangement of the type mentioned above, is coupled directly to the output 
of the temperature feeler. 
It is a substantial advantage of a circuit arrangement according to the 
invention that in it, due to the resistor arrangement being directly 
connected to the output of the temperature feeler, the compensation of the 
temperature dependence of the sensitivity of the piezoresistive pressure 
sensor can be performed independently of the compensation of the 
temperature dependence of the null point of the pressure sensor because 
both compensations take place without mutual influence due to the 
decoupling without reaction. Thereby, the time required for adjusting the 
circuit arrangement with respect to the desired overall compensation can 
be kept small, whereby the number of heating ovens required for the 
compensation can likewise be kept small, which in turn facilitates the 
mass production of pressure measuring transducers considerably. 
In the circuit arrangement according to the invention, it has been found to 
be advantageous if the resistor arrangement contains two voltage dividers 
of which one voltage divider is connected with its terminal end to an 
input of the output amplifier, and the other voltage divider is connected 
with its one terminal end to the other input of the output amplifier, and 
if, in dependence on the temperature behavior of the null point of the 
pressure sensor, either the first or the second voltage divider is 
connected via contacts, e.g., jumpers, directly with its other terminal 
end, to the output of the temperature feeler while the remaining voltage 
divider is connected with its other terminal end to the output of the 
temperature feeler via a balancing resistor. The jumpers allow taking into 
consideration from the start the temperature-dependent behavior of the 
null point of the respective pressure sensor, the trend of which is known, 
so that the compensation of the temperature dependence of the null point 
can be performed in a simple manner by changing the balancing resistor in 
a predetermined output temperature range of, for instance, 35.degree. C. 
to 45.degree. C. 
Piezoresistive pressure sensors usually have a heavily curved temperature 
dependence of the sensitivity as well as of the null point over the 
temperature range of interest. Therefore, it is, as a rule, necessary for 
compensation to subdivide the entire temperature range into several 
sections. A lower section of the entire temperature range can be 
compensated with respect to the temperature dependence of the null point 
in the circuit according to the invention, advantageously by the provision 
that, via further contacts at the taps of the voltage dividers, a further 
balancing resistor can be connected which is arranged in the 
collector-emitter circuit of a transistor to the reference potential of 
the output amplifier, and that the transistor can be switched by a control 
resistor connected to the output of the temperature sensor. The control 
resistor is connected so that it switches, upon reaching a predetermined 
temperature of, for instance, +10.degree. C., the resistor connected 
thereto, whereby then the further balancing resistor becomes effective and 
decreases after adjustment, the correction voltage present at the inputs 
of the output amplifier in the required manner. 
To compensate the pressure sensor as to the temperature dependence of its 
null point in a range of relatively high temperature, it is advantageous 
in the circuit arrangement according to the invention if, via additional 
contacts at the taps of the voltage dividers, an additional balancing 
resistor can be connected which can be activated by a threshold circuit 
connected to the output of the temperature sensor. If the temperature to 
which the pressure sensor is subjected reaches a relatively high value of, 
for instance, 45.degree. C., the threshold circuit responds due to the 
relatively high output voltage of the temperature feeler, whereby, after 
the additional balancing resistor is adjusted via this resistor, the 
temperature-dependent correction voltage at the inputs of the output 
amplifier is changed to the extent required. 
Due to the appropriate balancing of all balancing resistors in the 
temperature ranges mentioned, the temperature behavior of the null point 
of the pressure sensor can thus be compensated by means of the resistor 
arrangement described over a relatively large temperature range in 
sections. 
In the circuit arrangement according to the invention, the temperature 
behavior of the sensitivity of the pressure sensor is first compensated in 
a manner known per se in a predetermined output temperature range of, for 
instance 25.degree. C. to 45.degree. by the provision that, via an 
adjustable resistor between the output of the temperature feeler and the 
input of the auxiliary amplifier, a voltage occurs by means of an 
appropriate control of this amplifier at the input diagonal of the 
pressure sensor such as is required for compensating the sensitivity. This 
adjustment is made jointly with the compensation of the null point drift 
of the pressure sensor in this temperature range. 
To compensate for the sensitivity also in the range of relatively low 
temperatures, a correction resistor can be connected, according to the 
invention, at the input of the auxiliary amplifier which is arranged in 
the collector-emitter circuit of a further transistor, connected to the 
reference potential of the output amplifier, and the further transistor 
can be switched by the control transistor. In this manner, provision is 
made in an inexpensive manner and without reaction, and decoupled from the 
compensation of the temperature behavior of the null point, so that the 
sensitivity of the pressure sensor in the lower temperature range can be 
compensated, where this compensation takes place logically jointly with 
the compensation of the null point drift in this temperature range. 
To compensate for the temperature behavior of the sensitivity in an upper 
temperature range, a further correction resistor can advantageously be 
connected, in the circuit arrangement according to the invention, to the 
input of the auxiliary amplifier and via the threshold circuit, which 
correction resistor is in connection with the output of the temperature 
sensor. This correction resistor is therefore activated in dependence on 
the magnitude of the output voltage of the temperature sensor at a 
relatively high temperature and insures, after appropriate adjustment, the 
changing of the output voltage of the auxiliary amplifier or the voltage 
at the input diagonal of the pressure sensor in such a manner that the 
temperature behavior is compensated also in the upper temperature range. 
The threshold circuit of the circuit arrangement according to the invention 
advantageously contains two diodes which are poled in opposite directions 
and the common circuit point of which is kept at a predetermined 
potential; to the one diode is connected the additional balancing resistor 
and to the other diode, the further correction resistor. 
The temperature sensor of the circuit arrangement according to the 
invention advantageously contains at least one transistor as the 
temperature-measuring element which is followed by an impedance 
transformer. Such a design of a temperature sensor is characterized by 
high linearity and very little current drain, which is advantageous 
particularly in an application with measuring transformers in 
two-conductor technique.

DETAILED DESCRIPTION 
With reference to the drawing figure, the circuit arrangement shown serves 
for compensating for the temperature dependence of the sensitivity and the 
null point of a piezoresistive pressure sensor 1 which comprises several 
piezoresistive elements 2, 3, 4 and 5 in a bridge circuit. The pressure 
sensor 1 is connected with its input diagonal 6, 7 in the output circuit 
of an auxiliary amplifier 8 which is connected to a positive pole 9 and to 
a negative pole 10 of a supply voltage source, not shown. 
Besides the auxiliary amplifier 8, the circuit arrangement contains, among 
other things, a temperature feeler 11 which contains two transistors 12 
and 13 as the temperature-sensing elements and which at the same time form 
an amplifier. A temperature feeler is thereby obtained which is 
distinguished by a very small current drain. Transistor 12 serves as a 
temperature sensor while transistor 13 serves as an impedance matching 
device. The temperature feeler 11 is connected with one input 14 to the 
positive pole 9 and via a reistor 15 to the negative pole of the supply 
voltage source. A voltage dependent on the temperature of the temperature 
feeler 11 is present at an output 16 of the temperature feeler 11. This 
voltage of the temperature feeler 11 is a measure of the temperature to 
which the pressure sensor 1 is exposed, because the temperature feeler 11 
is thermally coupled, i.e., is in physical proximity, with the pressure 
sensor 1. 
A further component of the circuit arrangement shown is a resistor 
arrangement 17 which contains two voltage dividers 18 and 19. One voltage 
divider 18 is connected with its one terminal end 20 to one input 21 of an 
output amplifier 22, while the other voltage divider 19 is connected, with 
its one terminal end 23, to a further input 24 of the output amplifier 22. 
The other respective terminal ends 25 and 26 of the two voltage dividers 
18 and 19 are connected to a terminal point 27 via switching elements 28 
and 29 in dependence on the temperature behavior of the null point of the 
respective pressure sensor 1, the trend of which is known. The jumper 28 
is effective in the case of positive behavior, and the jumper 29 in the 
case of negative behavior of the null point. The circuit point 27 is 
connected permanently to the output 16 of the temperature feeler 11. The 
resistor arrangement 17 further contains a balancing resistor 30 which is 
connected with its one end permanently to the circuit point 27 or the 
output 16 of the temperature feeler 11, while its other end is brought to 
a circuit point 31; via a jumper 32 (in the case of positive temperature 
behavior) or a jumper 33 (in the case of negative temperature behavior), 
the balancing resistor 30 can be connected either in series with the one 
voltage divider 18 or in series with the other voltage divider 19. 
Each of the two voltage dividers 18 and 19 is provided with a tap 34 and 
35, respectively. To these taps 34 and 35, a further balancing resistor 38 
can be connected via further jumpers 36 and 37. This is accomplished by 
means of a transistor 39 which can be controlled by a preceding control 
transistor 40. This control transistor 40 is connected to a voltage 
divider which is formed by the resistors 41 and 42 and to which the 
voltage at the output 16 of the temperature feeler 11 is applied. The 
control transistor 40 is accordingly acted upon by a voltage depending on 
the temperature and switches the transistor 39 at temperatures (for 
instance, 10.degree. C., 15.degree. C., 20.degree. C.) which can be 
selected via jumpers 43, 44 and 45 assigned to different temperatures, in 
cooperation with resistors 46, 47 and 48. With the further balancing 
resistor 38, a compensation of the temperature behavior of the null point 
of the pressure sensor 1 is effected in a lower temperature range. 
In order to compensate for the temperature behavior of the null point of 
the pressure sensor 1 in an upper temperature range, a threshold circuit 
50 is provided which comprises, among other things, two diodes 51 and 52 
which are poled opposing each other. A common circuit point 53 of the two 
diodes 51 and 52 is maintained by a voltage divider including, among other 
things, resistors 54 and 55, at a predetermined potential. At the end 
facing away from the common circuit point 33, the diode 52 is connected to 
an additional balancing resistor 56 which is effective if the voltage at 
the output 16 is likewise relatively high due to a relatively high 
temperature. 
The auxiliary amplifier 8 already mentioned above is connected with its 
input 60 to the output of the temperature feeler 11 via an adjustable 
resistor 61 and a fixed resistor 62. In addition, there can be connected 
to the input 60 of the auxiliary amplifier 8 via a further transistor 63, 
a correction resistor 64 which can be connected via further jumpers 65, 66 
and 67 to the resistors 46 to 48. The further transistor 63 is connected, 
like the transistor 39, by the control transistor 40 at a relatively low 
temperature so that thereby the voltage at the input of the auxiliary 
amplifier 8 and thereby also the voltage fed from the output of the 
auxiliary amplifier 8 of the feed diagonal 6-7 of the pressure sensor 1 is 
corrected. 
A further correction resistor 68 is connected to the anode terminal of the 
diode 51 of the threshold circuit 50 and via fixed resistor 62, to the 
output 16 of the temperature feeler 11. In this manner, the temperature 
behavior of the sensitivity of the pressure sensor 1 in a higher 
temperature range can be compensated, because, via this further correction 
resistor 68, the input voltage of the auxiliary amplifier 8 is changed due 
to the connection via the resistor 55. 
In the adjustment of the circuit arrangement shown for compensating for the 
temperature dependence, the circuit arrangement is designed first in such 
a manner that it meets the stated requirements at a temperature of, say 
25.degree. C. Subsequently, the circuit arrangement with the pressure 
sensor 1 is subjected to a temperature of 45.degree. C. At this 
temperature, the balancing resistor 30 is changed on the one hand so that 
the temperature behavior of the null point remains constant in the 
temperature range from 25.degree. C. to 45.degree. C. and on the other 
hand, the adjustable resistor 61 is set so that also the sensitivity in 
this temperature range is constant. 
Above the temperature of 45.degree. C., the threshold circuit 50 responds 
by making the diodes 51 and 52 conductive. The additional balancing 
resistor 56 is now trimmed so that the null point remains constant also at 
this temperature of, for instance, up to 65.degree. C. Likewise, the 
further correction resistor 68 is trimmed, so that the null point remains 
constant also at this temperature of, for instance, up to 65.degree. C. 
Likewise, the further correction resistor 68 is adjusted so that the 
temperature behavior of the sensitivity remains constant in this 
temperature range. 
Below 25.degree. C., the transistors 39 and 63 are set in operation, where 
the set-in point can be adjusted by the jumpers 43 to 45 as well as 65 to 
67, for instance, to 10.degree. C., 15.degree. C., or 20.degree. C. By 
adjusting the further balancing resistor 38, the temperature behavior of 
the null point can be kept constant also in the range below 25.degree. C., 
while by adjustment of the correction resistor 64, the sensitivity can be 
maintained constant and independent of the temperature in this temperature 
range. 
The described adjustment work for compensating for the temperature behavior 
of the sensitivity and the null point of the pressure sensor 1 can be 
performed independently of each other with respect to the sensitivity of 
the null point, whereby the adjustment work can be carried out relatively 
simply and thereby, with little expenditure of time. 
In the foregoing specification, the invention has been described with 
reference to a specific exemplary embodiment thereof. It will, however, be 
evident that various modifications and changes may be made thereunto 
without departing from the broader spirit and scope of the invention as 
set forth in the appended claims. The specification and drawings are, 
accordingly, to be regarded in an illustrative rather than in a 
restrictive sense.