JPH04284644A - Piezosensor circuit - Google Patents

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezo sensor circuit for supplying a test current to a piezo resistor in a semiconductor piezo sensor such as a pressure sensor so that the detection characteristics of the sensor can be tested or diagnosed as needed.

0002

[Prior Art] As is well known, piezo elements made of semiconductors such as silicon have the advantage of being easy to miniaturize or miniaturize and have extremely high detection sensitivity. At the same time, it can be easily incorporated into a small semiconductor chip, so recently piezo sensors using this type of piezo sensor have been widely used as pressure sensors and acceleration sensors, for example.

However, although such semiconductor piezo sensors have very high sensitivity, they may be affected by contamination from the surrounding environment, and their detection characteristics may gradually change over a long period of use. Increasingly, devices are equipped with circuits for testing or diagnosing their detection characteristics at any time or automatically. Since a piezoresistor or a strain gauge is usually used as a piezoelectric element for a sensor, it is advantageous to carry out testing and diagnosis of the sensor with a test current being supplied to the piezoresistor. 3 shows a conventional example of a piezo sensor circuit equipped with the test function shown in FIG. 2. FIG.

As shown on the left side of FIG. 2, the main body of the sensor consists of, for example, four piezoresistors 11 to 14, and is connected to a bridge circuit 10 for high precision detection. As usual, with a voltage V applied to one diagonal of the bridge circuit 10, a differential voltage signal from the other diagonal is applied to the detection control circuit 20 via a high gain amplifier 21, and from this, pressure and acceleration are detected. The detection signal DS is taken out in analog form or in the form of digital detection data.

A test current circuit 70 is provided for testing the piezoresistors 11 to 14, and a test current I is applied to the bridge circuit 10 based on a test command TS. This test current I is applied to the piezoresistors 11 and 3 on one side of the bridge circuit 10, as shown in the figure, with the supply of voltage V to the bridge circuit 10 cut off.
It is preferable to apply it in parallel to the series circuit of the piezoresistors 12 to 14, which are the two sides. In this case, when the resistance value of each piezoresistor is r, a differential voltage of Ir/2 is applied to the amplifier 21, and when the amplification factor of the amplifier 21 is K, the detection signal D
Since S is supposed to be KIr/2, it can be determined whether the piezo sensor is healthy or not depending on whether the value is within a predetermined range.

In practice, a test function is built into the detection control circuit 20, and a test command TS is sent to the test current circuit 70 at the beginning of use of the piezo sensor or periodically.
Usually, the detection control circuit 20 determines whether the value of the detection signal DS is correct or not, and a test command TS is given to the test current circuit 70 from the outside especially when necessary.

[0007]

[Problem to be solved by the invention] By incorporating the piezo sensor circuit as described above into the piezo sensor,
In principle, it is possible to increase the reliability of the sensor by testing whether the piezoresistance is normal or not at any time or periodically, but in reality, the accuracy of testing the piezoresistance is at least higher than the detection accuracy of the piezo sensor. The problem is that it is difficult to obtain the accuracy required for testing piezoresistors.

The cause of this problem is that the resistance value of the piezoresistor made of semiconductor is temperature dependent, and since the test current circuit is also a semiconductor circuit, the test current is temperature dependent, and due to the synergistic effect of these, the detection signal DS is The value is highly variable depending on temperature. To solve this problem, for example, you can use a test voltage instead of the test current and generate it with a voltage divider circuit made of resistors to eliminate temperature dependence, but if the test object is a piezoresistor, you can actually It doesn't make much sense unless you test it under flowing conditions, and if the temperature dependence of piezoresistance remains the same, you'll be left out.

An object of the present invention is to solve this problem and provide a piezo sensor circuit that can reduce the temperature error of the test current applied to the piezo resistor and improve the test accuracy.

0010

[Means for Solving the Problems] According to the present invention, the present invention provides a piezoresistor for a sensor made of a semiconductor, a current mirror circuit that supplies a current driven by the piezoresistor as a test current, and a standard for the current mirror circuit. A semiconductor resistor that receives current and is made of a semiconductor of the same quality as the piezoresistor, a control circuit element that maintains a constant voltage drop due to the reference current of the semiconductor resistor, and a switch means that opens and closes the supply of the reference current to the current mirror circuit. This is achieved using a piezo sensor circuit.

Note that it is advantageous for the current mirror circuit to be of a resistance bias type using a semiconductor resistor as a bias resistor, and to set the driven current to be smaller than the reference current.

[0012] Furthermore, it is advantageous to construct the control circuit element by a transistor whose collector and emitter are connected in series with a semiconductor resistor and whose base potential controls the voltage drop within the semiconductor resistor to a constant level; is easiest to create using a voltage divider circuit using resistors.

[0013]

The present invention utilizes a current mirror circuit to solve the aforementioned problems. As is well known, a current mirror circuit has a current conversion function that generates a driven current that is exactly proportional to the reference current applied to it, but the present invention focuses on the fact that it can also have a voltage conversion function. The driven current of the current mirror circuit is applied to the piezoresistor as a test current, and a semiconductor resistor made of the same semiconductor as the piezoresistor is inserted in the current path of the reference current, and the voltage drop is controlled to be constant and proportional to it. A voltage drop is generated across the piezoresistor. Therefore, when the resistance value of the piezoresistor changes due to a change in the ambient temperature, the resistance value of the semiconductor resistor changes at the same time, and the reference current flowing through it changes, and a test current, which is a driven current proportional to this, flows through the piezoresistor. The change in resistance value due to temperature change is automatically compensated, and the voltage drop due to the test current of the piezoresistor, that is, the value of the detection signal of the piezo sensor during the test, is always maintained constant without being affected by changes in ambient temperature.

The control circuit element in the configuration described in the previous section is for controlling the voltage drop due to the reference current in the semiconductor resistor to a constant level, and the switch means controls the voltage drop for the piezo resistor by opening and closing the reference current in accordance with the test command. It starts and stops the test current.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention shown in FIG. 1 will be described below. FIG. 1 shows the test current circuit 70 in FIG. 2 previously described.
The figure shows a single piezoresistor 11 from which the test current I is supplied, but this can of course be replaced by the bridge circuit 10 of FIG.

In this embodiment, the current mirror circuit 30 shown surrounded by a dashed line in FIG. 1 is composed of a pair of pnp transistors 31 and 32, whose bases are commonly connected. On the other hand, in the illustrated example, the transistor 32 is diode-connected with its collector and base shorted. Transistor 31 is for reference current Ir, transistor 32 is for test current I which is a driven current,
The former is connected directly to the power supply potential point Vc, and the latter is connected via a resistor 34, respectively. In this embodiment, the test current I, which is the driven current, is set smaller than the reference current Ir, for example, by 1/1.
00, so the resistor 34 is a current limiting or current setting resistor for this purpose.

Note that the reference side transistor 31 and the driven side transistor 32 may be configured to have the same size, that is, the same emitter area, and the emitter current density of both transistors can be adjusted by setting the ratio of the reference current and the driven current as above. As is well known, by varying the power supply voltage V
It is possible to reduce fluctuations in the test current I, which is a driven current, with respect to fluctuations in c. Further, as shown in the figure, this test current I is passed through a piezoresistor 11 such as a semiconductor strain gauge through a transistor 33 attached to a current mirror circuit.
It is better to supply This transistor 33 returns the base current flowing from the emitter to the base of the reference side transistor 31 to the reference current Ir, thereby accurately maintaining the ratio of the reference current Ir to the test current I at a desired value.

The circuit shown in the lower left part of the figure is based on the reference current Ir.
In the example shown, the control circuit element 50 is used for setting and controlling the
, semiconductor resistor 40 , and switch means 60 are connected in this order. The semiconductor resistor 40 is a semiconductor of the same quality as the piezo resistor 11, and is usually a resistor built into the same chip.If the power supply voltage Vc is about several volts, it is set to have a resistance value of several tens of kilohms as a reference. Set the current Ir to about 0.1mA. The control circuit element 50 is for controlling the reference current Ir, and in this embodiment is composed of a single npn transistor.
As shown in the figure, there is a semiconductor resistor 40 between the collector and emitter.
connected in series with The switch means 60 is connected to the reference current Ir.
It is for opening and closing the test command TS and may be configured with an npn transistor and connected to the ground side as shown in the figure, for example.

In the present invention, the control circuit element 50 plays the role of controlling the voltage drop due to the reference current Ir of the semiconductor resistor 40 to a constant level, and in this embodiment, the reference voltage Vr is applied to the base of the control transistor for this purpose to perform this operation. make them do The base-emitter voltage of the control transistor is e5,
When the transistor of the switch means 60 is turned on, the collector
Assuming that the emitter voltage is E6, as can be easily seen, the reference voltage that should be applied to the base of this control transistor is V
r=RrIr+e5+E6. The control circuit element 50 having such a configuration operates to increase the reference current Ir when the resistance value Rr of the semiconductor resistor 40 decreases due to the influence of temperature, so as to always keep the voltage drop RrIr within the semiconductor resistor 40 constant. do. Note that since it is desirable that the temperature dependence of this reference voltage Vr be as small as possible, it is practical to create it through a resistive voltage divider circuit, for example.

In the piezo sensor circuit of the present invention configured as described above, the resistance value Rr of the semiconductor resistor 40 changes at the same time as the resistance value of the piezo resistor 11 shown in FIG. Since the voltage drop IrRr in the resistor 40 is controlled to be kept constant, the reference current Ir flowing therein changes, and the test current I, which is a driven current proportional to this, flows in the piezoresistor 11 to compensate for the change in resistance value. , the voltage drop across the piezoresistor 11 due to the test current I is always maintained constant regardless of temperature changes. Therefore, the piezoresistor 11 in FIG. 1 and the bridge circuit 1 in FIG.
The value of the detection signal DS during the 0 test is kept almost constant without being affected by temperature changes.

Note that in order to minimize the temperature dependence of this detection signal, the resistance value R of the above-mentioned setting resistor 34 for the test current I is
It is advantageous to optimize. As is well known in terms of the operation of the current mirror circuit 30, it is advantageous to select this resistance value R so as to satisfy the relationship dR/dT=R/T with respect to temperature T. The e5 and E mentioned above
6. Furthermore, since the temperature dependence of the amplifier 21 in FIG. 2 is somewhat related, the value of this resistor R is actually experimentally set so that the temperature dependence including all of them is minimized. When the reference voltage Vr is set as described above and the value of this resistor R is optimized, the variation in the value of the detection signal DS can be suppressed to about 0.04% within the temperature range of -40 to 140°C. .

[0022]

As explained above, the present invention provides a piezoresistor for a semiconductor sensor, a current mirror circuit that supplies a driven current of the piezoresistor as a test current, and a piezoresistor that receives a reference current of the current mirror circuit and has the same quality as the piezoresistor. The piezo sensor circuit is composed of a semiconductor resistor made of a semiconductor, a control circuit element that maintains a constant voltage drop due to the reference current of the semiconductor resistor, and a switch means that opens and closes the supply of the reference current to the current mirror circuit. Even if the resistance value of the piezoresistor changes, the resistance value of the semiconductor resistor changes at the same time, and the reference current flowing through it changes, and a driven current proportional to this is supplied to the piezoresistor as a test current, so the piezoresistor's resistance value changes. Changes in resistance due to changes in ambient temperature are automatically compensated, and fluctuations in the detection signal value of the piezo sensor during testing can be suppressed to 0.1% or less within the operating temperature range.

[0023] This makes it possible to accurately test or diagnose whether or not the piezo resistance is sound by applying a test command to the switch means of the piezo sensor circuit even for a high precision piezo sensor with a detection error of 1% or less. . By incorporating the circuit of the present invention into a piezo sensor and performing occasional or periodic performance tests, its long-term reliability can be greatly improved.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of essential parts of an embodiment of a piezo sensor circuit according to the present invention.

FIG. 2 is an overall circuit diagram of a piezo sensor circuit according to the present invention and prior art;

[Explanation of symbols]

10 Bridge circuit incorporating a piezoresistor 11 Piezoresistor 30 Current mirror circuit 40 Semiconductor resistor 50 Control circuit element or control transistor 60 Switch means or transistor 70 Test current circuit I Test current or driven current Ir of the current mirror circuit Standard of the current mirror circuit Current TS
test directive

Claims (3)

[Claims] Claim 1: A piezoresistor for a sensor made of a semiconductor;
A current mirror circuit that supplies the driven current as a test current to the piezo resistor, a semiconductor resistor made of the same semiconductor as the piezo resistor that receives the reference current of the current mirror circuit, and a voltage drop due to the reference current of the semiconductor resistor is kept constant. and switch means capable of opening and closing the supply of a reference current to the current mirror circuit, so that a test command can be given to the switch means at any time to supply a test current to the piezoresistor to test the sensor. A piezo sensor circuit characterized by: 2. The piezo sensor circuit according to claim 1, wherein the current mirror circuit is configured in a resistance bias type using a semiconductor resistor as a bias resistor, and the driven current is set smaller than the reference current. . 3. The piezo sensor according to claim 1, wherein the control circuit element is constituted by a transistor connected in series with a semiconductor resistor and whose base potential controls the voltage drop caused by the reference current to be constant. circuit.