JP2002098712A - Capacitive type physical quantity detection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To change easily a detection range of a physical quantity. SOLUTION: In this capacitive type physical quantity detection device equipped with a sensor element 1 having the capacity changing corresponding to the change of the physical quantity and a C-V conversion circuit 3 for converting the capacity change of the sensor element 1 into a voltage, the C-V conversion circuit 3 with a switched capacitor constitution has an operation amplifier 31, a switch S0 installed between input-output terminals of the operation amplifier 31, plural feedback capacitors 321-324, and switches S1-S5 for selecting the feedback capacitor connected in parallel with a switch S0 from among the plural feedback capacitors 321-324. An external signal is decoded by a decoder 7 and one of the switches S1-S5 is switched to the ON state, to thereby connect in parallel the desired feedback capacitor with the switch S0.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an acceleration, an angular velocity,
The present invention relates to a capacitance-type physical quantity detection device that detects a physical quantity such as pressure.

[0002]

2. Description of the Related Art Conventionally, as a device of this type, a carrier wave is applied to a fixed electrode, and a change in capacitance between the fixed electrode and the movable electrode caused when the movable electrode is displaced by a change in physical quantity is determined by a CV conversion circuit. There is a device in which a physical quantity is detected by converting to a voltage (see Japanese Patent Application Laid-Open No. 8-135717). The CV conversion circuit includes an operational amplifier having a movable electrode connected to an inverting input terminal, a switch provided between the inverting input terminal and the output terminal of the operational amplifier, and a feedback capacitor connected in parallel to the switch. A switched capacitor configuration having the following is used.

[0003]

In the above physical quantity detection device, the physical quantity detection range is fixed.
Therefore, when a physical quantity detection device having a different detection range is required, a separate device must be prepared. For example, when a plurality of acceleration sensors are installed in a vehicle using the physical quantity detection device as an acceleration sensor for an airbag, the detected acceleration differs depending on the detection target, for example, ± 10 to 30 G acceleration, ± 30 to 50 G acceleration. , ±
If it is necessary to detect an acceleration of 100 to 150 G and an acceleration of ± 200 G or more, a plurality of acceleration sensors set in respective detection ranges must be prepared. In this case, if a different acceleration sensor is used for each specification of the detection range, a new development of the acceleration sensor is required for each specification, which causes a problem that the number of development steps is increased.

The present invention has been made in view of the above problems, and has as its object to make it possible to easily change the detection range of a physical quantity.

[0005]

In order to achieve the above object, according to the first aspect of the present invention, a sensor element (1) whose capacity changes in accordance with a change in a physical quantity, and a capacitance change of the sensor element (1). And a CV conversion circuit (3) for converting the CV into a voltage.
The -V conversion circuit (3) has a switched capacitor configuration, and includes an operational amplifier (31) and an operational amplifier (3).
Switch means (S0) provided between input / output terminals of 1)
A plurality of feedback capacitors (321 to 324); and a selecting means (33) for selecting a feedback capacitor connected in parallel to the switch means (S0) from the plurality of feedback capacitors (321 to 324). And

According to the present invention, the detection range of the physical quantity can be easily changed by selecting the desired feedback capacitor connected in parallel with the switch means (S0) by the selection means (33).

The selection by the selection means (33) can be performed based on an external signal, as will be described later in an embodiment.

According to the second aspect of the present invention, the plurality of feedback capacitors (321 to 324) are formed on a semiconductor chip, and the plurality of feedback capacitors (321 to 324) are
It is characterized in that one common capacitor pattern is used and arranged in parallel so that each is formed as a feedback capacitor having a different capacity from the others.

According to the present invention, a plurality of feedback capacitors (321 to 324) can be formed using one common capacitor pattern, and a plurality of feedback capacitors (3 to 324) can be formed.
21 to 324) can be made more accurate.

According to the third aspect of the present invention, the selecting means (3
The feature 3) is characterized in that semiconductor switches of the same pattern connected in series to the respective common capacitor patterns are formed on a semiconductor chip.

According to the present invention, by forming the semiconductor switches with the same pattern, the influence of the parasitic capacitance of the switches can be eliminated.

The reference numerals in parentheses of the above-mentioned means indicate the correspondence with the concrete means described in the embodiments described later.

[0013]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a first embodiment of the present invention. FIG. 1 shows a configuration of a capacitive acceleration sensor according to an embodiment of the present invention. This acceleration sensor includes a sensor element 1 and a detection circuit 2.

The sensor element 1 comprises a fixed electrode 1a to which a carrier wave P1 is applied, and a carrier wave P2 having a phase opposite to that of the carrier wave P1.
And a movable electrode 1b that is displaced in proportion to the acceleration. The sensor element 1 operates as a differential capacitance. When the movable electrode 1b changes due to acceleration, the capacitance of the sensor element 1 changes. That is, the capacitance between the fixed electrode 1a and the movable electrode 1b is Ca, and the capacitance between the fixed electrode 1c and the movable electrode 1b.
If the capacity between b is Cb, the difference in capacity (Ca-Cb)
Changes in proportion to the acceleration.

The detection circuit 2 includes a CV conversion circuit 3, a sample and hold circuit 4, a low-pass filter (LPF) 5,
, An amplifier circuit (AMP) 6 and a decoder 7.

The CV conversion circuit 3 includes the sensor element 1
This converts the change in capacitance into a voltage, and includes an operational amplifier (operational amplifier) 31, a switch S0, and a feedback capacitance unit 3.
2 and a switch unit 33. The movable electrode 1b is connected to the inverting input terminal of the operational amplifier 31, the voltage of 1/2 of the power supply voltage Vdd is applied to the non-inverting input terminal, and the switch S0 is connected between the inverting input terminal and the output terminal. ing. The feedback capacitance unit 32 includes four feedback capacitors (capacitances of Cf1, Cf2, Cf3, and Cf1, respectively).
f4) 321 to 324, and any one of the switches S1, S2, S in the switch unit 32
When the switch S3 is turned on, the switch S0 is connected in parallel. One of the switches S1, S2, S3, and S4 is turned on by a signal from a decoder 7 described later. Hereinafter, the operation of the CV conversion circuit 3 will be described assuming that the capacitance of the feedback capacitor connected in parallel to the switch S0 is Cf.
This will be described with reference to the timing chart shown in FIG.

First, in the first period φ1, the carrier P1 is at the high level, the carrier P2 is at the low level, and the switch S0 is on. In this state, the movable electrode 1b
, A voltage of Vdd / 2 is applied, and the charge of the feedback capacitor is discharged. Charges of Q1 = −Ca · Vdd / 2 accumulate between the fixed electrode 1a and the movable electrode 1b, and Q2 = C between the fixed electrode 1c and the movable electrode 1b.
The charge of b · Vdd / 2 is accumulated.

In the second period φ2, the voltage levels of the carrier waves P1 and P2 are inverted, and the switch S0 is turned off. At this time, Q1 ′ is provided between the fixed electrode 1a and the movable electrode 1b.
= Ca · Vdd / 2 and Q2 ′ = − Cb · Vdd / 2 between the fixed electrode 1c and the movable electrode 1b. Therefore, the difference ΔQ between the charge (Q1 + Q2) accumulated on the movable electrode 1b at φ1 and the charge (Q1 ′ + Q2 ′) accumulated at the movable electrode 1b at φ2 is ΔQ =
(Q1 + Q2)-(Q1 '+ Q2') =-(Ca-C
b) · Vdd

Here, if the capacitances Ca and Cb are different, a charge of ΔQ is generated on the movable electrode 1b, but the voltage of the movable electrode 1b becomes Vdd by the operation of the operational amplifier 31.
/ 2, the charge of ΔQ accumulates on the movable electrode 1b side of the feedback capacitor, and the opposite polarity charge ΔQ ′ = (Ca−Cb) · Vd is applied to the electrode on the opposite side of the feedback capacitor.
d accumulates. As a result, the output voltage of the operational amplifier 31 becomes Δ
Q ′ / Cf + Vdd / 2. Therefore, the operational amplifier 3
1 outputs a voltage proportional to (Ca−Cb) / Cf.

The output voltage of the CV conversion circuit 3 is sampled and held by a sample and hold circuit 4. The sample and hold circuit 4 includes a switch S5, a capacitor 41,
The switch S5 is turned on during a period φ2 to sample and hold the output voltage of the CV conversion circuit 3, as shown in FIG.

The voltage sampled and held by the sample hold circuit 4 is output from an output terminal Vout through a low pass filter 5 and an amplifier circuit 6.

In this embodiment, the capacity of the feedback capacitor can be changed by an external signal. That is, an external signal input from the external input terminals A0 and A1 is decoded by the decoder 7, and one of the switches S1 to S4 in the switch unit 3 is turned on by a signal from the decoder 7, and one of the feedback capacitors 321 to 324 is turned on. Is selected. In the decoder 7, decoding is performed according to the truth table shown in FIG.

Here, as the capacitances Cf1 to Cf4 of the feedback capacitors 321 to 324, Cf2 = 2Cf1, Cf3
= 3Cf1, Cf4 = 4Cf1, and if Cf1 is set to a value that can be detected up to 50G, the output voltage of the operational amplifier 31 becomes a voltage proportional to (Ca−Cb) / Cf. Up to 100G, C
If f3 is selected, up to 150G, if Cf4 is selected, 2G
The acceleration sensor can detect up to 00G.

When the detection circuit 2 is formed on a semiconductor chip, the feedback capacitor 32 and the switch 33 are
It can be formed in a pattern as shown in FIG. That is, the feedback capacitor 321 is formed in a Cf1 pattern (common capacitor pattern), and the switch S1
Are connected in series. The feedback capacitor 322 is formed by arranging two same patterns as Cf1 in parallel, and connecting the pattern of the switch S1 to each of them in series. Further, regarding the feedback capacitor 323, Cf
The same pattern as that of No. 1 is formed by arranging three in parallel, and the pattern of the switch S1 is connected to each of them in series. Also,
The feedback capacitor 324 is formed by arranging four same patterns as Cf1 in parallel,
1 are connected in series.

In such a configuration, when the feedback capacitor 321 is selected, two switches S1 connected to the two patterns of Cf1 are simultaneously turned on, and when the feedback capacitor 323 is selected, Cf1 is selected.
When the feedback capacitor 324 is selected, the four switches S1 connected to the four patterns of Cf1 are simultaneously turned on.

With this configuration, the feedback capacitor 3
22, 323, and 324, the feedback capacitor 321 is also used.
Since the same Cf1 pattern can be used,
The accuracy of the capacitance ratio of the feedback capacitors 321 to 324 can be improved. Further, since the switches S1 to S4 are also formed in one common S1 pattern, the influence of the parasitic capacitance of the switches can be eliminated.

In the above-described embodiment, the generation of the carrier waves P1 and P2 and the turning on and off of the switches S0 and S5 are shown in the period φ1 and the period φ2.
In addition to the period φ2, another period, for example, a period for performing a self-diagnosis may be included.

In the sensor element, the fixed electrodes 1a and 1c arranged opposite to the movable electrode 1b are
It is preferable to provide one, but it is also possible to configure one.

Further, a plurality of feedback capacitors 321 to 32
4, when selecting a desired feedback capacitor, the selection is not limited to the selection by the switches S1 to S5 connected in series with the plurality of feedback capacitors 321 to 324.
A desired feedback capacitor may be selected by another configuration.

Further, the present invention provides an angular velocity,
The present invention can also be applied to a device that detects a physical quantity such as pressure.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a capacitive acceleration sensor according to an embodiment of the present invention.

FIG. 2 shows carrier waves P1, P2 and switches S0, S in FIG.
6 is a timing chart showing the operation of the fifth embodiment.

FIG. 3 is a table showing a decoding process of a decoder 7 in FIG. 1;

FIG. 4 is a diagram showing a specific configuration of a feedback capacitance unit 32 and a switch unit 33 in FIG. 1;

[Explanation of symbols]

1: sensor element, 1a, 1c: fixed electrode, 1b ...
Movable electrode, 2 detection circuit, 3 CV conversion circuit, 4 sample-hold circuit, 5 low-pass filter, 6 amplifier circuit, 7 decoder, 31 operational amplifier, 32 feedback capacitor, 33 switch Parts, S0 to S5... Switches (switch means).

Claims (3)

[Claims] 1. A sensor element (1) whose capacity changes according to a change in a physical quantity, and the sensor element (1)
And a CV conversion circuit (3) for converting a change in capacitance into a voltage, wherein the CV conversion circuit (3) has a switched capacitor configuration and includes an operational amplifier (3). 31), a switch means (S0) provided between the input and output terminals of the operational amplifier (31), and a plurality of feedback capacitors (321 to 324).
And a selecting means (33) for selecting a feedback capacitor connected in parallel to the switch means (S0) from the plurality of feedback capacitors (321 to 324). A plurality of feedback capacitors (321 to 3);
24) is formed on a semiconductor chip, and the plurality of feedback capacitors (321 to 324) use one common capacitor pattern, and by arranging them in parallel, each becomes a feedback capacitor having a different capacity from the others. The capacitance type physical quantity detection device according to claim 1, wherein the capacitance type physical quantity detection device is formed as described above. 3. The semiconductor chip according to claim 2, wherein said selecting means is formed on said semiconductor chip as semiconductor switches of the same pattern connected in series to each of said common capacitor patterns. Capacitive physical quantity detector.