Capacitance detection system and method

Details Capacitance detection system and method Capacitance detection system and method

1999-10-08

2001-12-04

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Impedance, admittance or other quantities representative of...

Lumped type parameters

C324S658000, C324S669000

Reexamination Certificate

active

06326795

ABSTRACT:

TECHNICAL FIELD
The present invention generally relates to a capacitance-voltage conversion system and method, and more particularly to a system and method of detecting a capacitance of a sensor which varies in response to a physical amount applied to a sensor, by converting the sensor capacitance to a voltage.
BACKGROUND ART
FIG. 1
illustrates a schematic diagram of a capacitance variation detection circuit of a prior art described in Japanese Patent Public Disclosure (Kokai) No. 6-180336, to which a static capacitance of a sensor formed of a diaphragm and an electrode facing each other is connected. The static capacitance varies when the diaphragm moves in response to a physical pressure or the like applied thereto. The prior circuit shown in
FIG. 1
has been proposed to solve a problem that a voltage applied to a sensor comprising a diaphragm and an electrode opposite thereto causes the electrode to come in contact with the diaphragm in response to an electrostatic attractive force, when the sensor is formed through fine machining on a semiconductor.
In
FIG. 1
, the reference numerals
1
and
2
designate voltage input and output terminals of the capacitance variation detection circuit, respectively. An input voltage Vin is supplied to the input terminal
1
, and an output voltage Vout is output from the output terminal
2
. The reference numerals
3
denotes an operational amplifier,
4
and
5
resistors, and
6
a switch. The input terminal
1
is connected to an inverting and non-inverting input terminals of the operational amplifier
3
through the resistor
5
and a static capacitance of the sensor S, respectively. An output of the operational amplifier
3
is connected to the output terminal
2
and to the inverting input terminal through the resistor
4
. The non-inverting input terminal is grounded through the switch
6
.
In the detection circuit of
FIG. 1
, the switch
6
is closed during an initialization period to charge the sensor capacitance to the voltage Vin supplied to the input terminal
1
, and is opened when a measurement of the sensor capacitance is made. During the opening state of the switch
6
, since the capacitance of the sensor S is connected to the non-inverting input terminal of a high input impedance, the charge accumulated on the capacitance is not discharged. On the other hand, as a physical change is applied to the sensor S by varying pressure to the diaphragm forming the sensor S, for instance, the static capacitance of the sensor S changes, causing a change in a voltage across the sensor capacitance. This voltage change is amplified by the operational amplifier
3
, a gain of which is determined by the resistors
4
and
5
, and appears at the output terminal
2
.
In supplementing the foregoing description using equations, it should be assumed that resistances of the resistors
4
and
5
are Rf and Ri, the original static capacitance of the sensor S is Cs, and voltages at the non-inverting and inverting input terminals of the operational amplifier
3
are v
+
and v
−
, respectively. Now, when the switch
6
is closed, the output voltage Vout is expressed by the following equation:
Vout=−Vin*Rf/Ri
  (1)
Assuming that the sensor capacitance changes from Cs to Cs′ and the output voltage of the operational amplifier
3
changes from Vout to Vout′ after the switch
6
is opened for measurement, Vout′ is represented as follows:
Vout′={
1+[1+(
Rf/Ri
)](
Cs/Cs
′)
}*Vin
  (2)
Here, with
Vout′−Vout=&Dgr;V, and Cs′−Cs=&Dgr;Cs, &Dgr;V=[
1+(Rf/Ri)]* &Dgr;Cs/(Cs+&Dgr;Cs)*Vin  (3)
is satisfied between &Dgr;V and &Dgr;Cs.
As mentioned above, since the output voltage Vout varies in response to the gain of the amplifier (that is the ratio Rf/Ri of the resistors
4
and
5
) as well as the sensor capacitance Cs, it is not necessary to apply a high voltage as the input voltage Vin to the sensor capacitance.
With a low input voltage Vin, an electrostatic attractive force to the diaphragm may be relatively small. Therefore, the detection circuit shown in
FIG. 1
may solve the problem that the electrode comes in contact with the diaphragm due to the electrostatic attractive force.
SUMMARY OF THE INVENTION
However, it still implies another problem regarding a parasitic capacitance in the prior detection circuit illustrated in FIG.
1
. That is, a parasitic capacitance Cp is generally formed at a point connecting the sensor S to the operational amplifier
3
. The parasitic capacitance Cp is connected to the switch
6
in parallel, and may be in a range from about one to a hundred pF, when the sensor S and the operational amplifier
3
are implemented on separate chips. On the other hand, the sensor capacitance Cs may be in a range from one to several hundreds fF. In taking such a parasitic capacitance into account, when the sensor capacitance Cs changes, the charge of the capacitance Cs is distributed to the parasitic capacitance Cp. Accordingly, a change in a voltage across the sensor capacitance Cs becomes extremely small, thus resulting in deteriorated noise immunity.
A voltage displacement &Dgr;v
+
at the non-inverting input terminal of the amplifier
3
is expressed by the following equation:
&Dgr;v
+
=(v
+
−Vin)*&Dgr;Cs/(Cp−Cs−&Dgr;Cs)  (4)
In the equation (4), since &Dgr;Cs/(Cp−Cs−&Dgr;Cs) is one per several hundreds, &Dgr;v
+
is also one per several hundreds, thus taking an extremely small value. In order to obtain a large voltage displacement &Dgr;v
+
, it is considered that the input voltage Vin to the operational amplifier
3
is increased and/or a sensibility of the sensor S is increased.
However, in case that the input voltage Vin is increased, it causes a problem that the diaphragm and electrode may contact each other, as mentioned above. On the other hand, if a gain of the amplifier is increased to obtain a large sense sensibility, it may cause the output voltage Vout to saturate, resulting in no change in the output voltage Vout even though the sensor capacitance is varied. If the input voltage Vin supplied to the operational amplifier having a large gain is reduced to prevent the output voltage Vout from saturation, another problem arises in that a control of such a change in a small input voltage itself may complicated and difficult.
The present invention has been made to solve problems as mentioned above inherent to a prior detection circuit as shown in FIG.
1
. Therefore, it is an object of the present invention to provide a capacitance detection system capable of deriving an output voltage which varies in response to a sensor capacitance if a parasitic capacitance exists.
Another object of the present invention is to provide a capacitance is detection system capable of deriving an output voltage which is substantially proportional to a sensor capacitance if a parasitic capacitance exists.
The capacitance detection system according to the present invention is usable as a static capacitance detection circuit for detecting a static capacitance of a sensor comprising a diaphragm and an electrode facing each other, which varies in response to a physical variation applied to the sensor.
In order to achieve the objects, a capacitance detection system according to the present invention which provides an output corresponding to a capacitance of a sensor comprising (a) a voltage input connected to receive an input voltage which is changed, and (b) an operational amplifier having an inverting input connected to the voltage input through a first resistor, a non-inverting input connected to the voltage input through a sensor and to a reference voltage through a first switch, and an output connected to the inverting input through a circuit including a second resistor and a second switch connected in parallel to each other.
It is preferable for the capacitance detection system to further compr

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