Electricity: power supply or regulation systems – Including an impedance
Reexamination Certificate
1999-10-04
2001-02-27
Nguyen, Matthew (Department: 2838)
Electricity: power supply or regulation systems
Including an impedance
C323S369000
Reexamination Certificate
active
06194888
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an impedance-to-voltage converter and an associated converting method which are capable of highly accurate conversion of an impedance into a voltage utilizing an operational amplifier in an imaginary short-circuit state.
BACKGROUND ART
FIG. 1
generally illustrates the configuration of a static capacitance-to-voltage converter described in Laid-open Japanese Patent Application No.
61-14578
. This static capacitance-to-voltage converter has been proposed to solve a problem of the prior art which suffers from the inability of accurate voltage conversion due to the fact that a stray capacitance of a cable used to connect with an unknown static capacitance is superimposed on the unknown static capacitance, and that these static capacitances may vary due to movements and bending of the cable or the like. As illustrated in
FIG. 1
, an alternate current (AC) signal generator OS and an operational amplifier OP are connected with an unknown capacitance Cx whose cables are covered with shielding lines s to reduce the influence of stray capacitances Csl, Cs
2
, Cs
3
. Specifically, an output and an inverting input of the operational amplifier OP are connected through a feedback circuit formed of a parallel circuit including a resistor Rf and and a capacitor Cf. The unknown capacitance Cx has one end connected to the inverting input of the operational amplifier OP through a shielding line s, and the other end connected to the AC signal generator OS through another shielding line s. Both of the shielding lines s and a non-inverting input of the operational amplifier OP are grounded.
With the configuration described above, since substantially no potential difference exists between the two inputs of the operational amplifier OP, the stray capacitance Cs
2
is not charged. Also, since the stray capacitance Cs
3
is regarded as a coupling capacitance of both the shielding lines s, the stray capacitance Cs
3
can be eliminated by grounding the shielding lines s. In this way, the influence exerted by the stray capacitances of the cables for connecting the unknown capacitance Cx is reduced by using the shielding lines s, so that a charge equal to that induced on the unknown static capacitance Cx is induced on the capacitor Cf of the feedback circuit, resulting in an output proportional to the unknown static capacitance Cx produced from the operational amplifier OP. Stated another way, assuming that an output voltage of the AC signal generator OS is Vi, an output voltage Vo of the operational amplifier OP is expressed by −(Cx/Cf)Vi, so that the converter of
FIG. 1
may be used to convert the unknown static capacitance Cx into the voltage Vo from which the unknown static capacitance Cx can be derived together with the known values Cf and Vi.
SUMMARY OF THE INVENTION
The known static capacitance-to-voltage converter described above, however, implies a problem that as an unknown static capacitance Cx is smaller, the influence of stray capacitances becomes prominent, so that the static capacitance Cx cannot be accurately converted into a voltage. In addition, since the feedback circuit of the operational amplifier OP is formed of a parallel circuit including the resistor Rf and the capacitor Cf, separate steps are required to form a resistor and a capacitor for actually integrating necessary components into a converter in a one-chip form, causing disadvantages of a complicated manufacturing process and an increased chip size. Furthermore, since the capacitor cannot be applied with an AC signal when one electrode of the static capacitance Cx is being biased at a certain potential, a conversion of the static capacitance Cx into a voltage cannot be performed.
The present invention has been proposed to solve the problems mentioned above, and it is therefore an object of the present invention to provide an impedance-to-voltage converter and an associated converting method which are capable of highly accurate conversion of an impedance into a voltage utilizing an operational amplifier in an imaginary short-circuit state to eliminate the influence of stray capacitances between a line connected to a non-inverting input of the operational amplifier and a shielding line surrounding the line.
To achieve the above object, the present invention provides an impedance-to-voltage converter which comprises:
an operational amplifier having an inverting input, a non-inverting input and an output, said operational amplifier placed in an imaginary short-circuit state between said inverting input and said non-inverting input;
an impedance element connected between said output and said inverting input containing a connection line having one end connected to said impedance element, and the other connected to said inverting input;
a circuit element having a known impedance;
a signal line having one end connected to said inverting input, and the other connected to said circuit element;
a shield surrounding at least a portion of said signal line and/or said connection line;
an alternate current voltage generator connected to said non-inverting input.
Also, to achieve the above object, the present invention provides a method of converting an impedance into a voltage to gain alternate current voltage corresponding to the change of the impedance of impedance element, comprising the step of:
providing an operational amplifier having an inverting input, a non-inverting input and an output;
connecting an impedance element between said inverting input and said output;
connecting a circuit element having a known impedance to said inverting input;
providing shield for surrounding at least a portion of a connection line connected between said impedance element and said inverting input, and/or a signal line connected between said circuit element and said inverting input;
connecting said shield and said non-inventing input;
applying said non-inverting input with an alternate current voltage.
The impedance element may be any of a variety of sensors including a strain sensor, a geomagnetic sensor, a capacitive sensor and so on, and the impedance of such an impedance element may be at least one of resistance, inductance, capacitance, and conductance of a transistor.
The shield preferably surrounds whole of said signal line and said connection line.
In the present invention, the alternate current voltage output from the operational amiplifier may be integrated to output a direct current voltage representative of the impedance of the impedance element.
Since the operational amplifier is in an imaginary short-circuit state between the inverting input and the non-inverting input, it is possible to eliminate a stray capacitance between the connection line for connecting an impedance element to the inverting input and the shield surrounding the connection line, and a stray capacitance formed between the signal line and the shield surrounding the signal line. Therefore, an alternate current voltage corresponding to the impedance of the impedance element is output from the operational amplifier without suffering from such stray capacitances between the connection line, the signal line and the shield, however long they are.
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Monticelli et al. “Op-amp circuit measures diode-junction capacitance” Electronics Week, vol. 48
Harada Muneo
Hirota Yoshihiro
Matsumoto Toshiyuki
Barnes & Thornburg
Nguyen Matthew
Sumitomo Metal Industries Limited
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