Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2002-08-02
2004-07-27
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S691000, C324S525000
Reexamination Certificate
active
06768318
ABSTRACT:
FIELD OF INVENTION
The present invention relates to electronic circuits. More particularly, the present invention relates to a signal conditioning circuit for determining the resistance of resistive sensors, including thermistors.
BACKGROUND OF THE INVENTION
There are types of resistors that have a resistance characteristic that varies with respect to changes to a certain property. The resistance can then be used to measure that property. For example, a thermistor has a resistance that varies with temperature. Thus, the thermistor can be used to measure temperature by measuring the resistance of the thermistor. There are other types of resistors available that are sensitive to different variables, such as pressure or light.
One prior art system for measuring a resistance is illustrated in circuit
100
of FIG.
1
. Circuit
100
features an excitation circuit and an amplifier section. The excitation circuit is configured to excite a resistive sensor and a reference resistor, while the amplifier is configured to output a result that is proportional to the difference in resistance between the reference resistance and the resistance of the sensor.
The excitation circuit comprises a voltage source
102
, a resistor
110
and a resistor
112
, a set resistor (or reference resistor)
114
, and a resistive sensor, e.g., a thermistor
116
. Resistors
110
and
112
may be identical in configuration, i.e., matched resistors, such that known biases are applied to set resistor
114
and thermistor
116
. This bias of set resistor
114
creates a voltage that is propagated to an input terminal
125
of instrumentation amplifier
120
. The current flowing across thermistor
116
creates a voltage that propagates to an input terminal
123
of instrumentation amplifier
120
.
The amplification section comprises an instrumentation amplifier
120
. Instrumentation amplifier
120
is typically configured as a differential amplifier that amplifies the difference in voltage between the voltage at input terminal
123
and the voltage at input terminal
125
and generates a signal at the output terminal
124
of instrumentation amplifier
120
. This voltage difference is proportional to the difference in resistance between thermistor
116
and set resistor
114
. A typical instrumentation amplifier may have a gain of approximately 100. Set resistor
114
has a known resistance, while the temperature/resistance characteristics of thermistor
116
and the gain of instrumentation amplifier
120
are also known. Due to these known characteristics, the temperature being sensed by thermistor
116
can be calculated. However, a significant drawback of circuit
100
is that it is important for resistors
110
and
112
to be matched to provide a known bias, often requiring expensive precision resistors to be included.
An alternative layout for a prior art circuit
400
of measuring a resistance is shown in
FIG. 4
, where the excitation of set resistor
414
and thermistor
416
is accomplished through the use of current sources
410
and
412
, i.e., voltage source
102
, and resistors
110
and
112
are replaced with current sources
410
and
412
. However, there may be difficulty in matching current sources
410
and
412
to provide known, equal currents.
The measurement of temperature can be important in a variety of applications. For example, one use of thermistors is in the field of optical networking. An optical network system may use lasers to transmit light through a fiber optic cable. The lasers are typically kept at a predetermined temperature, in order to have the laser transmit light of a predetermined wavelength. One method that can be used to control the temperature is to use a thermoelectric cooler and a thermistor mounted on the laser diode. The thermistor will change in resistance when there is a change in temperature. The thermistor may be coupled to the thermoelectric cooler in such a way that the amount of cooling increases when the temperature becomes too high and decreases when the temperature lowers to a desired level. However, prior art measurements systems for such applications can be quite complex.
There is a desire for a simpler and more compact system and method for testing and/or measuring the resistance in resistive sensors. In addition, to determine the difference between a set resistor and a thermistor or other resistive sensor, it would be desirable to have the currents exciting the set resistor and the resistive sensor be as closely matched as possible, i.e., to minimize the difference in excitation sources, without requiring precision resistors, matched resistors, or the difficult matching of current sources.
SUMMARY OF THE INVENTION
The method and circuit according to the present invention addresses many of the shortcomings of the prior art. In accordance with one aspect of the present invention, a circuit is provided that can facilitate accurate resistance measurements.
In accordance with an exemplary embodiment of the present invention, a self-contained signal conditioning circuit can be provided that contains a mechanism for testing and/or measuring resistance in a resistive sensor by connecting the resistive sensor and a reference resistor, e.g., a set resistor, to the self-contained signal conditioning circuit. Such a signal conditioning circuit may contain an amplification stage coupled to the set resistor, with a similarly configured amplification stage coupled to the resistive sensor. The current being supplied to the set resistor and to the resistive sensor can be monitored and the difference between the amount of current being supplied to the set resistor and the amount of current being supplied to the resistive sensor can be sensed. This difference in current is proportional to the difference in resistance between the set resistor and the resistive sensor. This difference in current may be converted to a voltage signal, if so desired.
REFERENCES:
patent: 4232271 (1980-11-01), Dobkin et al.
patent: 4833422 (1989-05-01), Atwell
patent: 5075633 (1991-12-01), Bowers
patent: 5276405 (1994-01-01), Mazzucco et al.
“Novel Current-Mode Instrumentation Amplifier,” Electronics Letters, Feb. 2, 1989, vol. 25, No. 3, pp 228-230, Feb. 1989.
“Low Noise, Precision Instrumentation Amplifier,” Analog Devices, inc., 1998, pp 1-22, no month available.
Botker Thomas L.
Brown John M.
Burt Rodney T.
Brady W. James
Nguyen Vincent Q.
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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