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.
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.
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.