文档介绍：? 1998 Microchip Technology Inc. DS00684A-page 1 M AN684 INTRODUCTION There is a variety of temperature sensors on the market all of which meet speci?c application needs. The mon sensors used to solve these application prob- lems include the thermocouple, Resistive Temperature Detector (RTD), Thermistor, and silicon based sensors. For an overview parison of these sensors, refer to Microchip’s AN679, “Temperature Sensing Technologies”. This application note focuses on circuit solutions that use thermocouples in the design. The signal condition- ing path for the thermocouple system will be discussed in this application note followed plete application circuits. THERMOCOUPLE OVERVIEW Thermocouples are constructed of two dissimilar metals such as Chromel and Constantan (Type E) or Nicrosil and Nisil (Type N). The two dissimilar metals are bonded together on one end of both wires with a weld bead. This bead is exposed to the thermal environment of interest. If there is a temperature difference between the bead and the other end of the thermocouple wires, a voltage will appear between the two wires at the end where the wires are not soldered together. This voltage monly called the thermocouple’s Electromotive Force (EMF) voltage. This EMF voltage changes with temperature without any current or voltage excitation. If the difference in temperature between the two ends (the weld bead versus the unsoldered ends) of the thermo- couple changes, the EMF voltage will change as well. There are as many varieties of thermocouples as there are metals, but binations work better than others. The list of thermocouples shown in Table 1 are most typically used in industry. Their behaviors have been standardized by the National Institute of Stan- dards and Technology (NIST).The particular document from anization that is pertinent to thermocou- ples is the NIST Monograph175, “Temperature-Electro- motive Force Reference Functions and Tables for the Letter-Designated Thermocouple Types Base