Project Analog - January 2008 - (Page 11) the furthest end of the thermocouple away from the solder bead will need to be measured. This reference temperature is usually sensed using an RTD, a thermistor or a silicon-based temperature sensor. rtd The RTD is a resistive element constructed from metals such as platinum, nickel or copper. The particular metals that are chosen exhibit a predictable, relatively linear change in resistance with temperature. Additionally, these metals have the basic physical properties that allow for easy fabrication. The temperature coefficient of resistance of these metals is large enough to render measurable changes with temperature. Unlike thermocouples, a RTD circuit requires a constant current source for biasing, along with an instrumentation circuit in order to convert the change in resistance to a voltage or frequency prior to digitization. RTDs have excellent repeatability and can provide a precise temperature-monitoring solution over a wide temperature range. Tradeoffs 11 · projeCt analog · jan 08 include cost, design complexity and increased system power consumption due to the presence of multiple active components. therMiStor Thermistors are made using semiconductor materials and are available in two varieties - with either a Negative Temperature Coefficient (NTC) or a Positive Temperature Coefficient (PTC). Like RTDs, a thermistor’s resistance will change with temperature. PTC thermistors will have an increase in resistance when the temperature increases, while NTC thermistors will decrease in resistance when temperature increases. Thermistors have several advantages over RTDs. The change in resistance of a thermistor is much more significant when compared to a RTD. This higher sensitivity makes the thermis- tor attractive in terms of accurate measurements. Additionally, thermistors have a quick thermal response and are inexpensive when compared to RTDs. The biggest disadvantage of thermistors is that they are not as linear as RTDs and require a thirdorder polynomial for linearizing the resistance across temperature. Silicon baSed SenSorS Silicon Integrated Circuit (IC) temperature sensors offer another alternative to solving temperature measurement problems. The advantages of silicon IC temperature sensors include user-friendly output formats and ease of installation in the PCB assembly environment. Some IC sensors include extensive signal-processing circuitry, providing a digital I/O interface that works directly with a microcontroller. On the other hand, the accuracy and temperature range of silicon-based temperature sensors do not match those of the other types of sensors discussed in this article. A silicon IC temperature sensor can operate over a nominal temperature range of -55°C to 150°C. Some devices go beyond this range, while others operate over a narrower range. concluSion Of the temperature sensors on the market today, the thermocouple, RTD, thermistor and silicon IC sensors continue to dominate. The thermocouple is most appropriate for higher temperature sensing, while the RTD is best suited for lower temperatures where good linearity is desirable. The thermistor is typically used for applications with smaller temperature ranges, but it offers greater accuracy than the thermocouple or the RTD. Silicon IC-based temperature sensors simplify designs, while offering relatively high accuracy over a temperature range of -55°C to 150°C. They also provide many integrated features that enhance system flexibility and performance. Contents Viewpoint Wireless sensor networks Temperature sensing technology Analog news Microchip analog page Thermal management overview Sample center microchipDIRECT Reference designs/ app notes Technical training • http://www.microchip.com http://www.microchip.com/analog http://www.microchip.com/analog http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2543¶m=en021419&pageId=79 http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2543¶m=en021419&pageId=79 http://sample.microchip.com/Default.aspx?testCookies=true http://www.microchipdirect.com/catalogselection.aspx?returnURL=default.aspx http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1423
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