1998 Microchip Technology Inc. DS00687A-page 1. M. AN687 ... This application note focuses on circuit solutions that use Platinum RTDs in the design. Initially ...
Precision Temperature Sensing with RTD Circuits Bonnie C. Baker Microchip Technology Inc.
INTRODUCTION One of the most widely measured phenomena in the process control environment is temperature. Common elements such as Resistance Temperature Detectors (RTDs), thermistors, thermocouples or diodes are used to sense absolute temperatures as well as changes in temperature. For an overview and comparison of these sensors, refer to Microchip’s AN679, “Temperature Sensing Technologies”. Of these technologies, the platinum RTD temperature sensing element is the most accurate and stable over time and temperature. RTD element technologies are constantly improving, further enhancing the quality of the temperature measurement (see Figure 1). Typically, a data acquisition system conditions the analog signal from the RTD sensor, making the analog translation of the temperature usable in the digital domain. This application note focuses on circuit solutions that use Platinum RTDs in the design. Initially, the RTD temperature sensing element will be compared to the negative temperature coefficient (NTC) thermistor, which is also a resistive temperature sensing element. In this forum the linearity of the RTD will be presented along with calibration formulas that can be used to improve the off the shelf linearity of the element. If more information is needed concerning the thermistor temperature sensor, refer to Microchip’s AN685, “Thermistors in Single Supply Temperature Sensing Circuits”. Finally, the signal conditioning path for the RTD system will be covered with complete application circuits from sensor or microprocessor.
RTD OVERVIEW The acronym “RTD” is derived from the term “Resistance Temperature Detector”. The most stable, linear and repeatable RTD is made of platinum metal. The temperature coefficient of the RTD element is positive. This is in contrast to the NTC thermistor that has a negative temperature coefficient as shown graphically in Figure 2. An approximation of the platinum RTD resistance change over temperature can be calculated by using the constant 0.00385W/W/˚C. This constant is easily used to calculate the absolute resistance of the RTD at temperature. RTD ( T ) = RT D 0 + T ´ RT D 0 ´ 0.00385 W / W/° C where RTD(T) is the resistance value of the RTD element at temperature (Celsius), RTD0 is the specified resistance of the RTD element at 0˚C, and T is the temperature environment that the RTD is placed (Celsius). 100
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