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ADT70GN Datasheet(PDF) 8 Page - Analog Devices |
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ADT70GN Datasheet(HTML) 8 Page - Analog Devices |
8 / 14 page ADT70 REV. 0 –8– FUNCTIONAL DESCRIPTION The ADT70 provides excitation and signal conditioning for resistance-temperature devices (RTDs). It is ideally suited for 1 k Ω Platinum RTDs (PRTDs), which allow a much wider range of temperature measurement than silicon-based sensors. Using a low cost PRTD, the ADT70 can measure temperatures in the range of –50 °C to +500°C. The two main components in the ADT70 are the adjustable current sources and the instrumentation amplifier. The current sources provide matching excitation currents to the PRTD and to the Reference Resistor. The instrumentation amplifier com- pares the voltage drop across both the PRTD and Reference Resistor, and provides an amplified output signal voltage that is proportional to temperature. Besides the matching current sources and the instrumentation amplifier, there is a general purpose op amp for any application desired. The ADT70 comes with a +2.5 V reference on board. RGA RGB INIA +INIA INST AMP SHUT- DOWN GND SENSE OUTIA AGND DGND VS +INOA INOA OUTOA +VS 2.5V REF IOUTA IOUTB MATCHED CURRENT SOURCES NULLA NULLB BIAS 2.5VREFOUT ADT70 SHUTDOWN Figure 26. Block Diagram What is an RTD? The measurable temperature range of the ADT70 heavily de- pends on the characteristics of the resistance-temperature detec- tor (RTD). Thus, it is important to choose the right RTD to suit the actual application. A basic physical property of any metal is that its electrical resis- tivity changes with temperature. Some metals are known to have a very predictable and repeatable change of resistance for a given change in temperature. An RTD is fabricated from one of these metals to a nominal ohmic value at a specified tempera- ture. By measuring its resistance at some unknown temperature and comparing this value to the resistor’s nominal value, the change in resistance is determined. Because the temperature vs. resistance characteristics are also known, the change in tempera- ture from the point initially specified can be calculated. This makes the RTD a practical temperature sensor, which in its bare form is a resistive element. Several types of metal can be chosen for fabricating RTDs. These include: Copper, balco (an iron-nickel alloy), nickel, tungsten, iridium and platinum. Platinum is by far the most popular material used, due to its nearly linear response to tem- perature, wide temperature operating range and superior long- term stability. The price of Platinum Resistance Temperature Detectors (PRTDs) are becoming more competitive through the wide use of thin-film-type resistive elements. Temperature Coefficient of Resistance The temperature coefficient (TC, also referred to as α) of an RTD, describes the average resistance change per unit tempera- ture from the ice point to the boiling point of water. TCR C RR CR ΩΩ ° ()= − °× 100 0 0 100 R0 = Resistance of the sensor at 0 °C R100 = Resistance of the sensor at +100 °C TCR = Thermal Coefficient of Resistance. For example, a platinum thermometer measuring 100 Ω at 0°C and 138.5 Ω at 100°C, has TCR 0.00385 Ω/Ω/°C . TCR C = Ω− Ω Ω× ° = 138 5 100 100 100 0 00385 . . The larger the TCR, the greater the change in resistance for a given change in temperature. The most common use of TCR is to distinguish between curves for platinum, which is available with TCRs ranging from 0.00375 to 0.003927. The highest TCR indicates the highest purity platinum and is mandated by ITS-90 for standard platinum thermometers. Basically, TCRs must be properly matched when replacing RTDs or connecting them to instruments. There are no technical advan- tages of one TCR over another in practical industrial applica- tions. 0.00385 platinum is the most popular worldwide standard and is available in both wire-wound and thin-film elements. Understanding Error Source The ADT70 uses an instrumentation amplifier that amplifies the difference in voltage drop across the RTD and the reference resis- tor, to output a voltage proportional to the measured temperature. Thus, it is important to use a reference resistor that has stable resis- tance over temperature. The accuracy of the reference resistor should be determined by the end application. The lead resistance of wires connecting to the RTD and the refer- ence resistor can add inaccuracy to the ADT70. If the reference resistor is located close to the part, while the RTD is located at a remote location connected by wires, the lead-wires’ resistance TEMPERATURE – C 0 50 0 10 25 25 50 75 100 125 20 30 40 50 VOUT OF IN AMP = 300mV VCC = 5V SINGLE SUPPLY = LOW TO HIGH TURNING ON VSHUTDOWN = HIGH TO LOW TURNING OFF VSHUTDOWN Figure 25. System Response Time from Shutdown vs. Temperature |
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