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ADT70GN Datasheet(PDF) 9 Page - Analog Devices |
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ADT70GN Datasheet(HTML) 9 Page - Analog Devices |
9 / 14 page ADT70 REV. 0 –9– would contribute to the difference in voltage drop between the RTD and the reference resistor. Thus, an error in reading the ac- tual temperature could occur. Table I. Copper Wire Gauge Size to Resistance Table. Lead-wire AWG Ohms/foot at +25ºC 12 0.0016 14 0.0026 16 0.0041 18 0.0065 20 0.0103 22 0.0162 24 0.0257 26 0.0413 28 0.0651 30 0.1027 From Table I the amount of lead-wire resistance effect in the circuit can be estimated. For example, connect 100 feet of AWG 22 wire to a 100 Ω Platinum RTD (PF element). The lead-wire resistance will be: R = 100 ft 3 0.0162 Ω/ft = 1.62 Ω. Thus the total resistance you have with the PRTD will be: R TOTAL . . =+ = 100 1 62 101 62 ΩΩ Ω Since the 100 Ω reference resistor is assumed to be relatively close to the ADT70, the lead-wire resistance is negligible. This shows 1.62 Ω of inaccuracy. From the PRTD’s data sheet, the PRTD’s sensitivity rating ( Ω/°C) can be used with the lead-wire resistance to approximate the accuracy error in temperature degree ( °C). Following the ex- ample above, the sensitivity of the 100 Ω PRTD is 0.385 Ω/°C (taken from PRTD data sheet). Hence the approximate error is: Error C C =° = ° 162 0 385 421 . / . / . ΩΩ assuming the reference resistor is constant at 100 Ω throughout the temperature range. As shown above, this is a significant inaccuracy, especially for ap- plications where the PRTD would be hundreds of feet away from the ADT70. To reduce lead-wire error it is recommended to use a larger sensitivity RTD; 1 k Ω instead of 100 Ω. Furthermore, in the application circuit section, Figure 28 illustrates how to elimi- nate such error by using the part’s general purpose op amp. Self-Heating Effect Another contributor to measurement error is the self-heating ef- fect on the RTD. As with any resistive element, power is dissi- pated in an amount equal to the square of the excitation current times the resistance of the element. The error contribution of the heat generated by this power dissipation can easily be calculated. For example, if the package thermal resistance is 50 °C/W, the RTD nominal resistance is 1 k Ω and the element is excited with a 1 mA current source, then the artificial increase in temperature ( ∆ºC) as a result of self-heating is: ∆° = × CI R PACKAGE 2 0 θ ∆° = () ×Ω× ° CmA C W 1 1000 50 2 / ∆° = ° CC 005 . where: PACKAGE = thermal resistance of package R0 = value of RTD resistance APPLICATION INFORMATION As shown in Figure 27, using a 1 k Ω PRTD, 1 kΩ reference resistor, 49.9 k Ω resistor between RG A (Pin 11) and RGB (Pin 12), and shorting BIAS (Pin 4) with VREFOUT (Pin 3) together, the output of OUTIA (Pin 14) will have a transfer function of VmV R OUT PRTD RESISTANCE REFERENCE RESISTANCE =Ω × () 1 299 . / ∆ − RGA RGB +INIA INIA INST AMP SHUT- DOWN GND SENSE OUTIA AGND DGND VS SHUTDOWN +INOA INOA OUTOA +VS 2.5V REF IOUTA IOUTB MATCHED CURRENT SOURCES NULLA NULLB BIAS 2.5VREFOUT ADT70 49.9k VOUT @ 5mV/ C INDEPENDENT OP AMP 50k +5V 1k PRTD 1k REF RESISTOR POTENTIOMETER IS USED TO ACHIEVE HIGHER PRECISION OF MATCHING CURRENT. 1V < VS < 5V Figure 27. Basic Operational Diagram |
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