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AD595 Datasheet(PDF) 8 Page - Analog Devices |
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AD595 Datasheet(HTML) 8 Page - Analog Devices |
8 / 8 page AD594/AD595 REV. C –8– OUTLINE DIMENSIONS Dimensions shown in inches and (mm). TO-116 (D) Package 0.100 (2.54) BSC 0.700 ±0.010 (17.78 ±0.25) 0.180 ±0.030 (4.57 ±0.76) 0.085 (2.16) 0.035 ±0.010 (0.89 ±0.25) 0.047 ±0.007 (1.19 ±0.18) 0.017 +0.003 –0.002 0.43 +0.08 –0.05 ( ( 0.125 (3.18) MIN 0.31 ±0.01 (7.87 ±0.25) 0.01 ±0.002 (0.25 ±0.05) 0.095 (2.41) 0.30 (7.62) REF 0.430 (10.92) 14 1 7 8 0.040 (1.02) R PIN 1 0.265 (6.73) 0.290 ±0.010 (7.37 ±0.25) Cerdip (Q) Package 14 1 7 8 PIN 1 0.77 ±0.015 (19.55 ±0.39) 0.310 (7.87) 0.260 ±0.020 (6.6 ±0.51) SEATING PLANE 0.100 (2.54) BSC 0.035 ±0.010 (0.889 ±0.254) 0.180 ±0.030 (4.57 ±0.76) 0.125 3.175) MIN 0.032 (0.812) 15 ° 0 ° 0.300 (7.62) REF 0.148 ±0.015 (3.76 ±0.38) 0.010 ±0.001 (0.254 ±0.025) 0.018 (0.457) 0.600 (15.24) BSC THERMOCOUPLE BASICS Thermocouples are economical and rugged; they have reason- ably good long-term stability. Because of their small size, they respond quickly and are good choices where fast response is im- portant. They function over temperature ranges from cryogenics to jet-engine exhaust and have reasonable linearity and accuracy. Because the number of free electrons in a piece of metal de- pends on both temperature and composition of the metal, two pieces of dissimilar metal in isothermal and contact will exhibit a potential difference that is a repeatable function of tempera- ture, as shown in Figure 14. The resulting voltage depends on the temperatures, T1 and T2, in a repeatable way. CONSTANTAN IRON UNKNOWN TEMPERATURE ICE POINT REFERENCE T1 T2 CONSTANTAN Cu Cu V1 Figure 14. Thermocouple Voltage with 0 °C Reference Since the thermocouple is basically a differential rather than absolute measuring device, a know reference temperature is required for one of the junctions if the temperature of the other is to be inferred from the output voltage. Thermocouples made of specially selected materials have been exhaustively character- ized in terms of voltage versus temperature compared to primary temperature standards. Most notably the water-ice point of 0 °C is used for tables of standard thermocouple performance. An alternative measurement technique, illustrated in Figure 15, is used in most practical applications where accuracy requirements do not warrant maintenance of primary standards. The reference junction temperature is allowed to change with the environment of the measurement system, but it is carefully measured by some type of absolute thermometer. A measurement of the thermo- couple voltage combined with a knowledge of the reference temperature can be used to calculate the measurement junction temperature. Usual practice, however, is to use a convenient thermoelectric method to measure the reference temperature and to arrange its output voltage so that it corresponds to a ther- mocouple referred to 0 °C. This voltage is simply added to the thermocouple voltage and the sum then corresponds to the stan- dard voltage tabulated for an ice-point referenced thermocouple. CONSTANTAN IRON T1 V1 CuNi– Cu Cu T3 V1' = V1 FOR PROPERLY SCALED V3' = f(T3) V3 ' V1 ' V2 Figure 15. Substitution of Measured Reference Temperature for Ice Point Reference The temperature sensitivity of silicon integrated circuit transis- tors is quite predictable and repeatable. This sensitivity is exploited in the AD594/AD595 to produce a temperature re- lated voltage to compensate the reference of “cold” junction of a thermocouple as shown in Figure 16. Cu CONSTANTAN IRON T1 Cu T3 Figure 16. Connecting Isothermal Junctions Since the compensation is at the reference junction temperature, it is often convenient to form the reference “junction” by connect- ing directly to the circuit wiring. So long as these connections and the compensation are at the same temperature no error will result. |
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