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AD592CN Datenblatt(PDF) 5 Page - Analog Devices |
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AD592CN Datenblatt(HTML) 5 Page - Analog Devices |
5 / 8 page AD592 REV. A –5– +V AD592 R 100 Ω 950 Ω VOUT = 1mV/K Figure 4. Basic Voltage Output (Single Temperature Trim) To trim the circuit the temperature must be measured by a ref- erence sensor and the value of R should be adjusted so the out- put (VOUT) corresponds to 1 mV/K. Note that the trim procedure should be implemented as close as possible to the temperature highest accuracy is desired for. In most applications if a single temperature trim is desired it can be implemented where the AD592 current-to-output voltage conversion takes place (e.g., output resistor, offset to an op amp). Figure 5 illus- trates the effect on total error when using this technique. AFTER SINGLE TEMPERATURE CALIBRATION ACCURACY WITHOUT TRIM +1.0 +0.5 –25 +105 +25 0 –0.5 –1.0 TEMPERATURE – oC Figure 5. Effect of Scale Factor Trim on Accuracy If greater accuracy is desired, initial calibration and scale factor errors can be removed by using the AD592 in the circuit of Figure 6. 8.66k Ω R1 1k Ω 97.6k Ω R2 5k Ω 7.87k Ω AD741 VOUT = 100mV/ oC +5V AD1403 V– AD592 Figure 6. Two Temperature Trim Circuit With the transducer at 0 °C adjustment of R1 for a 0 V output nulls the initial calibration error and shifts the output from K to °C. Tweaking the gain of the circuit at an elevated temperature by adjusting R2 trims out scale factor error. The only error remaining over the temperature range being trimmed for is nonlinearity. A typical plot of two trim accuracy is given in Figure 7. SUPPLY VOLTAGE AND THERMAL ENVIRONMENT EFFECTS The power supply rejection characteristics of the AD592 mini- mizes errors due to voltage irregularity, ripple and noise. If a supply is used other than 5 V (used in factory trimming), the power supply error can be removed with a single temperature trim. The PTAT nature of the AD592 will remain unchanged. The general insensitivity of the output allows the use of lower cost unregulated supplies and means that a series resistance of several hundred ohms (e.g., CMOS multiplexer, meter coil resistance) will not degrade the overall performance. +2.0 +1.0 –25 +105 +25 0 –1.0 –2.0 TEMPERATURE – oC 0 +75 Figure 7. Typical Two Trim Accuracy The thermal environment in which the AD592 is used deter- mines two performance traits: the effect of self-heating on accu- racy and the response time of the sensor to rapid changes in temperature. In the first case, a rise in the IC junction tempera- ture above the ambient temperature is a function of two vari- ables; the power consumption level of the circuit and the thermal resistance between the chip and the ambient environ- ment ( θ JA). Self-heating error in °C can be derived by multiply- ing the power dissipation by θ JA. Because errors of this type can vary widely for surroundings with different heat sinking capaci- ties it is necessary to specify θ JA under several conditions. Table I shows how the magnitude of self-heating error varies relative to the environment. In typical free air applications at +25 °C with a 5 V supply the magnitude of the error is 0.2 °C or less. A common clip-on heat sink will reduce the error by 25% or more in critical high temperature, large supply voltage situations. Table I. Thermal Characteristics Medium θ JA (°C/watt) τ (sec)* Still Air Without Heat Sink 175 60 With Heat Sink 130 55 Moving Air Without Heat Sink 60 12 With Heat Sink 40 10 Fluorinert Liquid 35 5 Aluminum Block** 30 2.4 NOTES * τ is an average of five time constants (99.3% of final value). In cases where the thermal response is not a simple exponential function, the actual thermal re- sponse may be better than indicated. **With thermal grease. |
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