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AD592BN Datasheet(PDF) 4 Page - Analog Devices |
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AD592BN Datasheet(HTML) 4 Page - Analog Devices |
4 / 8 page AD592 REV. A –4– THEORY OF OPERATION The AD592 uses a fundamental property of silicon transistors to realize its temperature proportional output. If two identical transistors are operated at a constant ratio of collector current densities, r, then the difference in base-emitter voltages will be (kT/q)(ln r). Since both k, Boltzman’s constant and q, the charge of an electron are constant, the resulting voltage is directly Proportional To Absolute Temperature (PTAT). In the AD592 this difference voltage is converted to a PTAT current by low temperature coefficient thin film resistors. This PTAT current is then used to force the total output current to be pro- portional to degrees Kelvin. The result is a current source with an output equal to a scale factor times the temperature (K) of the sensor. A typical V-I plot of the circuit at +25 °C and the temperature extremes is shown in Figure 1. SUPPLY VOLTAGE – Volts 378 248 06 1 2345 298 +105oC +25oC –25oC UP TO 30V Figure 1. V-I Characteristics Factory trimming of the scale factor to 1 µA/K is accomplished at the wafer level by adjusting the AD592’s temperature reading so it corresponds to the actual temperature. During laser trim- ming the IC is at a temperature within a few degrees of 25 °C and is powered by a 5 V supply. The device is then packaged and automatically temperature tested to specification. FACTORS AFFECTING AD592 SYSTEM PRECISION The accuracy limits given on the Specifications page for the AD592 make it easy to apply in a variety of diverse applications. To calculate a total error budget in a given system it is impor- tant to correctly interpret the accuracy specifications, non- linearity errors, the response of the circuit to supply voltage variations and the effect of the surrounding thermal environ- ment. As with other electronic designs external component se- lection will have a major effect on accuracy. CALIBRATION ERROR, ABSOLUTE ACCURACY AND NONLINEARITY SPECIFICATIONS Three primary limits of error are given for the AD592 such that the correct grade for any given application can easily be chosen for the overall level of accuracy required. They are the calibra- tion accuracy at +25 °C, and the error over temperature from 0 °C to +70°C and –25°C to +105°C. These specifications cor- respond to the actual error the user would see if the current out- put of an AD592 were converted to a voltage with a precision resistor. Note that the maximum error at room temperature, over the commercial IC temperature range, or an extended range including the boiling point of water, can be directly read from the specifications table. All three error limits are a combi- nation of initial error, scale factor variation and nonlinearity de- viation from the ideal 1 µA/K output. Figure 2 graphically depicts the guaranteed limits of accuracy for an AD592CN. TEMPERATURE – oC +1.0 +0.5 –25 +105 0 +25 +70 0 –0.5 –1.0 MAXIMUM ERROR OVER TEMPERATURE TYPICAL ERROR CALIBRATION ERROR LIMIT MAXIMUM ERROR OVER TEMPERATURE Figure 2. Error Specifications (AD592CN) The AD592 has a highly linear output in comparison to older technology sensors (i.e., thermistors, RTDs and thermo- couples), thus a nonlinearity error specification is separated from the absolute accuracy given over temperature. As a maxi- mum deviation from a best-fit straight line this specification rep- resents the only error which cannot be trimmed out. Figure 3 is a plot of typical AD592CN nonlinearity over the full rated tem- perature range. TYPICAL NONLINEARITY +0.2 +0.1 –25 +105 0 +25 +70 0 –0.1 –0.2 TEMPERATURE – oC Figure 3. Nonlinearity Error (AD592CN) TRIMMING FOR HIGHER ACCURACY Calibration error at 25 °C can be removed with a single tempera- ture trim. Figure 4 shows how to adjust the AD592’s scale fac- tor in the basic voltage output circuit. |
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