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AD590L Datasheet(PDF) 7 Page - Analog Devices |
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AD590L Datasheet(HTML) 7 Page - Analog Devices |
7 / 16 page AD590 Rev. C | Page 7 of 16 The AD590 is basically a PTAT (proportional to absolute temperature)1 current regulator. That is, the output current is equal to a scale factor times the temperature of the sensor in degrees Kelvin. This scale factor is trimmed to 1 µA/K at the factory, by adjusting the indicated temperature (that is, the output current) to agree with the actual temperature. This is done with 5 V across the device at a temperature within a few degrees of 25°C (298.2K). The device is then packaged and tested for accuracy over temperature. CALIBRATION ERROR At final factory test, the difference between the indicated temperature and the actual temperature is called the calibration error. Since this is a scale factory error, its contribution to the total error of the device is PTAT. For example, the effect of the 1°C specified maximum error of the AD590L varies from 0.73°C at –55°C to 1.42°C at 150°C. Figure 6 shows how an exaggerated calibration error would vary from the ideal over temperature. IACTUAL 298.2 298.2 TEMPERATURE (°K) ACTUAL TRANSFER FUNCTION IDEAL TRANSFER FUNCTION CALIBRATION ERROR Figure 6. Calibration Error vs. Temperature The calibration error is a primary contributor to maximum total error in all AD590 grades. However, since it is a scale factor error, it is particularly easy to trim. Figure 7 shows the most elementary way of accomplishing this. To trim this circuit, the temperature of the AD590 is measured by a reference temperature sensor and R is trimmed so that VT = 1 mV/K at that temperature. Note that when this error is trimmed out at one temperature, its effect is zero over the entire temperature range. In most applications there is a current-to-voltage conversion resistor (or, as with a current input ADC, a reference) that can be trimmed for scale factor adjustment. 1 T(°C) = T(K) –273.2. Zero on the Kelvin scale is “absolute zero”; there is no lower temperature. 5V R 100 Ω VT = 1mV/K AD590 950 Ω + – + – + – Figure 7. One Temperature Trim ERROR VERSUS TEMPERATURE: WITH CALIBRATION ERROR TRIMMED OUT Each AD590 is tested for error over the temperature range with the calibration error trimmed out. This specification could also be called the “variance from PTAT,” because it is the maximum difference between the actual current over temperature and a PTAT multiplication of the actual current at 25°C. This error consists of a slope error and some curvature, mostly at the temperature extremes. Figure 8 shows a typical AD590K temperature curve before and after calibration error trimming. AFTER CALIBRATION TRIM 2 0 –2 –55 150 TEMPERATURE (°C) CALIBRATION ERROR BEFORE CALIBRATION TRIM Figure 8. Effect to Scale Factor Trim on Accuracy ERROR VERSUS TEMPERATURE: NO USER TRIMS Using the AD590 by simply measuring the current, the total error is the variance from PTAT, described above, plus the effect of the calibration error over temperature. For example, the AD590L maximum total error varies from 2.33°C at –55°C to 3.02°C at 150°C. For simplicity, only the large figure is shown on the specification page. NONLINEARITY Nonlinearity as it applies to the AD590 is the maximum deviation of current over temperature from a best-fit straight line. The nonlinearity of the AD590 over the −55°C to +150°C range is superior to all conventional electrical temperature sensors such as thermocouples, RTDs, and thermistors. Figure 9 shows the nonlinearity of the typical AD590K from Figure 8. |
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