AD590
+
5V
+
–
EXPLANATION OF TEMPERATURE SENSOR
SPECIFICATIONS
AD590
+
The way in which the AD590 is specified makes it easy to apply
it in a wide variety of applications. It is important to understand
the meaning of the various specifications and the effects of the
supply voltage and thermal environment on accuracy.
R
100Ω
V
= 1mV/K
–
T
950Ω
–
Figure 9. One Temperature Trim
The AD590 is a PTAT1 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.
ERROR VS. 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 10 shows a typical AD590K
temperature curve before and after calibration error trimming.
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 8 shows how an exaggerated
calibration error would vary from the ideal over temperature.
2
BEFORE
CALIBRATION
TRIM
CALIBRATION
ERROR
0
ACTUAL
TRANSFER
FUNCTION
AFTER
CALIBRATION
TRIM
I
ACTUAL
IDEAL
TRANSFER
FUNCTION
CALIBRATION
ERROR
–2
–55
150
298.2
TEMPERATURE (°C)
Figure 10. Effect to Scale Factor Trim on Accuracy
ERROR VS. 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.
298.2
TEMPERATURE (°K)
Figure 8. Calibration Error vs. Temperature
The calibration error is a primary contributor to the maximum
total error in all AD590 grades. However, because it is a scale
factor error, it is particularly easy to trim. Figure 9 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
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 11
shows the nonlinearity of the typical AD590K from Figure 10.
reference) that can be trimmed for scale factor adjustment.
1 T(°C) = T(K) − 2ꢀ3.2. Zero on the Kelvin scale is absolute zero; there is no
lower temperature.
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