Understanding the Specifications–AD590
EXPLANATION OF TEMPERATURE SENSOR
SPECIFICATIONS
ERROR VERUS TEMPERATURE: WITH CALIBRATION
ERROR TRIMMED OUT
The way in which the AD590 is specified makes it easy to apply
in a wide variety of different applications. It is important to
understand the meaning of the various specifications and the
effects of supply voltage and thermal environment on accuracy.
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 (i.e., 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
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” since 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 5 shows a typical AD590K
temperature curve before and after calibration error trimming.
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 3 shows how an exagger-
ated calibration error would vary from the ideal over temperature.
Figure 5. 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
Figure 3. 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 4 shows the most
elementary way of accomplishing this. To trim this circuit the
temperature of the AD590 is measured by a reference tempera-
ture sensor and R is trimmed so that V
T
= 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.
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 6
shows the nonlinearity of the typical AD590K from Figure 5.
Figure 6. Nonlinearity
Figure 4. One Temperature Trim
1
T(°C) = T(K) –273.2; Zero on the Kelvin scale is “absolute zero”; there is no
lower temperature.
Figure 7A shows a circuit in which the nonlinearity is the major
contributor to error over temperature. The circuit is trimmed by
adjusting R
1
for a 0 V output with the AD590 at 0°C. R
2
is then
adjusted for 10 V out with the sensor at 100°C. Other pairs of
temperatures may be used with this procedure as long as they
are measured accurately by a reference sensor. Note that for
+15 V output (150°C) the V+ of the op amp must be greater
than 17 V. Also note that V– should be at least –4 V: if V– is
ground there is no voltage applied across the device.
–5–
REV. B
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