Understanding the Specifications–AD590
ERRO R VERUS TEMP ERATURE: WITH CALIBRATIO N
ERRO R TRIMMED O UT
EXP LANATIO N O F TEMP ERATURE SENSO R
SP ECIFICATIO NS
Each AD590 is tested for error over the temperature range with
the calibration error trimmed out. T his specification could also
be called the “variance from PT AT ” since it is the maximum
difference between the actual current over temperature and a
PT AT multiplication of the actual current at 25°C. T his 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.
T he 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.
T he AD590 is basically a PT AT (proportional to absolute
temperature)1 current regulator. T hat is, the output current is
equal to a scale factor times the temperature of the sensor in
degrees Kelvin. T his 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. T his is done with
5 V across the device at a temperature within a few degrees of
+25°C (298.2K). T he device is then packaged and tested for
accuracy over temperature.
CALIBRATIO N ERRO R
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 PT AT . 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
ERRO R VERSUS TEMP ERATURE: NO USER TRIMS
Using the AD590 by simply measuring the current, the total
error is the “variance from PT AT ” 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.
NO NLINEARITY
Nonlinearity as it applies to the AD590 is the maximum
deviation of current over temperature from a best-fit straight
line. T he nonlinearity of the AD590 over the –55°C to +150°C
range is superior to all conventional electrical temperature
sensors such as thermocouples. RT Ds and thermistors. Figure 6
shows the nonlinearity of the typical AD590K from Figure 5.
Figure 3. Calibration Error vs. Tem perature
T he 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. T o trim this circuit the
temperature of the AD590 is measured by a reference tempera-
ture 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.
Figure 6. Nonlinearity
Figure 7A shows a circuit in which the nonlinearity is the major
contributor to error over temperature. T he circuit is trimmed by
adjusting R1 for a 0 V output with the AD590 at 0°C. R2 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.
Figure 4. One Tem perature Trim
1T (°C) = T (K) –273.2; Zero on the Kelvin scale is “absolute zero”; there is no
lower temperature.
REV. B
–5–
ADI [ ADI ]
