Applying the AD590
GENERAL APPLICATIONS
Figure 10. Variable Scale Display
Figure 12. Differential Measurements
Figure 10 demonstrates the use of a low cost Digital Panel
Meter for the display of temperature on either the Kelvin,
Celsius or Fahrenheit scales. For Kelvin temperature Pins 9, 4
and 2 are grounded; and for Fahrenheit temperature Pins 4 and
2 are left open.
The above configuration yields a 3 digit display with 1°C or 1°F
resolution, in addition to an absolute accuracy of
±
2.0°C over
the –55°C to +125°C temperature range if a one-temperature
calibration is performed on an AD590K, L, or M.
desired temperature difference. For example, the inherent
offset between the two devices can be trimmed in. If V+ and
V– are radically different, then the difference in internal
dissipation will cause a differential internal temperature rise.
This effect can be used to measure the ambient thermal
resistance seen by the sensors in applications such as fluid level
detectors or anemometry.
Figure 13. Cold Junction Compensation Circuit for
Type J Thermocouple
Figure 11. Series & Parallel Connection
Connecting several AD590 units in series as shown in Figure 11
allows the minimum of all the sensed temperatures to be
indicated. In contrast, using the sensors in parallel yields the
average of the sensed temperatures.
The circuit of Figure 12 demonstrates one method by which
differential temperature measurements can be made. R1 and R2
can be used to trim the output of the op amp to indicate a
Figure 13 is an example of a cold junction compensation circuit
for a Type J Thermocouple using the AD590 to monitor the
reference junction temperature. This circuit replaces an ice-bath
as the thermocouple reference for ambient temperatures
between +15°C and +35°C. The circuit is calibrated by
adjusting R
T
for a proper meter reading with the measuring
junction at a known reference temperature and the circuit near
+25°C. Using components with the TCs as specified in Figure
13, compensation accuracy will be within
±
0.5°C for circuit
temperatures between +15°C and +35°C. Other thermocouple
types can be accommodated with different resistor values. Note
that the TCs of the voltage reference and the resistors are the
primary contributors to error.
REV. B
–7–
AD [ ANALOG DEVICES ]
