INTRODUCTION
The AD590 is a two-terminal integrated circuit temperature transducer that produces an output proportional to absolute temperature. For supply voltages between +4 V and +30 V the device acts as a high impedance, constant current source supplying 1 A/K. Laser trimming of the chip’s thin-film resistors is used to calibrate the device to an output of 298.2 A at 298.2K (+25℃).
A typical application for the AD590 is a remote temperature-to-current transducer. Figure 1 shows a thermometer circuit that measures temperature from –55℃ to +100℃and whose output voltage is 100 mV/℃. Since the AD590 measures absolute temperature (its nominal output is 1A/K), the output must be offset by 273.2A in order to read out in degrees Celsius. The output current of the AD590 flows through a 1k resistance, developing a voltage of 1 mV/K. The output of the AD580 2.5V reference is divided down by resistors to provide a 273.2 mV offset, Which is subtracted from the voltage across the 1k resistor by an AD524 instrumentation amplifier. The amplifier provides a gain of 100, so that the output range corresponding to –55℃ to +100℃ is –5.5 V to +10 V (100 mV/℃). An operational amplifier can substitute for the instrumentation amplifier, although care must be taken when designing with the op amp since the gain at the two input terminals will be different.
