IC temperature sensors generally are preferred in data-acquisition systems when the accuracy demands aren't extreme. Sensor, signal-processing, and data-conversion electronics are all integrated in the IC along with an easy to use digital-I/O interface, all at low cost. Such ICs require a supply voltage in the range of +3 V to +5 V, negative to common. Also, they usually draw very low supply current.
But biasing the sensor is more difficult in a telecom system, where the only supply voltage available is usually the -48 V systems bus. If no other positive voltage supply is available, designers normally must include a dc-dc converter powered from -48 V.
The circuit depicted in the figure solves this problem without requiring an extra power supply. It lets the IC sensor operate from -48 V. At the same time, with the addition of a few components, it presents an interface to the digital inputs of programmable-logic controllers (PLCs) or other digital I/O that require logic signals ranging from 0 V to some positive level.
As shown, the output can sink up to 1.6 mA and swing from 0 V to about +40 V (the maximum VCE for a 2N3904 transistor). That capability meets the requirements for digital I/O in most industrial systems (+15 or +5 V).
This circuit includes an absolute temperature-to-period converter (IC1). Connected as shown, it provides a conversion constant of 640 s/°K. (At room temperature, the period is about 190 ms, providing a frequency of 5.2 Hz.) This scheme also can accommodate other similar sensors, such as temperature-to-frequency converters.
