Temperature sensors must sometimes operate at locations whose return potentials differ considerably from that of the data-acquisition system's common—that is, equipotential—ground. In consequence, the temperature sensor's support circuitry must provide galvanic isolation between the sensor and its data-acquisition system. Also, the data-acquisition system seldom provides an isolated source of power for the sensor. The circuit in Figure 1 solves both problems by isolating the sensor's signal and power supply.

The complementary, fixed-frequency square-wave outputs of a power-transformer driver—IC₁, a Maxim MAX845—drive a Halo Electronics  TGM-010P3 1-to-1-to-1 transformer with dual primary windings and a single untapped secondary winding (Reference 1). The secondary winding feeds a Graetz-bridge rectifier that generates approximately 4.5V to power IC₂, a Maxim MAX6576 sensor. Combining a temperature sensor, signal-processing electronics, and an easy-to-use digital-I/O interface in a low-cost package, the MAX6576 draws little current from a single supply source and maintains its specified accuracy over a 3 to 5V supply-voltage range.

If you connect the sensor as Figure 1 shows, it operates as an absolute temperature-to-period converter and provides a nominal conversion constant of 10 µsec/°K, which, at room temperature, yields a period of approximately 2.980 msec—a frequency of 335 Hz. You can adjust the conversion constant from 10 to 640 µsec/°K. Note that longer conversion constants allow more signal-integration time to minimize noise effects. The sensor's symmetrical square-wave output drives NPN transistor Q₂'s base through R₄, a 10-kΩ resistor. A 390Ω resistor, R₃, serves as Q₂'s collector load and connects to the same lines that deliver power to the temperature sensor. When Q₂ conducts, it draws an asymmetrical power-supply current that exceeds the supply current during the sensor output's positive half-cycle.

In IC₁'s sensor output-to-ground return on the data-acquisition system's side, resistor R₂ and capacitor C₂ shunt Q₁'s base-emitter junction. The values of R₂ and C₂ ensure that the sum of IC₂'s current and transformer T₁'s magnetizing current cannot drive Q₁ into conduction. When Q₂ conducts, it draws about 12 mA from the isolated 4.5V power-supply line. Reflecting to the primary, Q₂'s conduction current flows from the 5V supply into IC₁, out through its ground terminals, and partly through R₂. The voltage drop across R₂ exceeds Q₁'s base-emitter voltage threshold and supplies sufficient base current to turn on Q₁.

Thus, when Q₂ conducts, so does Q₁, which copies IC₁'s isolated square-wave output to Q₁'s collector circuit. As the waveforms of figure 2 and figure 3 show, Q₁'s output rise and fall times, jitter, and propagation delay total about 2 µsec. The equivalent measurement error due to timing jitter amounts to less than 0.1°K at the fastest conversion constant of 10 µsec/°K. Varying the circuit's supply voltage through a range of 4.5 to 5.5V introduces an error of less than 0.1°K. The output at Q₁'s collector can sink several milliamperes at a voltage excursion of 0 to 5V.

This design can accommodate temperature-to-frequency converters and other types of temperature sensors. For further information on IC₁ and IC₂, review the devices' data sheets and the data sheet for the MAX845 evaluation kit (reference 2, reference 3, and reference 4).