Electronic Design

Solid-State Circuit Converts Temperature To Voltage

This circuit is a solid-state thermometer suitable for measuring temperatures from −40°C to 125°C (Fig. 1). By adjusting RZERO and RSCALE, the offset of the output voltage (VOUT) can be calibrated in accordance with any temperature scale. Celsius, for instance, requires an output change of 10 mV/°C. Therefore, RZERO would be adjusted for 0.0 V at 0°C, while RSCALE would be adjusted for −100 mV at −10°C.

The integrated temperature sensor (IC1) has a square-wave output whose frequency is proportional to its absolute temperature. To make use of this signal, the circuit employs the phase-locked loop (PLL) integrated within the waveform generator (IC2).

Since IC2 implements linear frequency tuning, its phase-detector output (PDO) is proportional to the input frequency and, therefore, also to temperature. Phase lock is then accomplished by applying the PDO output to the frequency-adjust input (FADJ). Since IC2's voltage-controlled oscillator (VCO) gain is negative, an external op amp is provided to invert the final output voltage.

To calibrate the circuit, the ambient temperature is adjusted to 0°C (or 0°F) and RZERO is adjusted until VOUT is zero. Then, with the ambient temperature at room temperature (25°C), RSCALE is adjusted until VOUT corresponds to the current room temperature (to 250 mV, for instance, if the scale is 10 mV/°C). For a scale of 10 mV/°C, the error versus temperature is less than ±1.5°C (Fig. 2).

The temperature sensor is an IC available in a small SOT23-6 package. Consequently, it's easily mounted on a pc board along with the other components. Offering a wide temperature range and small size, this thermometer is useful for both industrial and commercial applications.

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