Solid-State Circuit Converts Temperature To Voltage

Oct. 30, 2000
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...

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.

Comments

To join the conversation, and become an exclusive member of Electronic Design, create an account today!