Design Challenges and Component Selection of Infrared Digital Thermometers

Infrared (IR) digital thermometers have replaced traditional mercury thermometers due to their speed, accuracy, efficiency and cost advantages. Ear digital thermometers use a thermopile sensor to measure the infrared heat emitted by the eardrum, which reflects the temperature of the hypothalamus.

Thermopile infrared sensors consist of multiple thermocouples connected in series. Each thermocouple consists of two dissimilar metals that generate a voltage when the junction temperature is different. Thermocouples are located in the hot and cold zones of the thermopile. The cold end of the junction is welded to a temperature stabilizer and is therefore isolated from the ambient temperature. The hot end of the junction is exposed to incident radiation from the eardrum. To provide effective heat dissipation, the hot junction is thermally isolated from the cold junction. The cold junction of the thermocouple is usually measured using a thermistor to provide an accurate measurement of the ambient temperature. Some thermopile sensors have a built-in thermistor that can measure the ambient temperature, thus allowing the target temperature to be calculated.

The thermopile output signal has a very stable relationship with frequency, which is directly proportional to the intensity of the incident radiation. The detection sensor produces an output voltage in the range of 100μV to 900μV. Measuring such a weak signal is a severe challenge for the signal conditioning circuit.

Solution

Battery-powered portable applications such as infrared digital thermometers require low power consumption and single-supply operation. The high-precision AD8538 auto-zero amplifier features low power consumption, low input bias current, very low offset voltage, very low offset drift, and high open-loop gain to ensure optimal system performance. The AD8538 operates at low current, with a maximum supply current of 180μA, a maximum VOS of 13μV, and a typical TCVOS of 30nV/°C. The AD8538 has rail-to-rail input/output, operates at 2.7V, and is available in a 5-lead TSOT package.