Because of their speed, accuracy, effectiveness, and cost, infrared (IR) digital thermometers have replaced traditional mercury thermometers. Using a thermopile sensor, an ear digital thermometer measures the infrared heat of the eardrum, which reflects the temperature of the hypothalamus.
The thermopile infrared sensor consists of a number of thermocouples connected in series. Each thermocouple is made of two dissimilar metals, which produce a voltage when there’s a temperature differential across the junction. Thermocouples are placed across the hot and cold regions of a structure. The cold side of the junction is kept close to ambient temperature by bonding it to a temperature stable mass. The hot side of the junction is exposed to incident radiation of the eardrum. To provide effective heat sink, hot junctions are thermally isolated from the cold junctions. The cold junction of the thermocouple is commonly measured using a thermistor to provide an accurate ambient temperature measurement. Some thermopile sensors have a built-in thermistor that provides measurement of the ambient temperature, thus allowing the temperature of the target to be calculated.
The thermopile output signal is fairly constant and directly proportional to incident radiation. This construction produces output voltages in the range of 100 µV to 900 µV. Measuring such low level signals imposes great challenges on signal conditioning circuits.
For portable battery-operated applications such as IR digital thermometers, low power dissipation and a single power supply operation are required. The high precision AD8538 auto-zero amplifier offers low power consumption, low input bias current, ultralow offset voltage and offset drift, and high open-loop gain to ensure best system performance. Operating on a low supply current of 180 µA max, the VOS is 13 µV max and TCVOS is 30 nV/°C typical. The AD8538 has rail-to-rail input/output operation down to 2.7 V and is available in a 5-lead TSOT.