SBAA274A September 2018 – March 2023 ADS1118 , ADS1119 , ADS1120 , ADS112C04 , ADS112U04 , ADS1146 , ADS1147 , ADS1148 , ADS114S06 , ADS114S06B , ADS114S08 , ADS114S08B , ADS1219 , ADS1220 , ADS122C04 , ADS122U04 , ADS1246 , ADS1247 , ADS1248 , ADS124S06 , ADS124S08 , ADS125H02 , ADS1260 , ADS1261 , ADS1262 , ADS1263
Thermocouples are common temperature sensors used in a wide variety of commercial and industrial applications. While slightly less accurate than resistance temperature detectors (RTDs), thermocouples cover a wide temperature range, are self-powered, and have a fast response time. Their simple construction make them inexpensive and durable. Because of the small sensor voltage and low noise requirements, delta-sigma analog-to-digital converters (ADCs) are ideal data converters for measuring thermocouples. This application report gives an overview of thermocouples, discussing theory of operation, functionality, and methods in temperature measurement. Many circuits are presented showing thermocouple connections to precision ADCs. Different topologies focus on biasing thermocouples for the ADC input and for burn-out measurements.
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Thermocouples are temperature measurement sensors that generate a voltage that changes over temperature. Thermocouples are constructed from two wire leads made from different metals. The wire leads are welded together to create a junction. As the temperature changes from the junction to the ends of the wire leads, a voltage develops across the junction.
Combinations of different metals create a variety of voltage responses. This leads to different types of thermocouples used for different temperature ranges and accuracies. Choosing a thermocouple often is a function of the measurement temperature range required in the application. Other considerations include the temperature accuracy, durability, conditions of use, and the expected service life.
In 1820, Thomas Johann Seebeck discovered that when a metal bar is heated on one end, a voltage (known as the Seebeck voltage) develops across the length of the bar. This voltage varies with temperature and is different depending on the type of metal used in the bar. By joining dissimilar metals that have different Seebeck voltages at a temperature sensing junction, a thermocouple voltage (VTC) is generated.
The dissimilar metals are joined at a temperature sensing junction (TTC) to create a thermocouple. The voltage is measured at a reference temperature (TCJ) through the two metals. The leads of the thermocouple are required to be at the same temperature and are often connected to the ADC through an isothermal block. Figure 1-1 shows a thermocouple constructed from two dissimilar metals with the thermocouple leads connected to an isothermal block.
The connection of the thermocouple to an isothermal block is important for the temperature measurement. For an accurate thermocouple measurement, the return leads of different metals must be at the same known temperature.
Any connection between two different metals creates a thermocouple junction. Connections from the thermocouple to the ADC should be simple and symmetric to avoid unintentional thermocouple junctions. These additional junctions cause measurement errors.
As the thermocouple signal connects to the ADC integrated circuit, each step along the path can encounter several additional thermocouples. This becomes a measurement problem if there is a temperature gradient across the circuit. Each connection from wire terminal, to solder, to copper trace, to IC pin, to bond wire, to chip contact creates a new junction. However, if the signal is differential, and each of the thermocouple pairs are at the same temperature, then the thermocouple voltages cancel and have no net effect on the measurement. For high-precision applications, the user must ensure that these assumptions are correct. Measurement with differential inputs include unintentional thermocouple voltages that do not cancel if the thermocouples are not located close together, or if there is a thermal gradient on the board or device.