Challenges and solutions for thermocouple signal
By John Austin, Product Marketing Manager,
Analog and Interface Products Division,
Microchip Technology Inc.
The thermocouple is one of the oldest and most widely used components for
measuring temperature. Thermocouples
are generally found in applications that
require temperature measurements in
hostile environments, such as boilers, ovens,
What is a thermocouple? A thermocouple consists of
two wires made of dissimilar metals, joined together at
one end. The joined end is typically referred to as the
“hot” junction, while the open end is called the “cold”
junction. The differential voltage between the two wires
is used to calculate the temperature at the hot junction,
as shown in Figure 1.
Figure1: Simplified Thermocouple Diagram.
Images courtesy of Microchip Technology Inc.
All thermocouples measure microvolt-level signal
changes. The most common thermocouple types are
J, K, and T, and their room-temperature voltages vary
at 52 μV/°C, 41 μV/°C and 41 μV/°C, respectively.
Because their voltage signal is very small, it can be
difficult to extract from the system noise. Also, the
thermocouple output is not linear over temperature,
requiring the use of high-order equations to accurately
calculate the temperature. Furthermore, a thermocouple
measurement is only as accurate as its cold-junction
temperature measurement, adding more complexity to
an already complex system. Generally, thermocouple
signal conditioning is the largest investment in a
The differential voltage generated at the cold
junction is dependent on the temperature
differential between the hot junction and cold
junction. Therefore, in order to obtain an accurate
overall temperature reading, one must know the
temperature at the cold junction. This is known
as cold-junction compensation (CJC). The overall
temperature accuracy of the thermocouple solution
is limited by the temperature accuracy of its CJC.
Today, there are many solutions for measuring the
cold-junction temperature, such as RTDs, thermistors
and silicon-based IC temperature sensors. Thermistors
have fast responses and small packages, but they
require linearization and have limited accuracy over
wide temperature ranges. They also require current for
excitation, which can produce self-heating and increases
power consumption, thus limiting their use in many
portable or battery-powered applications. Resistance
temperature-detectors (RTDs) are accurate, stable and
reasonably linear devices. However, package size and
cost restrict their use in many applications. Silicon IC
temperature sensors now have temperature accuracies
better than 0.5°C. Silicon ICs are simple devices, and
require minimal external circuitry or thermal design
knowledge to implement. This simplicity, along with
improved temperature accuracy, has increased the
popularity of these devices in recent years.
Generally, discrete thermocouple solutions use
an instrumentation amplifier (INA) to extract the
thermocouple voltage, and the INA rejects voltages that
are common to each input of the device. Since most of
the noise will be common to each thermocouple lead,
the INA effectively filters the noise.
There are a variety of instrumentation amplifiers
available today. The traditional INA topology utilizes
two operational amplifiers, for the gain stage, which then
feed into a third operational amplifier configured as a
differential amplifier, as shown in Figure 2.