17.6.2 Thermocouple and IC Analog Temperature
30139599
FIGURE 40. Thermocouple with CJC
The LMP90xxx is also ideal for thermocouple temperature
applications. Thermocouples have several advantages that
make them popular in many industrial and medical applica-
tions. Compare to RTDs, thermistors, and IC sensors, ther-
mocouples are the most rugged, least expensive, and can
operate over the largest temperature range.
A thermocouple is a sensor whose junction generates a dif-
ferential voltage, VIN, that is relative to the temperature dif-
ference (Thot – Tcold). Thot is also known as the measuring
junction or “hot” junction, which is placed at the measured
environment. Tcold is also known as the reference or “cold”
junction, which is placed at the measuring system environ-
ment.
Because a thermocouple can only measure a temperature
difference, it does not have the ability to measure absolute
temperature. To determine the absolute temperature of the
measured environment (Thot), a technique known as cold
junction compensation (CJC) must be used.
In a CJC technique, the “cold” junction temperature, Tcold, is
sensed by using an IC temperature sensor, such as the
LM94022. The temperature sensor should be placed within
close proximity of the reference junction and should have an
isothermal connection to the board to minimize any potential
temperature gradients.
Once Tcold is obtained, use a standard thermocouple look-
up-table to find its equivalent voltage. Next, measure the
differential thermocouple voltage and add the equivalent cold
junction voltage. Lastly, convert the resulting voltage to tem-
perature using a standard thermocouple look-up-table.
For example, assume Tcold = 20°C. The equivalent voltage
from a type K thermocouple look-up-table is 0.798 mV. Next,
add the measured differential thermocouple voltage to the
Tcold equivalent voltage. For example, if the thermocouple
voltage is 4.096 mV, the total would be 0.798 mV + 4.096 mV
= 4.894 mV. Referring to the type K thermocouple table gives
a temperature of 119.37°C for 4.894 mV.
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