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Maxim > Design Support > Technical Documents > Reference Designs > A/D and D/A Conversion/Sampling Circuits > APP 5032
Maxim > Design Support > Technical Documents > Reference Designs > Measurement Circuits > APP 5032
Maxim > Design Support > Technical Documents > Reference Designs > Medical > APP 5032
Keywords: data acquisition system, DAS, thermocouple, platinum resistance temperature detector, delta sigma ADCs,
delta sigma, National Institute of Standards and Technology, NIST ITS-90, sigma delta, temp measurement
REFERENCE DESIGN 5032 INCLUDES: Tested Circuit Schematic Description Test Data
Modern Thermocouples and a High-Resolution Delta-
Sigma ADC Enable High-Precision Temperature
Measurement
By: Joseph Shtargot, Strategic Applications Engineer
Sohail Mirza, Applications Engineer Manager
Aug 17, 2011
Abstract:
Many industrial
and medical applications require temperature measurements with accuracies of ±1°C or
better, performed with reasonable cost over a wide range of temperatures (-270°C to +1750°C), and often with low
power consumption. Properly selected, standardized, modern thermocouples paired with high-resolution ADC data
acquisition systems (DASs) can cover this wide temperature range and ensure reproducible measurements, even in
the harshest industrial environments.
A similar version of this article appeared in the June 22, 2011 issue of EE Times magazine.
Introduction
Thermocouples are used in a wide range of temperature-sensing applications. Recent developments in thermocouple
designs, as well new standards and algorisms, have greatly extended their temperature ranges and precision.
Accuracies up to ±0.1°C are now possible over a very wide -270°C to +1750°C range. To utilize all the new
thermocouple capabilities, high-resolution thermocouple temperature-measurement systems are required. A low-noise,
24-bit, delta-sigma analog-to-digital converter (ADC) with the ability to resolve very small voltages perfectly fits this
task. When a data acquisition system (DAS) uses the evaluation (EV) kit for a 24-bit ADC, thermocouple temperature
measurements can be made across that wide temperature range. When the thermocouple, platinum resistance
temperature detector (PRTD), and ADC are integrated in a circuit, they enable a high-performance temperature-
measurement system. The ADC-based DAS can also be designed to operate at very reasonable cost and with low
power consumption, making it ideal for portable sensing applications.
A Primer on Thermocouples
Thomas Seebeck discovered the principle of a thermocouple in 1822. A thermocouple is a simple temperature-
measurement device consisting of a junction of two dissimilar metals, Metal 1 and Metal 2 (Figure 1). Seebeck
discovered that different metals will produce different electric potentials based on the temperature gradient applied to
them. If these metals are welded together on the temperature-sensing junction (T
JUNC
, also known as the hot
junction), the other differential unconnected junction (T
COLD
, which is kept at a constant reference temperature) will
show a voltage, V
OUT
, that is directly proportional to the applied temperature at the welded junction. This makes
thermocouples a voltage/charge generating device that does not require any voltage or current excitation.
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