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Analog Devices reference circuit CN0172 for high accuracy multichannel thermocouple measurement

Analog Devices reference circuits are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges.

In this Product Focus we will be looking at reference circuit number CN0172 which details a high accuracy multichannel thermocouple designed for precision temperature measurement using a signal chain of precision components including the ADT7320 ±0.25°C accuracy, 16-Bit Digital SPI temperature sensor  and the AD7793 3-Channel, low noise/power, 24-Bit Sigma Delta ADC with on-chip In-Amp and reference. The circuit addresses all the challenges for measuring thermocouple temperature with better than ±0.25°C accuracy including amplifying the small thermocouple voltages, reduction of noise, correction of nonlinearity and accurate cold junction compensation.

Each reference circuit from Analog Devices has a demo circuit board available to allow customers to fully test their design with real hardware helping to accelerate design cycles and time to market. For this reference circuit the EVAL-CN0172-SDPZ evaluation board is available which includes three K type thermo-couple connectors allowing measurement in the range of -200 C to +1200 C with high accuracy down to 0.25 degrees and a 16-bit digital temperature sensor for cold junction compensation.

Free evalboard   
Features
  • 3 channel K-type thermocouple measurement system
  • Typical temperature range of -200 C to +1200 C
  • Highly accurate temperature measurement to 0.25 degrees
  • 16-bit digital temperature sensor used for cold junction compensation
Applications
  • Field Instruments/ Smart Transmitters
  • Temperature Controllers
  • High Temperature
Resources

CIRCUIT FUNCTION AND BENEFITS

The function of the circuit shown in Figure 1 is to provide a high accuracy multichannel thermocouple measurement solution. Achieving a precision thermocouple measurement requires a signal chain of precision components that amplifies the small thermocouple voltage, reduces noise, corrects nonlinearity, and provides accurate reference junction compensation (commonly referred to as cold junction compensation). This circuit addresses all these challenges for measuring thermocouple temperature with better than ±0.25°C accuracy.

The circuit shown in Figure 1 shows how three K type thermo-couples are connected to the AD7793 precision 24-bit sigma-delta (Σ-Δ) analog-to-digital converter (ADC) to measure the thermocouple voltage. Because the thermocouple is a differential device rather than an absolute temperature measurement device, the reference junction temperature must be known to get an accurate absolute temperature reading. This process is known as reference junction compensation, commonly referred to as cold junction compensation. In this circuit, the ADT7320 precision 16-bit digital temperature sensor is used for the cold junction reference measurement and provides the required accuracy.

This type of application is popular where a cost-effective, accurate temperature measurement is required over the wide temperature ranges offered by thermocouples.

Analog Device

Figure 1. Multichannel Thermocouple Measurement System (Simplified Schematic: All Connections and Decoupling Not Shown) 

Download the full circuit note CN0172

Anglia are offering a FREE EVAL-CN0172-SDPZ Evaluation board, to register for yours please fill in the form below.

For a FREE Evaluation board please fill in the form below

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FREE samples for UK and Ireland customers only, subject to availability.

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