Analog Devices: CN0271 Reference Design for K-Type Thermocouple Measurement System with Integrated Cold Junction Compensation

Kit Overview   The thermocouple is a simple, widely used component for measuring temperature. It consists of a junction of two dissimilar metals. These metals are connected at one end to form the measurement junction, also called the hot junction. The other end of the thermocouple is connected to the metal lines that lead to the measurement electronics. This connection forms a second junction: the reference junction, also called the cold junction.   The EVAL-CN0271-SDPZEVAL-CN0271-SDPZ circuit is a complete thermocouple signal conditioning circuit with cold junction compensation followed by a 16-bit sigma-delta (Σ-Δ) analog-to-digital converter (ADC). The AD8495AD8495 thermocouple amplifier provides a simple, low cost solution for measuring K type thermocouple temperatures, including cold junction compensation.   A fixed gain instrumentation amplifier in the AD8495AD8495 amplifies the small thermocouple voltage to provide a 5 mV/°C output. The high common-mode rejection of the amplifier blocks commonmode noise that the long thermocouple leads can pick up. For additional protection, the high impedance inputs of the amplifier make it easy to add extra filtering.   The AD8476AD8476 differential amplifier provides the correct signal levels and common-mode voltage to drive the AD7790AD7790 16-bit, Σ-Δ ADC. The circuit provides a compact low cost solution for thermocouple signal conditioning and high resolution analog-to-digital conversion.   This circuit uses the EVAL-CN0271-SDPZEVAL-CN0271-SDPZ circuit board and the System Demonstration Platform (SDP-B) controller board (EVAL-SDP-CB1ZEVAL-SDP-CB1Z). The two boards have 120-pin mating connectors, allowing for the quick setup and evaluation of the performance of the circuit. The EVAL-CN0271-SDPZcontains the circuit to be evaluated, and the SDP-B controller board is used with the CN-0271 evaluation software to capture the data from the EVAL-CN0271-SDPZ circuit board.    Key Applications: Temperature Controllers

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Development Tools

• A PC with a USB port and Windows XP or Windows Vista (32-bit), or Windows 7 (32-bit)
• The EVAL-CN0271-SDPZEVAL-CN0271-SDPZ circuit evaluation board
• The SDP-B controller board (EVAL-SDP-CB1ZEVAL-SDP-CB1Z) or the SDP-S controller board (EVAL-SDP-CS1ZEVAL-SDP-CS1Z)
  • EVAL-SDP-CB1ZEVAL-SDP-CB1Z Features:

• The CN-0271 SDP evaluation software
• The 6 V power supply (EVAL-CFTL-6V-PWRZEVAL-CFTL-6V-PWRZ) or equivalent dc power supply

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Kit Features

Key Features for EVAL-CN0271-SDPZEVAL-CN0271-SDPZ Evaluation Board are as below:

• Complete thermocouple signal conditioning circuit with cold junction compensation followed by a 16-bit sigma-delta (Σ-Δ) analog-to-digital converter (ADC).
• The AD8495AD8495 thermocouple amplifier provides a simple, low cost solution for measuring K type thermocouple temperatures, including cold junction compensation.
  • Pretrimmed for J or K type thermocouples
  • Internal cold junction compensation
  • High impedance differential input
  • Standalone 5 mV/°C thermometer
  • Reference pin allows offset adjustment
  • Thermocouple break detection
• A fixed gain instrumentation amplifier in the AD8495 amplifies the small thermocouple voltage to provide a 5 mV/°C output.
• The AD8476 differential amplifier provides the correct signal levels and common-mode voltage to drive the AD7790 16-bit, Σ-Δ ADC.
• Features for AD8476AD8476:
  • Very low power: 330 μA supply current
  • Fully differential or single-ended inputs/outputs
  • Differential output designed to drive precision ADCs
    • Drives switched capacitor and Σ-Δ ADCs
    • Rail-to-rail output
  • VOCM pin adjusts output common mode
  • Robust overvoltage up to 18 V beyond supplies
• Features for AD7790AD7790:
  • Power
    • Supply: 2.5 V to 5.25 V operation, Normal: 75 µA maximum, Power-down: 1 µA maximum
  • RMS noise: 1.1 µV at 9.5 Hz update rate
  • 16-bit p-p resolution
  • Integral nonlinearity: 3.5 ppm typical
  • Simultaneous 50 Hz and 60 Hz rejection
  • Internal clock oscillator
  • Programmable gain amplifier
  • Rail-to-rail input buffer
  • VDD monitor channel

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Kit Contents

The Analog Devices EVAL-CN0271-SDPZEVAL-CN0271-SDPZ kit is supplied with below contents:

• Evaluation Board

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