3-Channel Thermocouple Temperature Measurement System with 0.25 В° C Accuracy (CN0172)
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.
Figure 1. Multichannel Thermocouple Measurement System (Simplified Schematic: All Connections and Decoupling Not Shown)
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