Thermocouple Temperature Measurement Solution Based on CM1103 and CM1106

01 Introduction

When two metal conductors made of different materials are joined together, a potential difference is generated at their ends if there is a temperature difference between the junction and the reference end. By measuring this thermoelectric potential, together with the reference temperature at the conductor ends and the characteristic parameters of the thermocouple, the actual temperature at the measurement junction can be determined.

Thermocouples are inexpensive, offer a wide temperature measurement range, have a simple structure, and are easy to use, making them widely applied in various industrial environments. The ITS-90 standard defines eight common thermocouple types along with their corresponding temperature ranges.

Among them, the K-type thermocouple is the most widely used. Taking it as an example: when measuring –270 °C, the thermocouple output voltage is –6.458 mV; at the full-scale temperature of 1372 °C, the output voltage is 54.886 mV. Across its entire measurement range, the output voltage changes by approximately 40 µV for every 1 °C change in temperature.

Because thermocouple output voltages are extremely small, the signal conditioning and data acquisition circuitry must provide very high precision. In addition, in industrial environments, the leads between the thermocouple (TC) and the data acquisition board are often long, which can introduce significant common-mode interference noise. Therefore, during signal acquisition and processing, it is necessary to extract and amplify the differential signal while minimizing the impact of common-mode noise on the measured differential signal.

02 CM1103 Features

• Low-power, high-precision, small-form-factor sigma-delta ADC analog front end

• Widely applicable to signal acquisition for various sensors, including temperature, optical, liquid, and gas sensors

• Sampling rate up to 2 kSPS with 4-channel programmable multiplexing

• Programmable gain amplifier (PGA) supporting flexible full-scale input ranges from ±256 mV to ±6.144 V

• High input impedance greater than 1 MΩ, allowing direct connection to a wide range of sensors and eliminating the need for additional signal conditioning and ADC driver circuits

• Noise-free resolution as low as 7.81 µV, sufficient for accurate measurement of the very small thermoelectric voltages generated by thermocouples

• Differential channel common-mode rejection ratio (CMRR) exceeding 100 dB, helping to reduce the impact of industrial interference on signal acquisition

03 Solution Overview

The CM1103 offers flexible channel configuration, with four input pins supporting up to two differential inputs or four single-ended inputs. As shown in the application diagram, in practical applications, one differential channel can be used to connect the thermocouple temperature sensor, while another single-ended channel can be connected to an NTC thermistor placed close to the cold junction. The TC and NTC channels can be configured with independent input gain and sampling rates, providing great convenience.

We also offer another product, the CM1106, which is based on the CM1103 and adds an on-chip temperature sensor. Through the SPI interface, the junction temperature of the CM1106 can be read in real time. For applications where the cold junction is located close to the ADC, selecting the CM1106 eliminates the need for an additional cold-junction measurement channel, enabling a higher number of temperature measurement channels.