Application of highly integrated analog front-end AFE AD7124 in RTD temperature measurement

“With the development of industrial production, temperature measurement and control are very important, and accurate measurement of temperature parameters is crucial to output quality, production efficiency, and safe and reliable operation. As a high-performance analog manufacturer, ADI has a complete temperature measurement/control solution, from passive resistance temperature detector RTD, thermocouple Thermocouple (TC), to direct current/voltage analog output, SPI/IIC semiconductor temperature sensor with digital interface output.

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With the development of industrial production, temperature measurement and control are very important, and accurate measurement of temperature parameters is crucial to output quality, production efficiency, and safe and reliable operation. As a high-performance analog manufacturer, ADI has a complete temperature measurement/control solution, from passive resistance temperature detector RTD, thermocouple Thermocouple (TC), to direct current/voltage analog output, SPI/IIC Semiconductor temperature sensor with digital interface output.

Vincent Chen, a technical support engineer of Excelpoint Shijian, an agent who has cooperated with ADI for more than 30 years, introduced to us ADI’s popular analog front-end AFE AD7124-4/8 suitable for industrial contact RTD temperature measurement.

Figure 1: AD7124-4/8 Internal Block Diagram

The rich features of the AD7124 optimized for RTD temperature measurement include the following:

1) 6 programmable matched excitation currents from 50 µA, 100 µA, 250 µA, 500 µA, 750 µA to 1 mA

2) xMux can switch two constant current sources to the required channels, and flexibly form a 2/3/4-wire PT100 temperature measurement

3) Simultaneous rejection of 50 Hz/60 Hz (25 SPS, 40mS single cycle settling, 62dB rejection)

4) Analog input buffer AIN buffer, plus 8 kinds of PGA programmable gain, 1-128 V/V, no need for external additional conditioning circuit

5) Proportional Ratiometric measurement to eliminate error sources such as the accuracy and drift of the excitation current source.

ADI’s official website can retrieve the tested laboratory circuit Circuit Note: “CN-0381, Fully Integrated 4-Wire RTD Measurement System Using Low Power, Precision, 24-Bit Sigma-Delta ADC”; and “CN0383, Using Fully Integrated 3-Wire RTD Measurement System for Low Power, Precision, 24-Bit Sigma-Delta ADCs”.

Using two-point calibration and linearization, the overall accuracy of the 3-wire system is better than ±1°C over the temperature range of −50°C to +200°C. The typical noise-free code resolution of the system is 17.9 bits in full power mode, with a sinc4 filter selected, and an output data rate of 50 SPS; in low power mode, with a post-filter selected, and an output data rate of 25 SPS Under the conditions of , the typical noise-free code resolution of the system is 16.8 bits.

Figure 2: Temperature Accuracy Measurement (Sinc4 Filter, Full Power Mode, 50 SPS, 25°C One-Time Calibration Only)

Analog Dialogue, a technical weekly published by ADI, analyzes the impact of RC passive components in the ADC front-end on measurement accuracy. For details, see “Analog Front-End Design Considerations for RTD Ratiometric Temperature Measurements” on ADI’s official website.

Figure 3: Analog Input Configuration for RTD Measurements Using the AD7124

A single AD7124-8 can support up to eight 3-wire RTDs. If the EMC immunity requirements are very high, it is recommended to use the connection on the right instead. The R/C value of the first-order passive filter can be taken as large. At this time, an AD7124-8 only supports 4-way 3-wire RTD.

Figure 4: Two connection methods for 3-wire system, focusing on channel density or EMC performance

The flexible hardware configuration of AD7124 needs to be combined with the same flexible software configuration to realize the final realization. There are a total of 8 independent settings (Channel Register is different from the physical channel, even 8/16 physical channels, using the same set of Config), each setting includes the following four registers:

• Configuration register Config_N

• Filter register FILTER_N

• Offset register OFFSET_N

• Gain register GAIN_N

Figure 5: Mixed differential and single-ended configuration (using multiple common settings)

ADI official website provides non-os driver code, a typical driver initialization code, as follows:

The following is the recommended ADC configuration process:

It is understood that with the technical support of Shijian, a number of well-known local industrial control manufacturers in the industry have adopted the multi-channel RTD/TC temperature measurement board designed by AD7124-8, which has been mass-produced and launched.