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. 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 detectors RTD, thermocouples Thermocouple (TC), to direct current/voltage analog output, SPI/IIC digital interface output semiconductor temperature sensors.

Vincent Chen, technical support engineer at Excelpoint, an agent that has been working with ADI for more than 30 years, introduced to us ADI's popular analog front end AFE AD7124-4/8 for industrial contact RTD temperature measurement.

Figure 1: AD7124-4/8 internal block diagram

AD7124 has rich features optimized for RTD temperature measurement:

1) 6 programmable matching 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 to flexibly form 2/3/4-wire PT100 temperature measurement

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

4) Analog input buffer AIN buffer, plus 8 PGA programmable gains, 1-128 V/V, no external conditioning circuit required

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

The tested and verified laboratory circuit Circuit Notes can be retrieved from the ADI official website: "CN-0381, Fully Integrated 4-Wire RTD Measurement System Using Low Power, Precision, 24-Bit Σ-Δ ADC"; and "CN0383, Fully Integrated 3-Wire RTD Measurement System Using Low Power, Precision, 24-Bit Σ-Δ ADC".

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 the sinc4 filter selected, and an output data rate of 50 SPS; in low power mode, with the post filter selected, and an output data rate of 25 SPS, 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, one-time calibration at 25°C only)

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

Figure 3: Analog input configuration for RTD measurements using the AD7124

One AD7124-8 can support up to 8 3-wire RTDs. If the EMC immunity requirement is very high, it is recommended to use the connection on the right side. The R/C value of the first-order passive filter can be larger. At this time, one AD7124-8 can only support 4 3-wire RTDs.

Figure 4: Two ways to connect three-wire systems, focusing on channel density or EMC performance

The flexible hardware configuration of AD7124 needs to be matched with the equally flexible software configuration to achieve the final implementation. There are a total of 8 independent settings (Channel Register is different from the physical channel, and even 8/16 physical channels can use 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 configurations (using a variety of common settings)

ADI's 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 Company, a number of well-known local industrial control manufacturers in the industry have adopted the multi-channel RTD/TC temperature measurement board designed with AD7124-8 and have put it into mass production and on the market.