Channel-to-channel isolated temperature input (thermocouple/RTD) for PLC/DCS applications

    The circuit shown in Figure 1 provides a dual-channel, channel-to-channel isolated thermocouple or RTD input suitable for programmable logic controllers (PLCs) and distributed control systems (DCSs). This highly integrated design uses a low power, 24-bit, Σ-Δ analog-to-digital converter (ADC) with rich analog and digital features, eliminating the need for additional signal conditioning ICs. 
 

    
   Figure 1. PLC/DCS channel-to-channel isolated temperature input (Simplified schematic: decoupling and all connections not shown) 
 

Circuit Description 
 

The 24-bit Σ-Δ ADC integrates a programmable gain array (PGA) and a voltage reference, providing a complete feature set for flexible interfacing to thermocouple or RTD sensors. 
 

Features include an on-chip reference, programmable gain array, excitation current, bias voltage generator, and flexible filtering with options for enhanced 50 Hz and 60 Hz rejection. The AD7124-4 is available in a small 5 mm × 5 mm LFCSP package, making it ideal for channel-to-channel isolation designs where space is an important consideration. It also includes several diagnostic features available to the user. 
 

The ADuM5010 isolated dc/dc converter provides an isolated 3.3 V supply using integrated isoPower® technology. The ADuM1441 is used to isolate the serial peripheral interface (SPI) of the AD7124-4. The AD7124-4 micropower isolator consumes only 4.8 μA per channel when idle, making it very energy efficient. The ADP2441 is a 36 V step-down dc-to-dc regulator that operates from an industry standard 24 V supply with a wide input voltage tolerance. The ADP2441 is used to step down the input voltage to 3.3 V to power all controller side circuits.
 

System Overview 
 

Channel-to-channel isolation is very advantageous in automation systems because a failure in a specific input channel will not affect other channels in the system. However, channel-to-channel isolated input modules present significant design challenges in terms of complexity, space constraints, and system cost. 
 

Thermocouple or RTD inputs are common inputs for industrial automation systems, so it is useful to design a temperature input module that can handle both. This flexibility minimizes the design effort for both input modules and provides flexibility to the module user. 
 

AD7124-4显著降低了设计复杂度，提供一个片上系统，能够执行热电偶和RTD传感器所需的全部测量功能。
 

Each channel of the circuit shown in Figure 1 is only 27 mm x 50 mm in size, which can be further reduced by mounting the device on both sides of the printed circuit board (PCB). This small size is achieved because the AD7124-4 is packaged in a small 5 mm × 5 mm LFCSP package and integrates almost all the required functions except isolation and additional front-end filtering and protection. The isolation circuit for data and power isolation occupies only 87 mm2, with a minimum combined width of 12.5 mm. 
 

Terminal Connection 
 

Figure 2 shows the terminal connections for each of the two input channels. These pins correspond to P1 and P2 in the hardware (see Figure 1). Thermocouple and 2/3/4-wire RTD connections are shown. 


 

Figure 2. Front-end filtering and circuit (simplified) 
 

Input filtering 
 

As shown in Figure 3, input common-mode noise filtering is implemented by R1, C1 and R2, C2 with a cutoff frequency of approximately 50 kHz. Differential noise filtering is implemented by R1, R2, and C3 with a cutoff frequency of approximately 2.5 kHz. It is particularly important to filter out any interference at the Σ-Δ modulator frequency (307 kHz in full power mode). It is recommended that the cutoff frequencies of these filters be adjusted to meet the system bandwidth requirements, with the common-mode filter cutoff frequency being approximately 10 times the differential filter cutoff frequency. 

 

Figure 3. Front-end filtering and circuit (simplified) 
 

Input protection 
 

To protect the inputs from overvoltage conditions, 3 kΩ resistors are placed on each input path of the AD7124-4. This resistor value limits the current generated by a 30 V dc overvoltage to less than 10 mA. 
 

Consider the case where 30 V is connected between AIN+ and AIN−. Looking in from AIN+, the 30 V sees R1 (3 kΩ), followed by the internal ESD protection diodes, followed by the 3 kΩ resistor looking out from AIN3 in parallel with the 3 kΩ resistor looking out from AIN4. Ignoring the internal ESD protection diodes, the total resistance between AIN+ and AIN− is 3 kΩ + 3 kΩ||3 kΩ = 4.5 kΩ. Therefore, the current through the AD7124-4 is limited to 30 V ÷ 4.5 kΩ = 6.7 mA. 
 

RTD Input 
 

The circuit shown in Figure 1 can be connected to 2-wire, 3-wire, or 4-wire RTDs. The maximum resistance that can be measured is 3.92 kΩ, so it is suitable for Pt100 and Pt1000 RTDs. Using current excitation, the resistance measurement is a ratiometric measurement between the RTD and a 3.92 kΩ precision reference resistor (RREF). As shown in Figure 3, the RTD measurement is made between AIN1 and AIN3, with REFIN1+ and REFIN1− used as the reference inputs for the measurement. The excitation current is set as follows: 
 

2-wire mode: Only excitation on AIN0 is active and is set to 250 μA. 
 

3-wire mode: The excitation currents on both AIN0 and AIN4 are active and are set to 100 μA each. 
 

4-wire mode: Only excitation on AIN0 is active and is set to 250 μA. 
 

Use a high-side current sensing technique. For lower RTD lead resistance values, this technique reduces the effects of current mismatch in 3-wire mode. For more information on 3-wire RTD configurations, see Circuit Note CN-0383. 
 

The reference resistor (RREF) was chosen to be 3.92 kΩ to allow for Pt1000 RTD measurements up to 850°C (RTD resistance at 850°C is 3.9048 kΩ). The value of RREF must be chosen based on the maximum expected resistance of the RTD. The accuracy of the RREF resistor directly affects the measurement accuracy, so a precision, low drift resistor must be used. 
 

The excitation current must be set to 250 µA in 4-wire mode and 100 µA in 3-wire mode. For 4-wire mode, assume the RTD value is 3.92 kΩ. The excitation current from AIN0 flows through RREF + RRTD + RRETURN = 3.92 kΩ + 3.92 kΩ + 3 kΩ = 10.84 kΩ. Therefore, the voltage at AIN0 is equal to 250 µA × 10.84 kΩ = 2.71 V. The AD7124-4 specifies an output compliance voltage of AVDD − 0.35 V at the excitation current output, which is 3.3 V – 0.35 V = 2.95 V. Because 2.95 V > 2.71 V, the 250 µA excitation current works properly even for the largest RTD resistance. 
 

For more information on 4-wire RTD configurations, see the Circuit Note CN-0381. 
 

In 3-wire mode, the pin compensation excitation current from AIN4 also flows through the 3 kΩ return resistor, generating an additional voltage at AIN0: 250 μA × 3 kΩ = 0.75 V. Therefore, the total voltage at AIN0 equals 2.71 V + 0.75 V = 3.46 V, which violates the headroom requirement. Therefore, in 3-wire mode, each excitation current must be reduced to 100 μA to provide sufficient headroom. 
 

The PGA gain can be used to increase the measurement resolution. For a Pt100 RTD, a gain of 8 is recommended (because the Pt100 value is 10 times smaller than the Pt1000 value). 
 

To achieve the required accuracy, the RTD itself must be linearized by the host controller via software; see Circuit Note CN-0383. 
 

Thermocouple Measurements 
 

As shown in Figure 3, the thermocouple is connected between the AIN+ and AIN− terminals. The AIN4 pin provides a bias voltage to the thermocouple of 3.3 V ÷ 2 = 1.65 V. The thermocouple voltage is measured between AIN1 and AIN3, and because the thermocouple signal is very small, a PGA gain of 32 or 64 is typically recommended. 


 

A 10 kΩ NTC thermistor is used for cold junction compensation. The reference voltage excitation VREF is obtained from REFOUT and is connected in series with a precision low drift 5.62 kΩ resistor to ground. The NTC resistor value can be calculated using the following equation: 
 

in: 
 

VNTC is the voltage measured between AIN1 and AIN3. 
 

VREF is the reference voltage provided by AD7124-4 REFOUT. 
 

The temperature difference between the terminal strip and the NTC temperature sensor will directly affect the temperature reading of the thermocouple input. Therefore, the NTC thermistor must be placed as close to the terminal strip as possible to maximize thermal coupling. 
 

To achieve the required accuracy, the thermocouples and NTCs must be linearized by the host controller via software; see Circuit Note CN-0384. 
 

diagnosis 
 

Provides a variety of system-level diagnostic capabilities, including:
 

    Reference voltage detection 

    Input overvoltage/undervoltage detection 

    CRC for SPI communication 

    Memory Map CRC 

    SPI read/write check 
 

    These diagnostic features provide a high level of coverage for possible faults that may occur in the input channels. 
 

isolation 
 

The ADuM1441 is available in a small 5 mm × 6.2 mm, 16-lead QSOP package (30 mm2). The data channels are isolated using the ADuM1441, a quad-channel micropower isolator that is very energy efficient. 
 

The ADuM5010 is a complete isolated development converter that uses isoPower technology to provide power isolation for circuits. The ADuM5010 is in a small 7.4 mm × 7.5 mm, 20-lead SSOP package (56.25 mm2).
 

Figure 4 shows the ADuM5010 circuit details. Ferrite beads are used on the secondary side of the power supply to suppress potential electromagnetic interference (EMI) radiation. The ferrite beads (Murata BLM18HK102SN1) are specifically selected for high impedance from 100 MHz to 1 GHz. 10 μF and 0.1 μF decoupling capacitors are also used. Both the ferrite beads and capacitors are connected to the ADuM5010 pins with short traces to minimize parasitic inductance and resistance. 


 

Figure 4. isoPower circuit with ferrite beads and decoupling capacitors. 
 

The stitching capacitors have been kept to a minimum area, as the ferrite beads have significantly reduced the radiation. The PCB area between the ADuM5010 power supply, GND pins, and the ferrite beads should be free of any ground planes or traces to minimize the capacitive coupling of high frequency noise into the ground plane. For more information on controlling the radiation of isoPower devices, see the AN-0971 Application Note. 
 

The R1 and R2 feedback resistors were selected according to the ADuM5010 data sheet for a 3.3 V output. 
 

Power consumption per channel 
 

The ADuM5010 is powered by the controller side supply and consumes 3.3 mA typical. The efficiency of the ADuM5010 is only 27% when fully loaded, so minimizing the current consumption on the field side will have a significant impact on the energy efficiency of the channel.
 

AD7124-4功耗约为994 μA（全功率模式、增益 = 32、TC偏置、诊断和内部基准电压源使能）。利用中功率或低功耗模式可以显著降低AD7124-4的功耗。 
 

For the ADuM1441, the total field-side power consumption is approximately 7.2 μA when idle and 552 μA when operating at 2 Mbps. If the interface is active 1/8 of the time, the total power consumption of the ADuM1441 is (552 μA × 0.125) + (7.2 μA × 0.875) = 75.3 μA. 
 

When operating in full power mode with a gain of 32, internal reference, and TC bias enabled, the measured power consumption for one input channel is 7.9 mA (from a 3.3 V supply on the controller side). 
 

Power Circuit 
 

The evaluation board is powered by a 4.5 V to 36 V dc supply and uses an on-board switching regulator to provide a 3.3 V supply to the system, as shown in Figure 5. The EVAL- SDP-CB1Z system demonstration platform (SDP) board provides the regulated 3.3 V for the digital interface.  
 

The ADP2441 includes features such as programmable soft start, regulated output voltage, switching frequency, and a power good indicator. These features are programmed through small external resistors and capacitors. 
 

The ADP2441 also includes protection features such as undervoltage lockout (UVLO) with hysteresis, output short-circuit protection, and thermal shutdown.
 

A 300 kHz switching frequency maximizes the efficiency of the ADP2441. Due to the very high switching frequency of the ADP2441, it is recommended to use a shielded ferrite core inductor with low core loss and low EMI. 
 

In the circuit shown in Figure 5, the switching frequency is set to approximately 300 kHz using a 294 kΩ external resistor. The 22 μH inductor value (Coilcraft LPS6235-223MLC) was selected using the downloadable ADP2441 Buck Regulator Design Tool. This tool selects the best component values ​​based on the required operating conditions (4.5 V to 36 V input, 3.3 V output, 1 A output current). The 1 A current was chosen to power other circuits on the host controller side if required. 

 

Figure 5. Power Supply Circuit (Simplified Schematic; Not All Connections Shown) 

 

Test Results 
 

For a detailed performance analysis of the thermocouple, 3-wire, and 4-wire RTD circuits, see Circuit Note CN-0381, Circuit Note CN-0383, and Circuit Note CN-0384, which present in-depth analysis and measurement results. 
 

Figure 6 shows a histogram of the EVAL-CN0376-SDPZ with a 25 SPS post filter, AIN+ shorted to AIN−, a gain of 32, and TC bias enabled. The data corresponds to 17.85 bits of noise-free resolution. 

 

Figure 6. Histogram of codes with AIN+ and AIN- inputs shorted (25 SPS post filter selected, gain = 32, TC bias enabled) 

 

Common changes 
 

If more channels are needed, the AD7124-8 can be used. The AD7124-8 has 8 differential inputs or 16 single-ended inputs. The AD7792 can also be considered as a low-cost option, although it has fewer features and lower performance.  
 

A SPIsolator™ such as the ADuM3151 is available as a data isolation option, which supports up to 17 MHz SPI transmission and has three general-purpose low-speed isolation channels built in.
 

The circuit shown in Figure 1 uses an NTC thermistor for cold junction compensation. Another option is to use the ADT7320 digital temperature sensor, which has an accuracy of 0.25°C. (See Circuit Note CN-0172). 
 

Circuit Evaluation and Testing 
 

The circuit shown in Figure 1 uses the EVAL-CN0376-SDPZ evaluation board and the EVAL-SDP-CB1Z SDP controller board. 
 

The EVAL-CN0376-SDPZ evaluation board has a PMOD-compatible header to enable integration with an external control board.
 

The CN-0376 Evaluation Software communicates with the SDP board to configure and capture data from the EVAL-CN0376-SDPZ evaluation board.
 

Equipment Requirements 
 

The following equipment is needed: 
 

PC with Windows® Vista (32-bit) or Windows 7 (32-bit) with a USB port 

EVAL-CN0376-SDPZ circuit evaluation board

EVAL-SDP-CB1Z SDP control board

CN-0376 Evaluation Software

Precision voltage and resistance source, or thermocouple, RTD simulator 

Power supply: 4.5 V to 36 V dc (100 mA) 
 

start using 
 

Download the CN-0376 Evaluation Software from ftp://ftp.analog.com/pub/cftl/CN0376/ and install it. Follow the on-screen instructions to install and use the software. Refer to the CN-0376 Software User Guide for more information. 
 

Test Setup Functional Block Diagram 
 

Figure 7 shows a functional block diagram of the test setup. 


 

Figure 7. Test setup functional block diagram 
 

set up 
 

The EVAL-CN0376-SDPZ evaluation board connects to the EVAL-SDP-CB1Z SDP board through a 120-pin mating connector that is available on both boards. 

The CN-0376 evaluation software and SDP board allow data analysis using a PC.
 

Apply a voltage in the range of 4.5 V to 36 V (24 V nominal) to the P3 connector. Be sure to set the P8 jumper to EXT (default) to power the board from the P3 supply. 
 

An external controller can also be used to communicate with the evaluation board (SPI communication through the PMOD header) and to power the evaluation board. If desired, the P8 jumper can be set to VCC_PMOD to provide 3.3 V power to the evaluation board through the PMOD connector. 
 

Precision voltage and resistance sources can be used as inputs to the analog front end to evaluate system performance. A thermocouple or RTD simulator can also be used. 


 

Figure 8 shows a photograph of the EVAL-CN0376-SDPZ circuit evaluation board.