Can a complete RTD module with overvoltage protection be designed?

In industrial environments, improper operation, incorrect connection wires, and exposed wires often lead to overvoltage faults, which can damage electronic devices and cause adverse consequences. Overvoltage protection capability is a key requirement for RTD modules. In addition to transient overvoltage protection, continuous overvoltage protection must also be considered in actual production processes.

This article focuses on how to provide a comprehensive solution for a multi-wire RTD module with overvoltage protection (based on AD7124), and introduces multiplexers and channel protectors with overvoltage protection and detection functions. This article can help designers understand this method and select appropriate components.

Ratio measurement is widely used in RTD modules because it can eliminate the error and drift of the excitation current source. Figure 1 is a typical schematic diagram of a 4-wire RTD measurement based on the AD7124-8.

Figure 1. 4-wire RTD ratiometric measurement based on the AD7124-8.

AIN0 provides the excitation current. The AD7124 integrates a reference voltage buffer and PGA. REFIN and AIN are both high impedance inputs, so the same current will flow through the RTD sensor and the reference resistor. The ADC conversion result is the ratio of the input voltage (V _RTD ) and the reference voltage (V _REF ), which is equal to the ratio of R _RTD and R _REF . If R _REF is a known high-precision and stable reference resistor, R _{RTD can be calculated using the R REF} value and the ADC conversion result .

With a 4-wire RTD configuration, the system can achieve high accuracy and reliability, and can eliminate the error caused by lead resistance. Correspondingly, its cost is higher than that of a 3-wire or 2-wire configuration. The 3-wire RTD sensor has a high cost-performance ratio. Figure 2 shows a 3-wire RTD measurement solution based on AD7124.

Figure 2. 3-wire RTD ratiometric measurement based on the AD7124-8

Two integrated, well-matched current sources facilitate 3-wire RTD measurements. V _REF and V _RTD can be expressed by the following two functions:

The AD7124 integrates two well-matched current sources, which means that IEXC0 is close to or equal to IEXC1, and the lead resistances RL1 and RL2 are very similar. The above function can be expressed as:

The conversion result can be expressed using the above two functions as:

According to this function, the RTD resistance value can be calculated from the conversion result and the reference resistance value.

For 2-wire RTDs, the errors caused by the lead resistance cannot be cancelled, but this type of RTD sensor costs less than other sensors; the AD7124-8 can be configured as a 2-wire RTD sensor, as shown in Figure 3.

Figure 3. 2-wire RTD ratiometric measurement based on the AD7124-8

In practice, many industrial customers require that many different types of RTD sensors be connected to the same port of the RTD module to facilitate the balance between the cost and performance of the RTD sensor. Figure 4 shows the universal interface of the RTD module, which can support RTD sensors with different line counts.

Figure 4. RTD interface for sensors with different line counts

For this requirement, such RTD modules need to be configured through software to support RTD sensors with different line counts. Figure 5 shows the block diagram of RTD sensors with different line counts based on AD7124-8 and switches. AD7124-8 supports 4-channel, 2-wire/3-wire/4-wire RTD measurements.

Figure 5. Measurement of RTD sensors with different line counts based on AD7124-8

The controller can be used to easily change the configuration for different sensors. Table 1 shows the switch and current source states for different configurations.

Table 1. Switch and IEXT status of RTD sensors with different line numbers

By calculating and selecting the appropriate resistor and capacitor values, noise performance can be optimized. The ADI official website article "Analog Front-End Design Considerations for RTD Ratiometric Temperature Measurement " can be used as a guide. In addition to optimizing noise performance, some additional measures are required to implement overvoltage protection.

First, some analog pins of the AD7124 are directly exposed to the external environment. According to the absolute maximum rating of the AD7124 at 25°C, the analog input voltage should be between –0.3V and AVDD+0.3V, which means that the module cannot provide protection when a high overvoltage occurs. Second, the three switches need to withstand high voltage.

Adding current limiting resistors to each pin of the AD7124 can provide overvoltage protection for the AD7124.

Figure 6 shows the analog pin architecture of the AD7124. There are two clamping diodes on each analog pin, which can be used to implement protection directly without introducing any additional leakage current.

Figure 6. AD7124-8 analog pin internal architecture

Figure 7 shows the schematic diagram of this method, with R1 to R4 placed in front of AIN1, AIN2, REF+, and REF–, respectively. This setup is used to eliminate noise. At the same time, these resistors can be used for current limiting; adding current limiting resistors in front of AIN0 and AIN3 can protect the remaining exposed analog pins of the AD7124.

Figure 7. Adding current limiting resistors in front of the ADC input pins.

These resistors and internal clamping diodes can protect against a certain degree of positive and negative overvoltage. When a positive or negative overvoltage fault occurs, the current will flow to AVDD or AVSS through the resistors and internal clamping diodes. According to the absolute maximum specification of the AD7124, the current value must be limited to less than 10mA. If RLimit is equal to 3 kΩ, the module can protect against ±30V continuous overvoltage.

However, when the module is operating in normal mode, a voltage drop will occur across RLIMIT. If the excitation current is 500μA, the voltage drop across RLIMIT will be 1.5V, and the sensor resistor and R _REF will be limited. Increasing RLIMIT can achieve better protection, but the sensor and reference resistor value range will be smaller. Based on this protection method, the compliance voltage will decrease as the overvoltage protection requirement increases. It is important to note the power dissipation of R _REF and RReturn, and the fault voltage will fall directly on these two resistors.

In addition to the AD7124-8 analog pins, the switches are also exposed to high voltage, so a device should be selected that can protect against ±30 V. In the past few years, photo MOS and relays have been used in these situations, but the high price and large package limit the application range.

The biggest disadvantage of using a current limiting resistor is the low compliance voltage on SOURCE+. Using discrete transistors and diodes, it is possible to implement overvoltage protection and increase the maximum allowed voltage on the SOURCE+ pin. Figure 8 shows a schematic of this approach.

Figure 8. Overvoltage protection using discrete transistors and diodes.

This structure allows the excitation current to always flow to the RTD sensor under normal conditions and prevents high overvoltage damage. Other analog input pins can be protected by current limiting resistors because the analog input pins have no compliance voltage limits.

If a large positive voltage is applied to the RTD sensor, D1 will prevent the current source from being affected by the positive high voltage. If a large negative voltage is applied to the RTD sensor, the PN junction between the collector and base of Q1 will be reverse biased, resulting in a high voltage drop across RB1 and the PN junction, preventing damage to AIN0.

In normal mode, D2 acts as a reverse biased diode, causing very little current to flow through the component. The current flowing through the emitter of Q1 to the base is very small, so the voltage drop across RB1 is negligible. This approach allows the compliance voltage to be higher than if a current limiting resistor was used, and provides protection against much higher fault voltages.

The disadvantages of using discrete components to protect this high-precision RTD module are obvious: it is not easy to choose the right components; these components will complicate the protection circuit; and they will occupy a large PCB area.

Although the leakage current of the AD7124 analog input pins is very small, the large resistors in series with these pins (such as R1 and R2) will produce significant errors, and the thermal noise of these resistors will reduce the resolution. In a practical design, the RTD module may have multiple channels, and the current source switches from one channel to another. The large resistor value will increase the settling time of the analog input RC combination, and the RTD module should spend more time charging the capacitors, such as C1, C2, and C3. It is difficult to balance protection functions and accuracy. Switches also need to be protected against high overvoltages.

In this case, using analog switches and multiplexers with fault protection can provide both switching and overvoltage protection. Figure 9 shows an example.

Figure 9. Fault-protected analog switches and multiplexers.

In Figure 9, three SPDT switches from the ADG5243F are used in front of the AD7124, and two variable resistors from the ADG5462F are used in front of AIN1 and AIN2. These protection functions can be implemented using the ADG5243F and ADG5462F, which have user-defined fault protection and detection functions.

Latch-up immunity, low leakage current, and industry-leading RON flatness are also advantages of these devices. Low leakage current and low resistance can improve the accuracy and noise performance of this RTD module.

If a positive or negative voltage is applied to the RTD interface, the voltage on the drain pin will be clamped at POSFV + VT or NEGFV – VT. If POSFV is set to 4.5V and NEGFV is set to AGND, the series resistor in the path to protect the AD7124 is much easier to choose. If the overvoltage occurs in the unpowered state, the switch remains in a high impedance state to help prevent damage to the device.

The detection features of these devices can be used for system diagnostics. The source input voltage of the ADG5243F and ADG5462F is continuously monitored. The active low digital output pin FF indicates the state of the switch. The voltage on the FF pin indicates if any of the source input pins has failed. The AD7124 provides many powerful system safety diagnostic features. The processor can combine the diagnostic features of these devices to build a more robust system.

The functional blocks and diagnostics in the AD7124 improve accuracy and robustness. Comparing the three overvoltage protection methods in RTD modules, it can be seen that using analog switches and multiplexers with overvoltage protection has many advantages.