Editor's note: I am pleased to bring you an excellent RAQ by Yao Zhao, a product application engineer with the China design center in ADI Beijing.
Is it possible to design a complete RTD module with overvoltage protection?
RTD (resistance temperature detectors) can provide excellent stability, accuracy, and reduce the impact of noise and interference. RTD sensors can be made up of either 2-wire, 3-wire, or 4-wire versions, and require an excitation current to produce an output voltage. AD7124-4/AD7124-8 contain two well-matched current sources, a PGA, reference buffers, and diagnostic functions, which are ideally suited for high reliability RTD modules.
Incorrect operations, careless connections, and exposed wires usually lead to overvoltage faults in industrial circumstances, which will damage the electronic devices and cause undesirable consequences that may occur from the damage. Overvoltage protection ability is a key specification for an RTD module. Outside of transient overvoltage protection, durable overvoltage protection must be considered during actual production.
This article will focus on how to provide a total solution for multiwire RTD modules with an overvoltage protection function, which is based on AD7124, and overvoltage protection, detection multiplexers and channel protectors. This article can help designers understand this method and choose appropriate devices.
For overvoltage protection functions, here are three optional solutions:
- Using series resistors in front of the pins of an ADC can help protect the AD7124 easily. These pins include analog input and excitation output pins, but the resistors will limit compliance voltage.
- The protection of the current source can be realized by discrete components. This solution can achieve higher overvoltage protection and larger voltage compliance. However, the switches and multiplexers are still externally exposed.
- ADI’s overvoltage protection and detection switches, multiplexers, and channel protectors (ADG52xxF and ADG54xxF) can be used for RTD module protection and different wires RTD sensor switching. These parts can provide +/-55 V fault voltage protection in both the powered and unpowered mode, and can implement fault detection with latch-up immunity. Their high-density package occupies a much smaller PCB area than traditional solutions.
RTD Module Based on AD7124
Ratiometric measurement is widely used in RTD modules, as it can remove the error and drift of the excitation current source. Figure 1 shows the typical diagram for 4-wire RTD measurement based on AD7124-8.
4-wire RTD ratiometric measurement based on AD7124-8.
AIN0 provides the excitation current, and the AD7124 integrated reference buffer and PGA, REFIN and AIN are high impedance inputs, so the same current flows through the RTD sensor and reference resistor. The ADC conversion result is the ratio between input voltage (VRTD) and reference voltage (VREF), which is equal to the ratio between RRTD and RREF. If RREF is a known, high precision, and stable reference resistor, RRTD can be calculated by the RREF value and ADC conversion result.
By using a 4-wire RTD configuration, the system can achieve high accuracy and reliability and the error due to lead wire resistance can be removed. Accordingly, the cost is higher than a 3-wire or 2-wire configuration. Figure 2 shows the 3-wire RTD measurement based on AD7124, which is a compromise between performance and cost.
3-wire RTD ratiometric measurement based on AD7124-8.
Two integrated, well-matched current sources are helpful for 3-wire RTD measurement. VREF and VRTD can be expressed by the following two functions:
AD7124 has two well matched current sources, which means IEXC0 is near or equal to IEXC1 and the lead resistors RL1 and RL2 are very similar. The functions can be expressed as:
Express the conversion code as a composition of these two functions:
According to this function, the RTD resistor value can be calculated by the conversion result and reference resistor value. Please refer to CN-0383 for details.
For a 2-wire RTD, the error due to lead resistance can’t be cancelled, but the cost of these type RTD sensors is lower than the others, and AD7124-8 can be configured as a 2-wire RTD sensor, as seen in Figure 3.
2-wire RTD ratiometric measurement based on AD7124-8.