Modern RTD-Based Sensors

The popular belief is to not fix things that aren’t broken. The idea is to not tamper with something performing reliably and proving its worth. This advice aptly applies to circuit designs using RTD sensors that efficiently and quietly measure temperature in industrial manufacturing facilities worldwide.

However, in meeting the requirements of Industry 4.0, where smart factories are the norm, it is now evident that the current RTD sensors in use are not fitting the purpose. Automation engineers today want industrial temperature sensors to be of smaller form factors, flexible with communications, and capable of remote reconfigurability. Incumbent solutions, sadly, are unable to support them. However, it is possible to easily redesign these sensors to equip them with the necessary features to meet the new industrial design.

The RTD industrial temperature sensor translates temperature, a physical quantity, into an electrical signal. The typical range of such sensors is between -200 °C and +850 °C, with a highly linear response across it. RTDs commonly use metal elements like copper, nickel, and platinum. Among these, PT1000 and PT100 platinum RTDs are the most popular. While an RTD can use either two, three, or four wires, the 3-wire and 4-wire versions are the most popular. Being passive devices, RTDs require an excitation current for producing an output voltage. A voltage reference generates this current, with an operational amplifier acting as a buffer for driving the current into the RTD, which produces an output voltage signal varying in response to changes in temperature. The voltage signal may vary from tens to hundreds of millivolts depending on the type of RTD in use and the measured temperature.

An AFE or Analog Front End conditions and amplifies the low amplitude voltage signal from the RTD before the ADC or Analog to Digital converter digitizes it. A microcontroller runs an algorithm over the digitized signal, compensating for any non-linearity in it. The microcontroller then sends the processed digital output to a communications interface for transmission to a process controller. A typical implementation of the AFE is by a signal chain of components with each performing a dedicated function.

This discrete approach requires a large PCB or printed circuit board for accommodating all the ICs and power and signal routing, setting a minimum size for the sensor enclosure. Rather, modern RTD-based sensors use a superior and more minimal approach—the AD7124-4, an integrated AFE.

The AD7124-4 is a compact IC in a single package. It includes a multiplexer for accommodating multiple-wire RTDs, a voltage reference, a programmable gain amplifier, and an ADC using the sigma-delta operating principles. The IC has the capability to provide the necessary excitation currents for the RTD. The entire arrangement effectively replaces five of the signal-chain components from the traditional setup. Not only does this significantly reduce the amount of board space necessary, but it also enables the sensor to use a much smaller enclosure.

Next comes the communications interface. Modern RTD-based sensors typically use the IO-Link which eliminates the use of expensive ASICs for implementing specific network protocols. IO-Link is a 3-wire industrial communications standard for linking sensors and actuators with all industrial control networks.