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FEATURE SENSORS & SENSING SYSTEMS


REDESIGNING RTD-BASED FOR THE SMART


In this article, we find out how a resistance temperature detector (RTD) industrial temperature sensor can be quickly redesigned to meet smart


factory demands for temperature measurement devices with smaller form factors, flexible communications, and remote configurability. This can be done using a highly integrated analogue front end (AFE) and an IO-Link transceiver. Brian Condell, product applications engineer, and Michael Jackson, end market specialist, both with Analog Devices, explain


Figure 1. An example of an RTD-based temperature sensor


TEMPERATURE SENSOR BUILDING BLOCKS


The building blocks of an RTD industrial temperature sensor are shown in Figure 2. An RTD translates a physical quantity


T


he old saying ‘if it ain’t broke, don’t fix it’ cautions against tampering with something that performs reliably and regularly proves


its worth. Arguably, this advice applies to the circuit design used in many RTD sensors that quietly and efficiently measure temperature in industrial manufacturing facilities worldwide. However, to meet the requirements of Industry


4.0, factories are becoming smarter, and it is becoming clear that many current RTD sensors will not be fit for purpose in these environments. Smaller form factors, flexible communications, and remote configurability are some features that automation engineers now demand from industrial temperature sensors, but incumbent solutions cannot support them. This article revisits the building blocks used in the design of many RTD-based temperature sensors and discusses the limitations that these impose on a sensor’s application. It then shows how to quickly redesign this type of sensor to equip it with the features required in this new industrial age.


(temperature) into an electrical signal and is typically used to detect temperatures between -200˚C and +850˚C, having a highly linear response over this temperature range. Metal elements commonly used in RTDs include nickel (Ni), copper (Cu), and platinum (Pt), with Pt100 and Pt1000 platinum RTDs being the most common. An RTD can consist of either two, three, or four wires, but the 3-wire and 4-wire versions are the most popular. Since they are passive


devices, RTDs require an excitation current to produce an output voltage. This can be generated using a voltage reference, buffered by an operational amplifier that drives current into the RTD to produce an output voltage signal that varies in response to changes in temperature. This signal varies from tens to hundreds of millivolts depending on the type of RTD used and the measured temperature, as shown in Figure 3.


The AFE amplifies and conditions the low amplitude RTD signal before the analogue-to- digital converter (ADC) digitises it for the microcontroller to run an algorithm to compensate for any non-linearity it contains. This sends the digital output to a process controller via a communications interface. The AFE is commonly implemented using a signal chain of components in which each performs a dedicated function, as shown in Figure 4.


Figure 3. A voltage signal produced by a Pt100 RTD in response to increasing temperature


Figure 2. A block diagram of an RTD industrial temperature sensor 24 DESIGN SOLUTIONS MAY 2024


Many existing temperature sensor designs use this discrete approach that requires a printed circuit board (PCB) large enough to accommodate the footprint of all the integrated circuits (ICs) and the signal and power routing and sets a de facto minimum size for the sensor enclosure. A superior and more straightforward approach uses an integrated AFE like the AD7124-4 shown in Figure 5. This compact IC is a complete AFE in a single package and includes a multiplexer, voltage reference, programmable gain amplifier, and a sigma-delta ADC. It also provides the excitation currents for the RTD, meaning it can effectively


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