With the advent of IoT applications, electric mobility and increasing industrial automation, the precise simulation of passive components, such as temperature sensors, is becoming increasingly important. Complex mechatronic problems require powerful software to perform difficult calculations, and efficient simulation models for electronic components. Since there are hardly any realistic models available for passive components, generic models are often used. Such simulations provide qualitatively correct results, but their accuracy is limited.
For a temperature sensor circuit based on an RTD resistor, however, Vishay provides a precise SPICE model that enables much more precise simulation than generic models.
The active part of the measuring circuit consists of an operational amplifier NJU7098A by New Japan Radio, which is characterized by an extremely low current consumption of just 2μA.
If this setup is to be used for temperature measurement, the circuit diagram can be described as shown in Image 1.
An SMD platinum sensor (PTS1206) from Vishay with accuracy class 1B serves as the input signal for the temperature measurement. This type of linear temperature sensor has become increasingly popular in the automotive industry since attaining its AEC-Q200 standard qualification, as it provides a good alternative to conventional SMD NTCs for applications involving high stability and temperature requirements. Another key advantage of the PTS over NTCs is the linearity of the electrical characteristic. Although NTCs are more sensitive than RTDs, they are not nearly linear enough over the wide temperature range from –40°C to +85°C, even after linearization.
Analog Devices does of course provide a usable LTspice model for this circuit, which is available to download from its website. In this, the PTS sensor is represented by a variable resistor.
Engineers who are familiar with simulations will notice a particular detail in this model: although this is a temperature measurement circuit with a specified overall accuracy of ±1°C, the variable temperature (global ambient temperature) does not appear anywhere – neither in the SPICE directives nor in the definition of the PTS.
To save users of New Japan Radio’s NJU7098A models from having to delve into the PTS data sheets, Vishay has explicitly included the SPICE model for DC temperature sweep. This extended model enables the following features:
- Adjusting/passing through the temperature
- Visualizing the influence of the TC tolerances of the PTS
- Fine-tuning/determining the feedback resistance
- Testing the circuit with Monte Carlo tolerances of all passive components (fixed resistors, PTS)
- Calculating the effective output voltage accuracy of the NJU7098A as a measure of temperature in °C (Image 2)
Image 2 shows that the circuit itself, including all component tolerances, has a linear temperature characteristic (top window) with a total accuracy of mostly ±1°C (bottom window) – which had to be proven.
The analysis could be taken deeper, such as with a dynamic temperature change over time for the PTS sensor. This would require a different SPICE model for the sensor however. This could be used to demonstrate an important effect: the response of the sensor over time. If the sensor turns out to be too slow for the planned application, a smaller sensor – such as in the 0805, 0603, or even smaller format – is a good alternative.
The example shows that a SPICE model provided by the sensor manufacturer complements the simulation model for the IC outstandingly well. It also shows that there are numerous possibilities for developments in the field of temperature sensor simulation.
The simulation described in this article can also be found in [1].
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