The Vishay NTCLE317E4103SBA 2 point sensor thermistor (accuracy of +/-0.5°C between 25 and 85°C) is simulated in LTspice XVII : the polynomial curve of Steinhart and Hart as well as the aforementioned accuracy are modeled as precisely as can be.
The presented circuit intends to provide a temperature reading from a thermistor voltage after sampling and digital quantization. This temperature reading is then compared to real temperature and the (simulated) sensor accuracy is then represented graphically.
Here is the circuit:
We present the results corresponding to a sample time period of 10 ms and we assume that the ADC bit number is 24. The measured temperature is represented by the voltage source V1 which swings between low and high temperature. The thermistor voltage V(NTC) is then sampled and converted to digital signal and then retransformed into a temperature knowing the Steinhart and Hart coefficients (A, B, C and D) of the NTCLE317E4103SBA thermistor.
LTspice is the ideal software to perform such simulation as the thermistor characteristics tolerances can be easily expressed in temperature terms via flexible ABS (Arbitrary Behaviour Sources). The netlist of the thermsitor modeling can be found in the software section of the project. We have adopted a Monte Carlo distribution : this means that each simulation run picks up one tolerance of the sensor randomly within the tolerance range. We analyze here the sensor accuracy alone but in a further simulation, every other circuit element could be studied with their own tolerances.
Once thirty runs are completed, we plot the temperature measurement error in function of the thermistor temperature V(therm) (transformed in °C) and we get the following graph.
This reproduces accurately the 2 point sensor precision (+/-0.5°C between 25°C and 85°C, and then increasing up to +/-1°C around 140°C).
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