Employ Kinetis KM3x MCU for PT100 sensing

This document demonstrates how to precisely measure the temperature difference between two PT100 sensors using the Kinetis MKM34Z128 processor. The measurement is to be used for a heating meter design. The heating meter design requires very precise temperature sensing in the range of 0°C to 200°C of absolute temperature, and the temperature difference from dt> 3°C, where the relative differential temperature error is better than 3%. A temperature difference error for low temperature difference (~3°C) may be measured with an error below 1.5%. This MCU offers four very precise 24bit sigma delta ADCs which may be used for temperature sensing. For accuracy, all components/parameters causing drifts should be avoided. The PT100 sensor resistivity is rated against a known normal resistor RN. A current mirror is used to ensure the same current is flowing through the sensor and the normal resistor. This topology doesn't require precise current or voltage source, only a stable normal resistor is needed. To calculate the temperature from the resistivity provided by the PT100 sensor, a 3rd order regression polynomial is used. All calculations are done in 32bit signed fractional fixed-point arithmetic.

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Originally published by Freescale Semiconductor Inc. at www.freescale.com as "Kinetis KM3x MCU PT100 Sensing".