Simulations
The simulations of the current source have been done by using LTSpiceXVII. The source file of the simulation are inside this repository at simulations/CurrentSource.
The schematic is shown in the image below.
NOTE: The maximal current as well as the current measurement branch has to be optimized after the first prototype.
There is a slight dependency of the output current on the load resistance - here the resistance of the PT100 - observable. From the simulation, the inital current is about 14.7 mA at 100 Ohm (0°C), which lowers to 12.7 mA at 150 Ohm (130°C).
Assuming a fully linear relation this results in 2 mA/30°C = 0.067 mA/°C. Nevertheless, this dependency is no reason to worry about, because the current through the PT100 gets measured and thus it can be analysed as well.
In case the current dependency is a problem - or a lower dependency is needed - R7 can be increased but the maximal current has to be optimised again.
The PWM filter simulation has been performed using a pulsed voltage source, and the settings, which would be used inside the STM32.
The files can be found in simulations/PWMFilter