I need to measure the angular speed of a DC motor. My idea is to fix a magnet on the spinning wheel and a hall effect sensor near to the wheel. What I need as output is a continuous voltage proportional to the frequency measured. If there is a practical way to achieve this using an IC could you please explain it to me? Or should I make the encoder myself? this second option feels a bit like re-inventing the wheel to be honest.
Look at the LM2907: Freq to Voltage Converter
This is the first device I found when I googled "frequency to voltage converter IC" I've never used one.
If you trigger a one-shot multivibrator from the digital output of a Hall sensor and low-pass filter the digital output (and buffer if necessary) you will get a voltage proportional to RPM. It is best if the multivibrator is operated from a well regulated supply voltage.
This works best if you get many pulses in the time you expect the voltage to react to changes in angular velocity.
Alternatively, it's always possible to do it digitally, and turn the resulting number into a voltage using a DAC.