Interesting ... probably we could start from pm dc motor.
1) Is your pm dc motor 'brushed' or 'brushless' type of dc motor?
2) What is your application?
3) What is the controller (microcontroller or DSP controller)? What is the processing speed of your controller?
4) If close loop, what kind of control you wish to apply? Speed control? Torque control? Or, position control (servo application)?
I think both, i.e. hardware and software, can be improved to measure more accurately. Noise (inteference, ground loop, etc) reduction is important, especially from the power switching drive.