Hi,
By measuring average Ampere draw you can get an approximate idea of motor speed,
I don't agree.
The average motor current is a measure of torque, not if speed.
But here we even can't say average motor current, because it depends on duty cycle, load behaviour, CCM vs DCM through freewheel diode.
There is a much better relationship between duty_cycle and RPM than average_switch_current and RPM.
I've done some tests in the past and found out that for a motor (complex load) with variable load there is no good relationship between
average_switch_current and average_motor_current, because there are too much unknown parameters.
But the setup is good to protect the transistor against overcurrent, but then one should use a fast reacting comparator, no averaging and ADC.
Maybe the OP is satisfied with the average switch current.
Then keep on nyquist:
first decide the sampling frequency that is useful for your application,
then calculate a useful anti aliasing filter.
Klaus