A calibration apparatus for calibrating a magnetic sensor configured to generate an output signal indicative of magnetic field strength when a bias signal is applied to it is disclosed. The apparatus includes a test magnetic field generator (MFG) to generate magnetic fields of known magnitude, and further includes a processor to control the MFG to generate a known magnetic field, control the sensor to generate a test output signal when the MFG generates the known magnetic field and a known bias signal is applied to the sensor, and determine how to change the bias signal based on a deviation of the measured test output signal from an expected output signal. Using a test MFG that produces known magnetic fields when known bias signals are applied to sensors allows evaluating and compensating for changes in sensitivity of the sensors by accordingly changing bias signals applied to the sensors.