In the AFM, the tip is attached to a spring in the form of a cantilever. As the tip moves over the surface, the cantilever bends back and forth in the z direction. A laser beam is directed onto the cantilever and as the cantilever bends, the movement of the reflected beam is detected by a photo diode. A feedback circuit integrates this signal and applies a feedback voltage to the z piezo (PZT) to exactly balance the cantilever bending. Since the probe force is proportional to the cantilever bending, this is constant. The image of the surface is built up as a series of scan lines, each displaced in the y direction from the previous one. Each individual line is a plot of the voltage applied to the z piezo as a function of the voltage applied to the x piezo.

In a further development at Quesant, the error signal E representing bending of the cantilever not corrected by the feedback is scaled and added to the Z signal to provide a mathematically correct image (I=E+Z).

The broadband mode allows the PID loop to be optimized for the best tracking of the probe but the image quality is superior because surface information is not lost due to the low-pass nature of the PID loop.
Under best conditions, the z-axis response Z will be about 3500 Hz, while the error response will be 25000 Hz. Under conditions where the PID gains must be reduced to prevent oscillation in portions of the image, the error component E will still provide the full bandwidth, and detail in the image will not be lost, even at the highest scan speeds.