Digital linearization and cancellation of capacitive coupling for a scanning tunneling microscope

This article describes the development and test of a digital control loop, to control the tip-to-specimen distance in a scanning tunneling microscope. This digital controller performs a frequency-independent linearization of the gap-to-current relationship, as well as the compensation of the undesirable capacitive coupling between the electrodes of the scan tube and the input of the current-to-voltage converter, two difficulties normally associated with analog controllers. In the described work, the control loop is implemented on an inexpensive fixed-point DSP, processing the signals at a 25 kHz sampling rate.