Since its invention in the 1990s single-molecule force spectroscopy has been increasinglyapplied to study protein (un-)folding, cell adhesion, and ligand–receptor interactions. Inmost force spectroscopy studies, the cantilever of an atomic force microscope (AFM) isseparated from a surface at a constant velocity, thus applying an increasing force to foldedbio-molecules or bio-molecular bonds. Recently, Fernandez and co-workers introduced theso-called force-clamp technique. Single proteins were subjected to a defined constant forceallowing their life times and life time distributions to be directly measured. Up to now, theforce-clamping was performed by analogue PID controllers, which require complexadditional hardware and might make it difficult to combine the force-feedback with othermodes such as constant velocity. These points may be limiting the applicabilityand versatility of this technique. Here we present a simple, fast, and all-digital(software-based) PID controller that yields response times of a few milliseconds incombination with a commercial AFM. We demonstrate the performance of ourfeedback loop by force-clamp unfolding of single Ig27 domains of titin and themembrane proteins bacteriorhodopsin (BR) and the sodium/proton antiporter NhaA.