Practitioners prefer analog transfer systems (filters) to digital computers for the test of experimental controllers with simulators or real plants. The analog filters must be converted to equivalent sampling filters for the final implementation on a process computer. The author gives some limits to known approaches for these transformations and introduces a "QTA-method" for the approximation of continuous filters by sampling filters in the frequency domain. The abbreviation "QTA" means "quotient transversal filter approximation". The application is demonstrated using a low pass filter, a PID controller, and the design of higher order differentiators. The proven secand and parabola differentiators are contained in the solution set. Using the example of the PD control of a double integrator it is shown that differentiators of higher orders yield better stability.