I. Introduction
The active filters are very common circuits used in a wide variety of electronic applications, from data acquisition and measurement to telecommunication systems [1]–[4]. Usually, these filters are implemented with operational amplifiers combined with fixed resistors and capacitors, but very often their parameters, such as corner frequency, quality factor and gain, need to be adjustable. In this case, the digital potentiometers may play an effective role whenever the filter characteristics have to be changed by altering the resistance values.