A significant advantage of high granularity and highly sensitive semiconductor pixel detectors is the possibility to directly observe single tracks of charged particles. In many cases, however, these particles are accompanied with unwanted background radiation overlapping traces of interest. The detection selectivity can be increased using a triggering approach usually provided by an external trigger from other detecting devices such as ionization chambers, scintillating or semiconductor detectors. A self-trigger from the same sensor would be highly desirable. Unfortunately the Medipix/Timepix devices are not equipped with such self-trigger feature. A solution which is presented in this contribution makes use of the analog signal taken from the common electrode of the pixelated sensor. This signal, called back-side-pulse, is amplified by a custom made charge sensitive preamplifier which, after shaping, can be used as fast trigger and as independent spectroscopic signal. The stability and energy resolution of this analog signal is, however, strongly affected by electromagnetic noise interference from the digital read-out chip and its interface. In this article we present the solution based on the construction of a hardware galvanic shielded extender which isolates and effectively suppresses such noise interference. The result enables selective self-triggering according to the deposited energy of the detected particle. The technique and operation of the prototype are demonstrated on measurements with heavy charged particles from radioactive α-sources 241Am and 239Pu.