A fast continuous sampling digitizer has been designed to acquire fast scintillating detector signal from cerium activated lantanum bromide (LaBr3(Ce)) scintillation crystal. These are foreseen in the FAMU experiment which is aimed at spectroscopic measurements of muonic hydrogen, possibly providing insights into the so-called proton radius puzzle. The board, named GSPS, is implemented as an FMC mezzanine which hosts two off-the-shelf sampling ADC used in interleaved timing architecture, achieving a 1 GS/s sampling rate (with a 12-bit nominal accuracy over the first Nyquist interval of frequencies, ranging from DC to 500 MHz). Interleaved technique allowed us to keep both lower production costs and simple acquisition system avoiding complex interface protocols like JESD204. The board will be described; the test setup and the used methodology to characterize the device will be explained; the achieved performances will be shown and discussed. In particular it will be pointed out how the two interleaved ADCs can be calibrated in order to get the best performance. The different contributions to both noise and distortion affecting the device will be analyzed applying general techniques for high-sampling-speed continuous ADCs. Lastly, future improvements will be introduced: in particular, the solutions we foresee to get a effective number of bits of about 10 over the whole first Nyquist interval will be enlightened.