  The Cherenkov Imaging Telescope Integrated Read Out Chip (CITIROC) is a
32-channel fully analogue front-end ASIC dedicated to the read-out of silicon
photo-multiplier (SiPM) sensors that can be used in a variety of experiments
with different applications: nuclear physics, medical imaging, astrophysics,
etc. It has been adopted as front-end for the focal plane detectors of the
ASTRI-Horn Cherenkov telescope and, in this context, it was modified
implementing the peak detector reading mode to satisfy the instrument
requirements. For each channel, two parallel AC coupled voltage preamplifiers,
one for the high gain and one for the low gain, ensure the read-out of the
charge from 160 fC to 320 pC (i.e. from 1 to 2000 photo-electrons with SiPM
gain = 10$^{6}$, with a photo-electron to noise ratio of 10). The signal in
each of the two preamplifier chains is shaped and the maximum value is captured
by activating the peak detector for an adjustable time interval. In this work,
we illustrate the peak detector operation mode and, in particular, how this can
be used to calibrate the SiPM gain without the need of external light sources.
To demonstrate the validity of the method, we also present and discuss some
laboratory measurements.
