Apparatus for converting a sequence of electrical current pulses into a sequence of steady voltage levels having magnitudes significant of the charge content of the respective pulses includes an integrator for receiving the current pulses, via an input amplifier and one input of an adding network, and operative to perform integration of each pulse over its duration. A first follow and hold circuit is connected to the output of the integrator via a first gate circuit and is operative to follow the integrator output and hold the steady voltage level at the completion of the integration of a pulse. The output of the first follow and hold circuit is connected via a second gate circuit to a second follow and hold circuit for delivering the steady voltage corresponding to each current pulse until another voltage level corresponding to a succeeding current pulse is received from the first follow and hold circuit. The output of the first follow and hold circuit is also connected to a second input of the adding network via a third gate circuit to feed to the integrator simultaneously with the integration of a pulse a reset signal proportional to the steady voltage level corresponding to the integrated value of the preceding current pulse.