in a fully differential switch cap integrator the output common mode is set by a switch cap common mode feedback circuit. the theory of this can be applied in various ways. basically apply charge to a cap and switch the cap into your high impedance node. just remember a charged cap being applied to a high z node acts like a battery causing that node to be the voltage across the cap. I have seen continuous time c feedback integrators set their input common mode this way( cmfb for the output and a switch cap setting the input cm = to the output cm)