Dielectric charging of insulating films in microelectromechanical systems (MEMS) has a crucial effect on the operation of those devices. A new method is presented in order to characterize the dynamics of the charge trapped in the dielectric layer of MEMS devices. This allows knowing the state of the charge at each sampling time without distorting the measurement. This approach allows one to model the expected behaviour of the trapped charge inside the dielectric as a response to a sigma-delta control of charge. The goodness of the proposed approach is obtained by matching the experimentally obtained closed loop response with the one predicted by the model obtained using the proposed characterization method.