1. An filter comprising: a digitally controlled switch; a voltage source for providing a predetermined voltage signal; a capacitor coupled to the voltage source through the digitally controlled switch for storing a charge at the predetermined voltage; an operational amplifier having an input node coupled to the capacitor and the switch, such that the node and the capacitor are charged by the voltage source when a connection through the digitally controlled switch is enabled and the input node is charged by the capacitor when the connection through the digitally controlled switch is disabled.