Bootstrap sampling circuit with accurately averaging pre-charge circuit

Abstract

A sampling circuit may include a sampling capacitance, an electronic sampling switch, and a switch controller. The electronic sampling switch may have a control input that controls whether the electronic sampling switch is in a sample state or a hold state. The electronic sampling switch may connect the sampling capacitance to an input signal while in the sample state and disconnect the sampling capacitance from the input signal while in the hold state. The switch controller may control the control input to the electronic sampling switch so as to cause the electronic sampling switch to be in the sample state during one period and the hold state during another period. While in the sample state, the switch controller may cause the impedance of the electronic sampling switch that is seen by the input signal to be substantially independent of the voltage of the input signal. The switch controller may include a pre-charge circuit that pre-charges the control input to the electronic sampling switch prior to each commencement of the sample state to approximately the average of the voltage of the input signal and the voltage on the sampling capacitance immediately prior to each commencement of the sample state. The amount of the pre-charging may be substantially independent of the voltage of the input signal.