Sigma delta converter with flying capacitor input

Abstract

A sigma delta analog to digital converter of the type adapted to be connected to a capacitive current input device, comprises: a sigma delta modulator having an input; and a flying capacitor constructed so as to be connected between the input of the sigma delta modulator and the capacitive current input device. In a preferred embodiment, the converter includes an input stage having an input coupled to the flying capacitor; a feedback path; a digital to analog converter (DAC) disposed in the feedback path and having a DAC output; and a DAC output capacitor and a second switch for alternately switching the DAC output capacitor between a first connection so as to transfer a signal from the output of the digital to analog converter to the DAC output capacitor and a second connection so as to transfer a signal from the DAC output capacitor to the input of the sigma delta modulator. The flying capacitor includes a charging capacitor and a flying capacitor switch, the flying capacitor being constructed so as to be alternately switched between a first connection so as to transfer a signal from the capacitive current input device to the charging capacitor and a second connection so as to transfer a signal from the charging capacitor to the input of the sigma delta modulator. The switching frequency of both switches is preferably operated at the same frequency fso as to define a common switching cycle for both the flying capacitor switch and the second switch, and synchronized so that the flying capacitor switch and the second switch are each switched to the first connection during one phase of the switching cycle and switched to the second connection during a second phase of the switching cycle. The converter has particular utility for any device having capacitive current input devices, such as CAT scanner detectors and electret microphones. The disclosure also describes and claims a method of converting a current provided by a capacitive current input device to a digital signal using a sigma delta analog to digital converter.