Dc restore scheme for capacitive sensing amplifiers with optimal noise filtering
AssigneeInvenSense, Inc.
InventorsLuca Collamati, Federico Mazzarella
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Abstract
A capacitive sensing charge amplifier, e.g., in a direct current (DC) feedback network, may incorporate a switching capacitor, including a first control switch and a second control switch, which receives a switching signal to charge and discharges the switching signal to a virtual ground of a sense amplifier. Noise incorporated into the output of the sense amplifier (e.g., a MEMS output signal) is filtered by a demodulation signal at a demodulator such that the period average noise at the demodulator output equals zero. The time varying nature of the switching capacitor resistance generally reshapes the system's post-demodulation noise to reduce its low frequency output noise.
Claims20
A microelectromechanical system (MEMS) device, comprising:
- a MEMS structure comprising a capacitive output corresponding to movement of the MEMS structure;
- a sense amplifier with a first node, a second node, and an output node, wherein a feedback capacitor and a switching capacitor are each connected in parallel between the first node and the output node;
- a demodulator coupled to the output node;
- and
- a signal generator configured to output a demodulation signal to the demodulator and a switching signal to a first control switch and a second control switch of the switching capacitor, wherein the first control switch is connected between a first side of the switching capacitor and the output node, and wherein the second control switch is connected between a second side of the switching capacitor and the first node.
A method, comprising:
- receiving a capacitive output from a MEMS structure at a first node of a sense amplifier;
- applying a switching signal to charge a switching capacitor based on a first portion of a duty cycle of the switching signal;
- applying the switching signal to discharge the switching capacitor to a virtual ground, wherein the virtual ground is the first node of the sense amplifier, and wherein the discharge of the switching capacitor is based on a second portion of the duty cycle of the switching signal;
- outputting, from an output node of the sense amplifier, a MEMS output signal;
- applying a demodulation signal to a demodulator, wherein the demodulator is coupled to the output node of the sense amplifier, and wherein a frequency of the switching signal is equal to or twice as great as a frequency of the demodulation signal;
- and
- demodulating the MEMS output signal based on the demodulation signal.
Dependencies are inferred from the claim text.