Differential Capacitive Readout Circuit Using Oversampling Successive Approximation Technique

Abstract

This paper designs a close loop $\Sigma - \Delta $ readout circuit for differential MEMS accelerometer. A technique named oversampling successive approximation (OSA) is employed to build basic amplifiers and integrators. This technique can largely reduce the gain error and thus low gain amplifier such as single stage amplifier is allowed to be used. As a result, the power consumption and chip area are reduced. However, the OSA-based amplifiers and integrators are vulnerable to the interference caused by charge injection and leakage current from the specific MOSFET switches. This drawback is analyzed in detail and the interference suppressing solutions are given. The OSA-based readout circuit is fabricated in a commercial 0.18 $\mu \text{m}$ BCD process. The measurement results show that the interference is reduced by 20 dB in the circuit with interference suppressing solutions compared with the circuit without interference suppressing solutions. And the noise floor is 24 $\mu \text{g}$ /rtHz. The readout circuit achieves a 0.07% gain error with a low power consumption of 0.5 mW and 9 MHz sampling rate.