Low‐offset differential output switched‐capacitor interface for micro‐capacitive acceleration sensors

Abstract

Abstract This paper presents two simple but effective switched capacitor based differential output capacitive sensor interfaces, designed and fabricated in United Microelectronics Corporation 180 nm complementary metal oxide semiconductor (CMOS) process technology. The first configuration combines chopper modulation and switched capacitor based correlated double sampling, while the second configuration utilises chopper modulation with a modified demodulation scheme to reduce noise and offset. The proposed designs utilise a fully differential operational amplifier (op‐amp) with single‐feedback loop, which reduces the mismatch between the single‐ended outputs in the differential configuration. The proposed configurations are analysed in perspective of noise to achieve high accuracy with less noise and integrated with supporting circuitry. The fabricated application specific integrated circuit (ASIC) is first characterised with the internal on‐chip capacitors and then integrated with micro‐electro‐mechanical systems (MEMS) based differential capacitive accelerometer sensor on a printed circuit board (PCB) for dynamic testing. The integrated PCB and a commercially available accelerometer (ADXL335) is mounted on a sub‐woofer system and the dynamic testing is carried out. The measurement results show that the sensor interface is capable of measuring sub‐fF capacitance change with zero offset less than 2 mV. The first configuration provides sensitivity of 720 mV/g, whereas the second configuration attains sensitivity of 385 mV/g. The noise floors are 400 μg/Hz, 325 μg/Hz, whereas the capacitance resolutions are 70 and 60 aF, respectively.

Keywords

• Time varying and switched networks
• Power electronics, supply and supervisory circuits
• Amplifiers
• CMOS integrated circuits
• Fabrication of MEMS and NEMS devices
• Microsensors and nanosensors