An Automatic Offset-Cancellation Circuit for Differential MEMS Capacitive Sensors

The inevitable mismatch between rest capacitances or additive parasitics in a differential MEMS sensor leads to the output offset voltage. The mismatch originates from MEMS process imperfections and asymmetric bonding of the differential sensor to the readout circuit. Unfortunately, these mismatches escalate the linear behavior of the readout circuit in the presence of the parasitic capacitance at the analog front end. In this work, we present an automatic offset cancellation technique that cancels the output offset while improving the deteriorated linearity due to the mismatches. The proposed method uses a coarse-fine routine and the well-known correlated double sampling amplifier to balance the injected charge from both sensor capacitors at 0 g state. The coarse tuning uses a digitally controlled capacitor bank, while a DC voltage is used for fine-tuning. The accelerometer has a scale factor of 0.2 pF/g within the \(\pm \,\hbox {10\,g}\) range, and the readout electrical sensitivity is designed to be 1 V/pF. A prototype is fabricated in a \(0.35\,\upmu \hbox {m}\) standard CMOS technology that occupies 1.6  \(\hbox {mm}^2\) (including pads) of silicon with a core size of \(0.54\,\hbox {mm}^2\) and consumes 4.6 mW power from a 3.3 V supply driving a 150 pF load. The reference voltage and clocks are generated internally and can be applied off-chip. Experimental results prove that the chip can cancel up to a \(\pm \,\hbox {3\,pF}\) mismatch between sensor capacitors with a resolution of 3 fF, improving the SFDR from 42 to 75.5 dB. After offset cancellation, the measured resolution is 176 aF within a 100 kHz bandwidth, corresponding to a dynamic range of 81.2 dB.