Oversampling Successive Approximation Technique for MEMS Differential Capacitive Sensor

Abstract

This paper proposed an over sampling successive approximation (OSSA) technique to build switched-capacitor capacitance-to-voltage convertor (SC-CVC) for readout circuit of MEMS differential capacitive sensor. The readout circuit employing the OSSA technique has significantly improved resistance to common-mode parasitic capacitance of the input terminal of the readout circuit. In the OSSA readout circuit, there are 5 main nonideal characteristics: holding error, recovery degradation, increment degradation, rise-edge degradation and charge injection which reduce the accuracy and the settling time of the circuit. These problems are explained in detail and their solutions are given in the paper. The OSSA readout circuit is fabricated in a commercial 0.18um BCD process. To show the improvement evidently, a reported traditional readout circuit is also reproduced and fabricated using the same process. Compared with the traditional readout circuit, the proposed readout circuit reduces the affect of common-mode parasitic capacitance on the accuracy of SC-CVC by more than 23.8 dB, reduces power dissipation by 69.3%, and reduces die area by 50%.

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“…Compared with (5), (10) shows that the error term 1/(1 + 1 / ) is to be diminished and it tends towards zero exponentially with the increase of N. This means that the error will be reduced to infinitesimal regardless of the A1 and the parasitic capacitances CP1 and CP2, but this is at cost of settling time. The capacitor COSA is the key component holding the error voltage, which should be charged accurately in order to guarantee the accuracy.…”

“…The accuracy of the OSA based CMCVC is similar to that of the OSA based DCVC. The process of diminishing the error caused by the parasitic capacitance is similar to that described by (6)- (10). The OSA based CMCVC should also be driven by the nested non-overlapping clocks rather than the traditional non-overlapping clock to achieve the high accuracy.…”

“…The readout circuit employs the switched-capacitor circuits [10], [11] rather than the oscillator based circuits [12]- [14] in consideration of power efficient. The analog front-end circuit structure of the readout circuit is shown in Fig.…”

“…The gains of DCVC and CMCVC are programmable to fit the sensing cell's capacitance, which are set by the programmable capacitor arrays CD and CCM. Secondly, it employs the "Oversampling Successive Approximation" (OSA) technique [10], in order to suppress the output DC error brought by large grounded parasitic capacitance (i.e., CP1 and CP2 of the sensing cell shown in Fig. 1(c)).…”

“…The OSA technique is described as follows [10]. As the MEFS sensing cell guides the high density electrical field into the ground, it introduces large parasitic capacitances to the ground, which appear as CP1 and CP2 in Fig.…”