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A continuous-time (CT) sigma-delta modulator (SDM) for condenser microphone readout interfaces is presented in this paper. The CT SDM can accommodate a single-ended input and has high input impedance, so that it can be directly driven by a single-ended condenser microphone. A current-sensing boosted OTA-C integrator with capacitive feedforward compensation is employed in the CT SDM to achieve high input impedance and high linearity with low power consumption. Fabricated in a \(0.35\)-\(\upmu\)m complementary metal-oxide-semiconductor (CMOS) process, a circuit prototype of the CT SDM achieves a peak signal-to-noise-and-distortion ratio of 74.2 dB, with 10-kHz bandwidth and \(801\)-\(\upmu\)W power consumption.

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  1. \(g_m\) can also be increased by increasing the transistor’s size. However, this increase is limited if the transistor is already in the subthreshold region. Furthermore, a transistor shows worse linearity in the subthreshold region (i.e., weak-inversion region) than in the strong-inversion region.


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Correspondence to Dingkun Du.

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Du, D., Odame, K. A microphone readout interface with 74-dB SNDR. Analog Integr Circ Sig Process 81, 241–252 (2014).

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