Correlated double-sampling (CDS) is widely used to suppress the effect of flicker noise in switchedcapacitor (SC) circuits. Once the flicker noise is suppressed by CDS, the noise of the SC circuits is ultimately determined by the thermal noise. In this work, we develop a method to calculate the thermal noise in SC integrators as functions of a variety of circuit parameters such as capacitor size and switch resistance; this methodology is then applied to a CDS integrator as well as a conventional integrator. We found that for the CDS integration scheme, in order to avoid significantly increasing the noise power of the integrator, the size of the CDS capacitor should be comparable to that of the sampling capacitor. We also found that if the CDS capacitor is sufficiently large, the noise power of a CDS integrator is almost the same as that of a conventional integrator with the same sampling capacitor size. These findings are explained based on the bandwidth of the transfer functions.

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A design of an on-chip ac-coupling preamplifier for mobile applications is presented. A microphone preamplifier should have a large input impedance to mitigate the effective-gain reduction caused by the microphone’s non-zero output impedance. From a review of previously reported microphone preamplifier structures in terms of input impedance, feasibility of on-chip ac-coupling, and noise performance, we chose an inverting amplifier structure with capacitive feedback. In addition, to provide dc bias path, we used off-state MOSFET switches as pseudo-resistors of very large resistance in the giga-ohm range. The large resistance enables on-chip ac-coupling with sufficient noise performance. A fast start-up is achieved by turning-on the switch for a short period during the preamplifier start-up. The gain of the preamplifier can be programmed from 0 dB to 21 dB with 3 dB steps. A 2-stage pseudo-class-AB amplifier was adopted to reduce power consumption. The proposed preamplifier was implemented using a 28 nm CMOS process and achieves 107 dB dynamic range in a 20 kHz bandwidth under 0 dB gain setting and balanced differential input signal. The preamplifier dissipates a power of 270 μW with a 1.8 V supply.

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