Sub-μW switched-capacitor circuits using a class-C inverter

In a switched-capacitor (SC) circuit, the major block is an operational transconductance amplifier (OTA) designed in order to form a feedback loop. However, the OTA is the block that consumes most of the power in SC circuits. This paper proposes the use of a class-C inverter instead of the OTA in SC circuits and a corresponding switches configuration for extremely low power applications. A detailed analysis and design trade-offs are also provided. Simulation and experimental results show that sufficient performance can be obtained even though a class-C inverter is used. The second-order biquad filter and the second-order SC sigma-delta (∑Δ) modulator based on a class-C inverter are designed. These circuits have been fabricated with a 0.35-μm CMOS process. The measurement results of the fabricated SC biquad filter show a 59-dB signal-to-noise-plus-distortion ratio (SNDR) for a 0.2-V_p-p input signal and 0.9-V dynamic ranges. The power consumption of the biquad filter is only 0.4μW with a 1-V power supply. The measurement results of the fabricated ∑Δ modulator show a 61-dB peak SNR for a 1.6-kHz bandwidth with a sample rate of 200kHz. The modulator consumes 0.8μW with a 1-V power supply.