1.2 V CMOS switched-capacitor circuits

In battery-powered portable systems, low-voltage CMOS integrated circuits are essential for low power consumption. While integrating analog and digital circuits on the same chip, it is preferred that both analog and digital circuits share the same voltage supplies. However, a low supply voltage forces severe constraints on the design of analog circuits and the conventional CMOS transmission gates may no longer be adequate or even functional as analog switches if the signal swing of the switch control is kept between the nominal supply voltages. Design techniques for 1.2 V CMOS switched-capacitor (SC) circuits are described. MOS transistors with low-threshold voltages are not required. The signal paths are fully differential to maximize noise immunity against disturbances from supplies and substrate, critical when a large number of digital circuits are on the same chip. The analog switches are implemented with nMOS transistors. An on-chip high-voltage generator is used to generate the high-voltage required to turn on the nMOS switches. The circuits use a 0.8 /spl mu/m n-well double-poly double-metal CMOS technology. The threshold voltages are 0.7 V for the nMOSTs, and -0.8V for the pMOSTs.