TABLE OF CONTENTS A switched-capacitor array (SCA) can be used to provide effectively a coarse tuning of the total capacitance and thus of the oscillation frequency of an oscillator. Basically, an SCA is constructed with several switched-capacitor branches in parallel, each of which consists of a unit capacitor in series with a MOS transistor as a digital switch. By digitally controlling the gates of the MOS switches, the total capacitance can be controlled, and the oscillation frequency can be tuned. The unit capacitors in an SCA can be designed to be of the same value to achieve good matching and uniformity or to be binary-weighted to minimize the number of branches required. Figure 3.57 shows the schematic of a binary-weighted switched-capacitor array.
The MOS switches in an SCA are not perfect. Finite turn-on resistance ( R on) limits the quality factor of the capacitor. In addition, non-zero drain-to-bulk capacitance ( C db) and drain-to-gate capacitance ( C dg) are connected in series with the unit capacitor ( C u) and presents a finite parasitic capacitance when the switches are turned off, which inevitably degrades the maximum achievable tuning range for the SCA. A smaller turn-on resistance and thus a larger Q can be realized by using a larger switch size. Nevertheless, the parasitic capacitance will be increased, and the tuning range becomes smaller. In addition, the effect of the drain-to-source capacitance...
