Sampled analog filtering using switched capacitors as resistor equivalents

The equivalence between resistor-capacitor (RC) filtering circuits and circuits containing switches and ratioed capacitors is examined. The authors have demonstrated the advantages of ratioed capacitors for integrated active filters experimentally, using a specially designed integrated MOS test chip. Polysilicon gate technology was used to fabricate the chip, with the capacitors being formed by two levels of polysilicon. For the theoretical description, and as a design aid, attention is drawn to the general equivalence between a resistor and a circuit element consisting of a capacitor and two switches. Nonideal effects, which set limits to the generality, are also discussed.