Parasitic Insensitive, Biphase Switched Capacitor Filters Realized With One Operational Amplifier Per Pole Pair

The use of active switched capacitor (sc) filters1"3 as constituents in large-scale integrated (LSI) subsystems3"10 has been rapidly expanding. Crucial to the realization of manufacturable metal-oxide-semiconductor (MOS) SC filters has been the development of parasitic insensitive sc networks.6,1112 As a consequence, it has become a commonly accepted notion that insensitivity to both top and bottom plate parasitic capacitances requires realization with one op-amp per pole (i.e., two op-amps per pole pair). However, since op-amps consume power, represent about 20 percent of a filter's die area, and are sources of noise and power supply feed, it is useful to consider techniques that reduce the number of op-amps required to implement a given transfer function. The purpose of this paper is to introduce practical techniques for achieving this, while retaining the crucial parasitic insensitivities mentioned previously. A straightforward technique for reducing opamp count is to time share or multiplex13,14 each op-amp among two or more storage capacitors. A problem with this approach is that each op-amp operates without feedback during the dead zones of the non685