Continuous-time and switched capacitor monolithic filters based on current and charge simulation

The authors are concerned primarily with low-pass filters. A typical section on an LCR network for polynomial low-pass filters, which are those not possessing transmission zeros at finite frequencies, is presented. For this kind of filter, a signal-flow-graph (SFG) representation may conveniently be obtained. The authors choose as variables for the SFG the inductor currents and capacitor voltages and hence obtain the SFG presented. It is noted that the branch weights in the SFG are either +1 or -1 unity branches or are of the integrator form 1/sTi, where Ti is the value of an inductor or capacitor in the LCR network and s is the complex frequency variable. The authors combine the +1 and -1 branches, which occur in pairs, at the inputs or outputs of the 1/s type integrator branches, obtaining an implementation in terms of differential input integrators. The authors show that the use of current or charge integrators allows them to carry out nodal voltage scaling while at the same time obtaining low sensitivity which is not possible using voltage integrators alone. For elliptic filters, use of a judicious combination of voltages and currents (or charges) provides the lowest sensitivity of any approach with acceptable component parameter spread