Capacitor charge control technique applied to digitally programmable IIR switched-capacitor filter

Abstract

This paper presents a novel procedure for the implementation of digitally programmable switched-capacitor recursive (IIR) filters. The use of a reduced number of poles assures low sensitivity in the passband, being better than that obtained with an elliptic filter. The response of the proposed structure - a couple of poles and four couples of zeros - is similar to a fifth order elliptic filter. Digital-to-analog conversion (DAC) techniques allow filter programming by controlling capacitor charge instead of adjusting a capacitor bank, reducing both silicon area and costs. Multiplexing individual operational amplifiers among 2nd order FIR modules reduces the number of required op-amps and, consequently, power dissipation. The effects of charge injection and operational amplifier offset voltages are greatly reduced by using a totally differential filter structure. Emphasis in solutions to parasitic grounded capacitance problems is presented.