Switched-capacitor ladder filters with offset-insensitive time-continuous feedback integrators

The possibilities of designing high-order switched-capacitor (SC) ladder filters which are insensitive to the DC offset voltage and at the same time guarantee that all operational amplifiers operate permanently in their active region are investigated. The proposed circuit technique is based on an offset-insensitive fully differential topology, where all operational amplifiers are maintained in the active region by time continuous feedback capacitors. The damping introduced by these additional capacitors leads to a lossy SC ladder filter. A comparative example between a lossless and a lossy 5th-order allpole ladder filter is given in terms of Monte-Carlo sensitivity simulation, showing how the suggested technique can show similar performances. The effect of the finite operational amplifier DC gain on the frequency response of the lossy SC ladder filters can be compensated by the use of a simple predistortion method. The technique seems to be attractive for GaAs technology, where real operational amplifiers can put serious limitations on monolithic analogue designs.