Considerations for fast settling operational amplifiers

The design considerations for fast-settling operational amplifiers (op amps) are significantly different between sampled-data switched-capacitor (SC) and conventional continuous-time applications. In SC circuits, the shape of the output voltage waveform of an op amp is of no consequence provided that the output settles to within a specified tolerance of its steady-state value prior to the next sampling instant. This feature allows for an optimum op amp frequency shaping to obtain a minimum small-signal settling time. The theory applies to any op amp that is well approximated by a two-pole model, including the conventional two-stage and single-stage folded-cascode topologies. As the commonly used equivalent-circuit Miller-effect model for frequency compensation has generally been improperly applied to two-stage transconductance amplifiers, it does not provide sufficient accuracy to achieve the optimum phase margin condition. Therefore, the use of equivalent-circuit models has been refined to provide greater accuracy and to eliminate some previous misconceptions