Performance comparisons of pseudo-N-path SC cells in filters with real operational amplifiers

Four pseudo-N-path switched capacitor (SC) cells are compared with regard to finite op amp gain and gain-bandwidth ratio. The results show that a three-path RAM-type cell displays the best performance with respect to the other cells studied. Unfortunately, the RAM-type cell is affected by clock-feedthrough noise, which can limit drastically its field of application. Some improvement can be achieved using fully differential techniques. Clock-feedthrough-free operation is typical for circulating-delay-type designs, but they are much more affected by op amp nonidealities than RAM-type designs. Therefore, in applications where the signal-to-noise ratio is a critical filter requirement, the RAM-type cell based on the z to -z ^N transformation can be used, since it is fully immune to clock-feedthrough noise and gives a sensitivity performance just slightly higher than that of the three-path RAM-type cell