Study of parasitic capacitance effect on low pass CMOS active filters

This paper clarifies a classic first-order low pass filter could not be considered as the classical one when we take into account the input parasitic capacitance of the CMOS amplifier, which contributes a zero into the transfer function of the low pass filter. Based on that analysis, interestingly a so-called virtual full pass filter is designed using the same structure, which frequency response is however limited by the open loop transfer function of the amplifier. It highlights the importance of taking into account the parasitics in high frequency circuit design. Analysis of parasitic capacitance of high-order low pass filters also proves it may decrease the order of the filters.