Relationship between common-mode rejection and differential-mode distortion in fully differential Gm-C filters

A relationship between common-mode rejection and differential-mode distortion of the fully differential ladder Gm-C filters employing pseudo-differential transconductors is investigated. The equation which relates differential-mode HD3 and common-mode gain of the Nth-order ladder Gm-C filters is derived. It has been found that in the cases where the inverter-based pseudo-differential transconductors are employed, the HD3 level of the odd-order filter is always lower than that of the even-order filter. By using the derived equation with the extracted parameters of certain inverter-based transconductors, it was found that by varying the common-mode gain from 0 to 1, the HD3 of the odd-order filters and even-order filters are decreased by 4.9 dB and increased by 3.1 dB, respectively. Simulation results are found to be in good agreement with the derived equation.