Analysis of Common-Mode Induced Even-Order Distortion in a Pseudo-Differential gm-C Filter

The effect of lacking input common-mode rejection of a pseudo-differential gmC-filter in a direct-conversion receiver is studied. The studied system consists of a differential mixer and a gm-C baseband filter with a passive pole at the input. In addition to the basic second-order nonlinearity due to mismatch, two additional error mechanisms specific to pseudo-differential circuits are found and analysed. Even-order distortion in the common-mode signal, which is due to the mixer, is found to leak back to the differential path. Furthermore, an interfering signal in the filter common-mode input will, even under perfect matching, produce signal distortion by modulating the transconductance.