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This paper investigates the sources of non-linearity of a self-balancing capacitive half-bridge sensor interface. It has been shown that the linearity of the interface output with respect to input acceleration is infinity at DC input signals when half-bridge sensor element is assumed as parallel plate capacitor pair. The linearity degrades as the input signal frequency gets higher. The source of the non-linearity is found out to be the phase difference between senor sensor element displacement and the interface output voltage. The analytical expression for the HD3 of the interface is derived. It has been seen that to increase the linearity of the interface, the phase difference can be decreased by increasing the cut-off frequency of the interface loop.