Setting ADC compare mode in a differential input. Possible?

Hello. I am having a hard time trying to configure the ADC of the SAM3X8E (Arduino DUE) as I want to detect the events that make the differential A6-A7 input of the ADC go out of a certain window (I know the pins A6-A7 correspond to A0-A1 of the board). I am using a piezoelectric membrane connected between A0-A1 and configured those inputs as differential. By the time I configure the Comparison Window out of a range and the CMPSEL bits to detect channel 6 (A6-A7 differential corresponds to CH6), I get continuous COMP Events interrupts. I am doing something wrong or the comparison mode in a differential configuration cannot be used?

Thanks