A circuit includes a first input terminal, a second input terminal, a third input terminal and an output terminal. A first summation node adds signals at the first and third input terminals. A second summation node subtracts signals at the second and third input terminals. A selector selects between the added signals and subtracted signals in response to a selection signal. The output of the selector is integrated to generate an integrated signal. The integrated signal is compared by a comparator to a threshold, the comparator generating an output signal at the output terminal having a first level and a second level. Feedback of the output signal produces the selection signal causing the selector to select the added signals in response to the first level of the output signal and causing the selector to select the subtracted signals in response to the second level of the output signal.