A circuit has a first window comparator determining whether a signal at a first input has a voltage higher than a first threshold but lower than a second threshold, and a second window comparator determining whether a signal at a second input has a voltage higher than the first threshold but lower than the second threshold. A logic circuit generates pulses in response to either the first window comparator determining that the signal at the first differential input has a voltage higher than the first threshold but lower than the second threshold or the second window comparator determining that the signal at the second input has a voltage higher than the first threshold but lower than the second threshold. A filter circuit receives the pulses from the logic circuit and generates a flag indicating that the signal is invalid, based upon pulses received from the logic circuit.