An apparatus comprises: a first comparator configured to: receive an input proximity signal indicating a proximity of a target device, receive a second reference signal associated with a second distance, make a first comparison of the input proximity signal to the second reference signal, and provide a first output proximity signal based on the first comparison; and a second comparator configured to: receive the input proximity signal, receive the first output proximity signal, make a second comparison of the input proximity signal using the first output proximity signal, and provide a second output proximity signal based on the second comparison.