Apparatus and methods for detecting passing magnetic articles using a first, True Power On State (TPOS) 
detector during a first time interval and transitioning to a second, running mode 
detector after the first time interval, when the output of the second 
detector is accurate. A phase 
comparator is responsive to the output 
signal of the first and second detectors and provides a 
control signal indicative of a change in the 
phase relationship between the two output signals. An output switch controlled by the phase 
comparator provides, as the detector output 
signal, the first detector output 
signal during the first time interval and the second detector output signal thereafter. A threshold signal associated with the TPOS detector is at a fixed level during a first portion of the first time interval and is adjusted during a second portion of the first time interval in response to an 
automatic gain control circuit in order to reduce the 
phase error associated with transitioning from the TPOS detector to the running mode detector. Also described are apparatus and methods for updating a running mode threshold signal that is a percentage of the peak-to-peak 
magnetic field sensor signal, so that unnecessary update events are reduced.