A multi-chamber cardiac pacing systems for providing synchronous pacing to at least the two upper heart chambers or the two lower heart chambers or to three heart chambers or to all four heart chambers employing one or more field density clamp (FDC) sense amplifiers for accurately sensing and timing cardiac depolarizations of the right and left heart chambers is disclosed. The synchronous pacing of one of the right and left heart chambers is provided on demand following expiration of programmable pace CDW and sense CDW that are started by both a paced event and a sensed event first occurring in the other of the right and left heart chambers. The delivery of the pacing pulse is inhibited by a sensed event detected in the other of the right and left heart chambers before the expiration of the corresponding CDW. In a four channel atrial and ventricular pacing system, the right and left atrial chambers are sensed and paced as necessary upon at the end of a V-A escape interval and right and left, pace and sense, AV delays are commenced for sensing ventricular depolarizations in the right and left ventricles. The four channel system is programmable to pace and sense in three selected heart chambers. Each FDC sense amplifier allows the timing of a short CDW from a paced event or a sensed event. Preferably, a pacing output stage is coupled with the FDC sense amplifiers to deliver pacing pulses to each heart chamber.