An ablation catheter has an ablation electrode at a distal end coupled to an ablation power source through a low impedance coupling. The ablation electrode also functions as a sensing electrode, for monitoring the endocardial signal and preferably also tissue impedance during the ablation procedure, and is coupled to an electrode monitor through a high impedance coupling. A timing element operates a plurality of switches to selectively isolate, dampen, or interconnect various signal paths during plural repetitive non-overlapping ablation and quiescent intervals which alternate at a rate substantially above a Nyquist sampling rate. RF energy is delivered to the ablation site during the ablation intervals. The local endocardial signal is measured during the quiescent intervals.