An apparatus includes detection circuitry and gating circuitry. The detection circuitry is configured to sense a radio frequency (RF) ablation signal that is applied to a heart by an intra-body probe, and to identify time intervals during which an amplitude of an ablation signal is within a predefined window. The gating circuitry is configured to gate an electrocardiogram (ECG) signal acquired in the heart, such that the ECG signal is sampled only within the identified time intervals.