A system includes signal acquisition circuitry, an oscillator circuit, and a processor. The signal acquisition circuitry is configured to receive from an intra-cardiac probe multiple intra-cardiac signals acquired by multiple electrodes of the probe, and to further receive a common ground signal for the multiple intra-cardiac signals. The signal acquisition circuitry is further configured to digitize the intra-cardiac signals relative to the common ground signal so as to produce multiple digital signals. The oscillator circuit is configured to generate an Alternating Current (AC) signal and to apply the AC signal to the common ground signal provided to the signal acquisition circuitry. The processor is configured to detect the AC signal in the multiple digital signals, and to assess, based on the detected AC signal, respective qualities of physical contact between the electrodes and cardiac tissue.
