A system includes signal acquisition circuitry, anoscillator circuit, and a processor. The signal acquisitioncircuitry is configured to receive from an intra-cardiacprobe multiple intra-cardiac signals acquired by multipleelectrodes of the probe, and to further receive a commonground signal for the multiple intra-cardiac signals. Thesignal acquisition circuitry is further configured todigitize the intra-cardiac signals relative to the commonground signal so as to produce multiple digital signals.The oscillator circuit is configured to generate anAlternating Current (AC) signal and to apply the AC signalto the common ground signal provided to the signalacquisition circuitry. The processor is configured todetect the AC signal in the multiple digital signals, andto assess, based on the detected AC signal, respectivequalities of physical contact between the electrodes andcardiac tissue.
