Electrocardiograms (ECGs) are one of the most important parameters in all kinds of patient monitors. Extracting high-quality ECG signals from 50-Hz interference is a difficult problem for many monitor manufacturers. In this paper, a novel ECG amplifier, which can efficiently restrain 50-Hz interference, as well as high-frequency interference, is detailed. Experiments are described that show that ECG modules with this kind of amplifier can get very high common mode rejection rates (CMRRs) and extract high-quality ECG signals, even under strong interference conditions.