Digital ECG systems are inherently non-linear, and usually cannot be properly tested using linear frequency or time domain testing. It is shown that the use of sinusoidal or triangular input signals to test plotted outputs may provide misleading results. Principal factors are the characteristics of pre-sampling and interpolation filters. In addition to validating sampling rate and bit precision, it is best to test system performance with real electrocardiographic input signals and verification of output waveform integrity. integrity.