Stair-case approximates of sine-waves have found wide range of applications including impedance measurements due to availability of integrated generators operating on the direct digital synthesis (DDS) principle. The DDS circuits produce signals of programmed frequency of a very good stability but the signals contain discontinuities because of their operating principle. In some measuring methods, especially those based on sampling of measuring signals, it is a source of errors originated from undesired components present in the signal spectrum. This problem has a practical meaning as resolution of modern analog-to--digital converters achieves 16 bits, which is much more than resolution of the DDS output digital-to-analog converters which is typically in the range of 10-12 bits. In the paper results of investigations of errors of magnitude and phase angle of output signal of digital phase-sensitive demodulator are presented. The number of elements of staircase approximation to sine-wave signal, number of signal samples, low-pass filter corner frequency and quantization error have been taken into consideration.