I keep maximal sampling rate, and only change frequency of sine wave. At low frequencies all seems okay. We use NI products for noise measurements, simultaneous sampling is needed for correlation or coherence spectrum measurements. I take both acquired signals and after some mathematical operations have complex coherence spectrum, actually, complex spectrum can be plotted as complex spectra modulus, its real part, or its phase. So I see peak in the modulus and real part spectra at frequency which is the same as sine wave frequency. If this frequency is low, there is no peak in phase spectra, but, peak occurs and increases as sine wave frequency reaches near MHz range and above. Mathematical part is proven many times, now only signal capture module is written for DAQ card.
Also now I think, that only frequency domain was looked for, so next time I should specially examine if skew is seen in time domain, as frequency is high... also its possible, (if non-simultaneous regime for 6111S is impossible, what sounds promising) that arrays handling can be made incorrectly.
I believe that tomorrow or so, after next my time in the laboratory, I will be able to post screens and I hopefully answer to problem.
