National Instruments NI 9233 - Manual (Page 12)

NI 9233 Operating Instructions and Specifications 12 ni.com
The relative phases of these signals also have a
frequency-dependent delay. The variation in the phase delay with
frequency is called the phase nonlinearity. Figure 6 shows the
phase nonlinearity for data rates above 25.65 kS/s and less than or
equal to 25.65 kS/s. The phase nonlinearity scales directly with the
oversample rate, so the two curves normalize the signal frequency
to the data rate.