The delta-sigma ADCs and DACs on DSA devices, C Series Delta-Sigma devices, and NI 433x devices employ digital filtering that imparts a delay of several sample intervals. The filter delays are equal in a homogeneous system, so these delays cancel out when performing phase measurements between channels. However, the filter delays differ in a heterogeneous system. This can introduce errors in phase comparisons between channels on different devices or between channels on similar devices running at different rates. Such phase errors are always deterministic, and you can account for them in software.