According to the Nyquist theorem, in order to resolve a signal component correctly, the sampling rate needs to be at least twice the signal frequency (i.e. to resolve a 50 Hz signal component, the sampling rate should be at least 100 samples per second [100 Hz]).
However, using this example, signal components higher than 50 Hz will appear as a lower frequency than they actually are when sampled at 100 Hz. This phenomenon is known as signal aliasing.
An anti-aliasing filter looks at the user-specified sampling frequency and removes any under-sampled frequencies that may appear in the signal (i.e. signal component frequencies >0.5 of the sampling frequency).
PowerLabs with Anti-Aliasing filter include the PowerLab 15T, 26T, 2/26, 4/26 and later.
What is an Anti-Aliasing Filter?