A device that attenuates signal content outside the desired bandwidth. It precedes the analog-to-digital converter (ADC) in instrumentation designs theoretically allowing only frequencies in the pass-band to be sampled by the ADC.
It consists of a low pass filter designed to stop frequencies higher than the half the sample rate, in order to minimize aliasing.
Anti-alias filters are specified according to the sampling rate and so if you change the sampling rate the anti-aliasing filter changes frequency as well.
There must be one filter for each input channel.