Many DAQ boards do not offer an internal lowpass filter. One of the most important reasons to need a pre-A/D filter is for anti-aliasing. If your signal has frequency content above your Nyquist frequency (1/2 your sample rate), that content will want to spill / bleed into your real measurements.
When your board doesn't offer internal filtering, and IF your signal has such high-freq content, it's vital to add hardware signal filtering in front of the A/D. An RC filter is simplest, not the most effective, but sometimes sufficient. Many other options are available.
-Kevin P
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