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
CAUTION! New LabVIEW adopters -- it's too late for me, but you *can* save yourself. The new subscription policy for LabVIEW puts NI's hand in your wallet for the rest of your working life. Are you sure you're *that* dedicated to LabVIEW? (Summary of my reasons in this post, part of a voluminous thread of mostly complaints starting here).
