Hello dav,
To block DC, you may generally place a capacitor in series with the signal and the channel input of the DAQ board. No matter what value you choose, the DC should be blocked completely. You'll have to experiment with different values to see what works best for you, but in general the relationship is as follows: The larger the capacitor, the better AC signals are passed through the capacitor. Lower frequencies need a larger capacitor than higher frequencies. The larger the value of the capacitor, the less the AC signal will be attenuated.
As a starting point, you might try a 1 uF and go from there.
Russell
Applications Engineer
National Instruments
http://www.ni.com/support
