Hi Naushica,
Where are you finding the autozero function you're trying to use? Autozero is generally included with instrument drivers rather than the DAQmx API, which you should be using for the DAQ board and SCXI chassis you are using.
All an autozero function will do is shift the whole signal you've already sampled either down (to compensate for a positive DC offset) or up to compensate for a negative offset. There won't be any resolution increase in doing this, in fact you will actually lose part of your range at either side of the signal, as the maximum signal you can digitize is effectively reduced whichever way you shift the signal away from.
The best way to block DC from entering your system is to use a capacitor on the input if possible - if you choose the value of this capacitor correctly it should block the DC offset and only allow the AC component through with as little attenuation as posssible. The value you use will be based upon the frequencies of AC you are inputting - have a look in electronic textbooks about coupling capacitors to work out the value.
I hope this helps!
Mark
National Instruments UK & Ireland
