Hey Yunsan,
Those programmable ranges are there to provide you with better resolution if you know the maximum/minimum voltage will be inside ±0.2V (for instance).
To demonstrate this, the ADC on the 9205 has 16 bits of resolution. At ±10V, the bin size for the device would be 20V/2^16 = 305 microvolts. This means that the device would be able to measure changes in increments of 305 microvolts. Now if the maximum voltage range is set to ±0.2V, the bin size would change to 0.4V/2^16 = 6.1 microvolts, which can be useful as long as you know that your measurement will not go outside of this range.
Another thing to consider is the Absolute Accuracy of the device at those programmable levels. According to the Operating Instructions on page 22, the AI Accuracy of the device changes with this setting as well. For example, at ±10V the Absolute Accuracy at Full Scale 6.23 mV, which means that while the given digital value will fall within a bin size of 305 microvolts, the actual reading of the device that falls within one of those bins could be up to ±6.23 mV off. Now if set to ±0.2V, the accuracy would be 174 microvolts. This means that again, the bin size is 6.1 microvolts but the actual reading could be up to 174 microvolts off. Information on other things to consider such as noise and sensitivity can be found in the document linked above.
So to answer your question, yes you can measure 30mV with either the setting of ±10V or ±0.2V. The difference in these settings for that example would be (taking just accuracy into consideration) that for ±10V your 30mV input would measure within the range of 30mV ± 6.23mV and for ±0.2V it would measure within the range of 30mV ± 0.174mV.
A suggestion would then be to use the smallest nominal range that you know you can get away with.
Regards,
Jake G.
National Instruments
Applications Engineer
