The range of the DAQ card does not specify the minimum voltage you can measure, but rather how much that analog-digital converter (ADC) needs to amplify your signal. The range of the ADC is -10 to +10 volts, and when a range of -0.2 to +0.2 volts is specified, the signal is amplified to meet the voltage range of the ADC. LabVIEW automatically selects the voltage range based on the minimum and maximum voltages you specify when you create a DAQmx task.
I think what you need to know is the smallest voltage change in your signal that you can detect. When using a 200 mV range, the smallest voltage change that that can be detected is 4 uV, which is given in the 9205 user manual. Thus, you should be able to measure your 10-50mV signal just fine. Sincerely,
Julianne K Systems Engineer, Embedded Systems Certified LabVIEW Architect National Instruments