Hello,
Although you do not mention what NI DAQ device you are using, I am assuming you have a standard Multifunction DAQ card. The BNC-2120 is not a measurement device; it is simply a terminal block that provides you with connectivity to your DAQ device. The SC-2345 is front-end signal conditioning hardware for a DAQ card and also is not a measurement device. Neither of these will allow you to measure capacitance. A standard DAQ card can be used for voltage measurements, but it will not perform capacitance measurements. Theoretically it could be possible for you to measure capacitance using a voltage measurement device. You would need to provide an excitation signal and sweep it across a range of frequencies, then measure back the voltage to determine the cutoff frequency. However, this could get tricky because you will need to take into account factors such as the internal capacitance of the DAQ device and the resistance of the circuit you are measuring. You probably won't get the most accurate measurements using a DAQ card to measure capacitance and it will take a considerable amount of programming to develop the algorithm to sweep the excitation signal and compute the capaciatnace.
The NI 4072 FlexDMM and LCR Meter can be used to perform capacitance measurements. I would strongly recommend that you consider using this card for your capacitance measurements. You can read NI Developer Zone Tutorial: Capacitance/Inductance Measurements to find out more about the algorithm that the NI 4072 uses to measure inductance, and also to find some general pointers on factors to consider when making capacitance measurements.
I hope this helps. Please let me know if you have any questions and I'd be happy to give you some more assistance.
