I tried to record a RF waveform ~800mV and 20kHz. I checked RF signal looked great with scope. However, I got very different result with NI USB-6356 Analog input. Can anyone give me some suggestions?
Please provide more information,
I observe that you've enabled 50-ohm termination on the scope, have you implemented the 50-ohm termination for the DAQ?
Thank you for your information!
I thought all the AI ports are 50-ohm. Would you please let me know why we need 50-ohm termination for DAQ? If I need to use ports 0~3, shall I terminate ports 4~7 with 50-ohm connector?
50-ohm termination is available only for high-speed or RF instrumentation, the DAQ you've is neither and hence required a 50-ohm termination at the input.
Typically all DAQ ADCs are high-impedance to measure small signals and not to load typical circuits. As mentioned in the specifications, the input impedance is very high.
I would definitely recommend a 50-ohm resistor at the AIs or else your RF signal is driven into high impedance causing harmonics and double the voltages.
Just a 50-ohm connector such as SMA/SMB will not be sufficient, you definitely need a 50-ohm resistor to gnd at the front of AI.