I am trying to use a NI USB-6002 DAQ to measure the noise produced by a number of different 24 volt power supplies. The NI USB-6002 DAQ is only able to measure about +10 to -10 volts so I am thinking I need to use a voltage divider. I setup a voltage divider using four 5.1 MΩ resistors but I was getting a LOT of noise.
I simplified the circuit to use a 9 volt battery and only one resistor (as shown in the image below) and I was still getting about 550 mv difference between the maximum and minimum voltages over a 1 second period (i.e. noise). I am sampling 1000 times per second and have tried using RSE (single ended) and Differential modes.
If the resistor is removed, there is only 1 or 2 mv of noise.
We noted from the DAQ's datasheet that the Input impedance is >1 GΩ but I don't really know what that means.
Can you please let me know what I am missing? I don't think I should be getting practically any noise from a battery and a resistor. My end goal is to measure the noise of a number of 24 volt power supplies.