Reading analog voltage data through USB-6215 using NI MAX v16

[OmidMech]

Hi everybody,

I have a power supply and I want to measure its voltage using analog input part of the DAQ (USB-6215). I have one screw terminal (16-input port) attached to the analog input part of the DAQ. I am using two ports of the terminal in order to connect two sides of the power supply to the DAQ. It has a voltage around 1.5V. When I am using the NI MAX software, and I am trying to read the values from the four ports, including two connected ones (phase and null for the power supply) and two unused ones, I am expecting to read zero (or close to zero) from three of the ports and around 1.5V from one port, but I am reading 1.5V from all four ports! Did I need to apply any settings while setting up? Thank you in advance for the help.

Best regards,

Omid

[crossrulz · Accepted solution]

If you are reading inputs that are unwired, this is expected.  The DAQ works by multiplexing the signals to the ADC.  The output of that mux has a capacitance.  So if there is nothing coming in, you will just read the voltage still on that capacitor.  If you want to read 0V, then wire those channels to your ground.

[OmidMech]

Thanks for your reply. Does this mean that I should also wire the null end of the device to the ground?

[crossrulz]

If you are referring to the negative side of the power supply, then a definite yes.