I have an NI 9215 with the following setup:
- ch1: sine wave input
- ch2: DC battery
- ch3: microphone input
All of these are floating signal sources and the NI 9215 offers differential measurement only. Significant noise appears on my signals. I am confident that the sources are not noisy, so I guess the noise comes from an improper setup. I have connected the wires to the AI+ and AI- on a channel basis, but I got really confused what I supposed to connect to the COM and how.
Plz help me out here.
NI publishes a document with recommendations for field wiring of DAQ devices. I did not search for it and do not recall the exact name but you should be able to find it.
Typically with differential inputs you need to place a large value resistor from each input to COM. The values of the resistors should be much larger than the source impedance of the things you are measuring and smaller than the input impedance of the DAQ device. 1 megohm is often a suitable value.
Let me correct myself... probably its not confusing once you got familiar with all these, but currently I dont really know what to do.
in my esperience the resistor values has to be 10Kohm<R<100Kohm.
1 MOhm seems too big to me.