NI 9215 really noisy measurements

[NotTodayBoss]

Hi,

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.

[johnsold]

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.

Lynn

[Andrea_Perin]

Hi,

probably Lynn was mentioning this documents:

Field Wiring and Noise Considerations for Analog Signals

Best regards

Andrea

[NotTodayBoss]

I believe its still confusing: this says I suppose to connect two 10k-100k resistor to both AI+ and AI- while this says I should connect a 1M res to AI-.

I dont have a 1M resistor now, but tried with a 650k, it saturated my input.

[NotTodayBoss]

Let me correct myself... probably its not confusing once you got familiar with all these, but currently I dont really know what to do.

[Andrea_Perin]

Hi NotTodayBOss,

in my esperience the resistor values has to be  10Kohm<R<100Kohm.

1 MOhm seems too big to me.

Best regards

Andrea