Resistor Differential Measurement

[DAI-AT]

Hello,

I want to connect a floating voltage signal source to a NI-9202 in differential configuration. In the Getting-Started-Guide of the NI-9202 it is stated, that the negative lead has to be connected to COM through a 1 MΩ resistor.

However, the tutorial Grounding Considerations (https://www.ni.com/en/support/documentation/supplemental/06/grounding-considerations---intermediate-...) recommends 10kΩ<R<100kΩ.

Why is that difference and which way should I follow?

Is the  "AI GND"-Connection, shown in the tutorial, equal to the COM port of the NI-9202?

Thanks for your answers.

[Henrik_Volkers]

Both is possible,

In short, what's best, depends on:

your Source, mainly output impedance and source impedances to EARTH/surroundings

your cable length and surrounding EMC

With two resistors the input impedance of your 9502 is 2R (instead of >10GOhm as stated in the spec), but bad vor high impedance sources (like ph probes)

If you have a low impedance source and longer cable, the 2 R solution help to minimize electric field influences.