Differential measurements of AC signals

[jrestrepo]

We have a NI 9205 with a cDAQ and we want to mesure 6 AC signals

Three AC signals are floating.  Voltage 0-5V rms

Three AC signals are earth grounded Voltage: 0-5V rms

If we want to make differential measurements in both group of signals, should we use bias resistor for every AC floating signal connected to COM, and every leg from each AC earth grounded signal to COM too.?

Is it possible to measure the AC grounded signals as NRSE connecting each grounded leg to AISENSE?

We notice that the signal connected between AI0+ and AI0- is enough to reference the ADC, and the rest can be connected as DIFF without any reference to COM.  Is it normal and safe to use just one signal reference to COM in our case?

What is better:  To measure the grounded signals as NRSE using AISENSE as common reference (3b AI chanels) or use differential measurement but connecting the all the -legs to COM (as reference to prevent the 9205 ADC from floating)?

[bpracek]

Hi!!!

If your module supports differential acquisition, always is better choose this type of measure to avoid noise.

You use it for both signal sources, grounded and floating.

In the floating case, in fact may be necessary use a bias resistor.

To plug your signals on the module always follow the instructions that you found on the manual.

On the link below, you'll find a good application note that explains detailed some wiring considerations for analog measures:

https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider...

Hope this helps!

Regards,

[jrestrepo]

Following the manual instructions and you recommendations we connected the signals (see attached file) . The floating signals with Bias resistors (22 K) to COM and the grounded signals to COM according to the attached file. We notice that connecting the floating signals to channel A0 and A8 everything works well, but when connecting the grounded signals to channels A0 and A8 the floating signals measurement gets saturated. Notice that the signals were connected interspersed . We are measuring 3 floating signals and 3 grounded.

We have notice that connecting only the A8 of the first floating signal to COM we obtain satisfactory results. Could you please help us with an explanation for this behavior and which connection do you recommend us.

Best regards,

Juliana