Ni 9234 ground reference.

[MC_automotive]

Hi,

I give you some information about physical event to measure:

• I have to make an acceleration measures with MEMS accelerometers.
• The Voltage amplitude of signals regard my physical model is in the order of 10-15 mV
• The frequency of the event is in the order to 1-10Hz

Unfortunately I couldn't make a hardware choice for this application and I should obtain a result with current hardware configuration.

Hardware:

• NI cDAQ-9174
• NI 9234

The problem concerns ground reference of the module. Accelerometers are supply with a +-12Vdc with a Voltage power supply. I connect cDAQ and accelerometers to the same power supply in order to obtain a noise decrease (in fact I have reduced of five-fold the phenomenon) because the National standard power supply is of poor quality. Unfortunately I obtain a DC offset at about 1.6 Volt and 0.5 v for both channels.

I tried to acquire with Ni 9215 in differential mode and these offsets have disappeared.

So i think that problem is due to pseudo-differential mode of NI 9234, and in particular because It was create a ground loop with accelerometers and because every AI- is connected to the cDAQ ground through a 50ohm resistor.

I can't use ICP (IEPE) sensors because the AC coupling mode (it's the unique that can be selected for IEPE excitation circuit in NI 9234) cuts low frequency.

Do you have some feedback about this problem?

Do you think that my supposition it be possible?

How can I eliminate problem of offset?

Thanks for your support,

[m.amoruso]

Hi,

i'm Mario Amoruso, an NI's Application Engineer. Your supposition is right, the Input impedence of the NI 9234 in DC mode is: Differential: 305 Ohms/AI- (shield) to chassis ground: 50 Ohms, as outlined in the NI 9234 Operating Instructions and Specifications sheet. The NI 9234 operates in a pseudodifferential mode, where the impedance between AI+ and AI- is 305kohms as stated. The difference between a true differential mode and a pseudodifferential mode is that the AI- is referenced to ground through some impedance; for the NI 9234, this impedance is 50ohms. The input impedance can be considered 305kohms because that is the impedance that the DUT will see when it is connected to the NI 9234. The 50ohm impedance to ground comes about since a common mode voltage can source/sink current to/from the DUT because it has a reference to ground in the NI 9234.

As describe in the second link, to eliminate the common-mode rejection the differential mode is right choise.

http://www.ni.com/pdf/manuals/374238c.pdf

http://www.ni.com/white-paper/6962/en#toc2