Rif.: ni 9234

[MC_automotive]

Hi,

I give you some information about physical event to measure:

• I have to make an acceleration measure 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 frequencies.

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,

[LiciaP]

HI,

did you try to connect the chassis ground to earth ground?

Also, what did you select for Iex source and coupling?

If you connect a waveform generator to the 9234, do you see the same offset?

Bye,

Licia

[MC_automotive]

In order:

-Yes, but I obtained the same result.

-I can't use AC coupling mode because events studied have low frequencies (I should avoid to cut frequencies upper to 0.3 Hz).

-I obtained a different result in fact the problem is due to the ground reference, but I wouldn't lose the advantages that I obtained with using the same power supply.

Thanks,

Marco

[LiciaP]

Hi Marco,

What is the value of excitation current required by your sensor? Did you set Iex to internal?

Bye,

Licia